A GROUP OF SIX PAPERS
PRESENTED AT THE LAKE PLACID CONFERENCE
OF THE SOCIETY OF MOTION PICTURE ENGINEERS
OUTLINING THE FUNDAMENTAL FACTORS INVOLVED
- IN THE PRODUCTION OF SOUND PICTURES

FALL MEETING OF THE
SOCIETY OF MOTION PICTURE ENGINEERS
LAKE PLACID, N. Y. SEPTEMBER, 1928

VOL. XII, PP. 633-643; 657-741 SEPTEMBER, 1928
THE BELL SYSTEM TECHNICAL JOURNAL
VOL. VIII, PP. 159-208 (Papers 3, 4, 5, 6-1) JANUARY, 1929

. The Quality of Speech and Music.....
_ General Principles of Sound Recording
_ Recent Advances in Wax Recording.

sounds probably played equally important parts. Although all
three have survived, the voice has become the one most universally
used. With the invention of the telephone and the phonograph
it became possible for man to communicate at a distance and to
perpetuate his speech. Now man may communicate at a distance
and perpetuate, not only his speech, but his grimaces and gestures
as well.

This progress has been accompanied by extensive investigations
into the physical nature of speech and hearing. It is the aim of the
present paper to consider the bearing of various data on the sensory
aspects of speech and music. A selected list of published papers
that have been consulted is attached.

The organs of speech (Fig. 1) consist of the lungs (not shown in
figure), the trachea or windpipe, and the mouth, nose and throat
cavities. The larynx is situated at the upper part of the windpipe
and contains two muscular ledges known as vocal cords. They are
so arranged as to form a straight slit through which the breath
passes.

In the process of speaking, the lungs by their bellows-like action
force streams of air in and out through the vocal passages. As the
streams of air pass through the slit formed by the vocal cords, they
are set in vibration so that the slit is alternately opened and closed.
A train of sound waves is thus set up in the lower part of the throat.
As these waves pass out into the air, certain resonant and transient
characteristics are impressed upon them by the vocal cavities and
the movements of the tongue, lips, etc. It is these variations which
we interpret as speech sounds.

These sounds are called unvoiced sounds and the vocal cords do
not enter into their production. They are produced by frictional
vibration set up in the mouth itself.

Both groups may be divided into two classes—those produced
by a continuous flow of air which may be called the continuants
and those produced by the sudden stoppage of the air which may .
be called the stops. In the former class are such sounds asa, v, f,
etc. In the latter class are such sounds as p, g, d, t, etc.

Although the vocal cords lend quality to the voice they are
not highly essential for producing the distinguishing characteristics
of the speech sounds. These are produced mainly by the mouth

and nose cavities. This is evidenced by the fact that we can under-
stand whispered speech, in which the vocal cords play no part.
As a matter of fact counterparts of the lungs and vocal cords can
be located quite outside the body and still produce intelligible speech

A piece of apparatus known as the artificial larynx is a good
illustration. An ordinary bellows serves as the lungs, a rubber tube
as the windpipe, and a little metal box as the larynx. Inside the
box is a reed which vibrates with the passage of air, much like the
old style automobile horn. The interrupted air stream is led into
the mouth by means of a short pipe which is attached to the metal

. This device is used by people who have undergone a surgical
operation known as tracheotomy. In this operation the larynx is
removed and the windpipe is terminated by a small hole in the
patient’s neck, through which he breathes. In this case, the bellows
is dispensed with, and the rubber tube is fitted over the hole, so
that, when he breathes a train of sound waves is produced. In order
that he may speak these sound waves are directed into his mouth.

The mechanism of hearing (Fig. 2) is divided into three general
parts, the outer ear, the middle ear and the inner ear. The outer
ear consists of the external part and the ear canal. The middle ear
contains three small bones which are called the hammer, the anvil,
and the stirrup. They connect the ear drum with the small window
or diaphragm ‘‘O”’ of the inner ear. In the inner ear is located the
organ of hearing. It consists of a spiral cavity ‘“‘S” surrounded by
bone. The cavity is filled with liquid. As may be seen from the
figure a spiral ledge projects into the cavity. The liquid above this

** The production of the simple speech sounds and also connected
speech was demonstrated with the above type of apparatus.

+ Reproduced, with permission, from Textbook of Physiology by William
H. Howell, M. D., W. B. Saunders Co., Philadelphia.

ledge is separated from the liquid below it by a flexible membrane.
The two windows “O” and ‘‘E”’ retain the liquid at the base. At the
apex, the membrane is pierced by a small hole so that the liquid
may flow from the upper side of the ledge to the lower side. When
the window at ‘“O” is pushed in, the window at ‘EK’ bulges out and
vice versa. The nerves of hearing are distributed along the flexible
membrane.

Sound enters the ear as successions of minute condensations
and rarefactions of the air, which are known as sound waves. They
cause minute increases and decreases in the air pressure near the
ear drum. These variations are called sound pressure and cause the
ear drum to vibrate. It is supposed that the liquid in the inner ear
surges up and down the spiral in accordance with the frequency
of vibration. If the frequency is low the pressure is relieved through
the flexible membrane near the apex. If the vibration frequency is
high, the pressure is relieved near the base. In other words, tones
of different frequency disturb the membrane in different positions
along its length. This separation enables us to identify the pitch of
the tone.

Speech waves are composed of a large number of tones of
different vibration frequency which change continually during utter-
ance. These tones cause disturbances which are spread out along
the membrane containing the nerves of hearing. The high tones
disturb one end of the membrane, the low tones disturb the other
end of the membrane. If one could see this membrane it might
resemble the keyboard of a player piano in operation. At one in-
stant the keys at one part of the scale are disturbed; at a later
instant the keys at some other part of the scale are disturbed. In
the case of the ear, the pattern of the disturbance on the membrane
is carried to the brain and interpreted as a speech sound.

Fig.3 shows the range of sound pressure and frequency which the
ear can perceive. The vertical axis represents the sound pressuref fT
and the horizontal axis represents frequency of vibration. In the
sensation of tone, loudness is caused by the sound pressure and pitch ©
is caused by the frequency of vibration. The lowest pitch that can
be sensed as a tone is of an order of 20 cycles, the highest is of an
order of 20,000 cycles. The lower curve represents the sound

pressures at which the various pure tones are just audible. It will
be noticed that the ear is less sensitive to the low notes, that is,
a greater pressure is required for these tones to become audible. The
upper curve represents the sound pressures at which the tones cause
a sensation of feeling or pain. The faintest tone that can be heard
and the loudest tone that can be heard stand in a ratio of one million
to one. The ear can detect any tone that falls in the area enclosed
by the two curves.

Studies on the wave forms of speech sounds have shown that
the pitch of man’s voice is of an order of 128 cycles (vibrations per
second), that of woman’s voice of an order of 256 cycles. In both
cases, overtones of the fundamental chord tone occur in the speech
sounds. These studies have shown frequencies as high as 8000 or
9000 cycles in various speech sounds. Studies on the interpretation
of speech sounds have also indicated the presence of tones covering
a large part of the audible frequency range. This can best be seen
by considering the parts of the auditory sensation area which are
particularly important to the interpretation of the various speech
sounds, as shown in Fig. 4.

Although in the above figure the speech sounds have been
grouped in sharply defined areas, it must be understood that
actually the sounds overlap these areas somewhat, and that the
indicated areas are those which are most important in the inter-
pretation of the sounds. The three voiced consonants which are
symbolized by the letters v, z, th (as in then), are exceptions and

belong in the unvoiced consonant area. The various speech sounds
cover a frequency range up to 8000 cycles, and a sound pressure
range of 60 sensation units. The area above 8000 cycles does not
contribute appreciably to speech interpretation. It does, however,
carry a large part of the sounds of breathing which take place during
speech.

In general, woman’s speech i is more difficult to ae se than
man’s. This may be due in part to the fact that woman’s speech
has only one-half as many tones as man’s, so that the membrane
of hearing is not disturbed in as many places. We may suppose,

therefore, that the nerve fibres do not carry as much data to the
brain for interpretation. The greatest differences occur in the case
of the more difficult consonant sounds. In woman’s speech these
sounds are not only fainter but require a higher frequency range
for interpretation. A range from 3000 to 6000 cycles for man’s

woman’s voice. Since the ear is less sensitive in the latter range
and the sounds are initially fainter, their difficulty of interpretation
is greater.

There is one other phenomenon of hearing which enters into
interpretation. When sounds containing a number of tones are
increased in loudness, the lower tones in the sound deafen the
auditor to the higher tones. This deafening or masking effect
becomes very marked when the sound pressure of the lower tones
is greater than twenty sensation units. In the case of speech, this

effect impairs the interpretation of the higher pitched sounds. The
optimum loudness for the interpretation of speech corresponds to
a cound pressure between 0 and 20 sensation units. If the sound
pressure is less than this, the fainter sounds are inaudible. If the
sound pressure is greater, the masking effects impair the inter-
pretation of these sounds. 3

As in the case of speech, musical sounds consist of a funda-
mental frequency and various overtones of the fundamental. The
tones, however, are sustained for appreciable lengths of time, and
when.they are changed, the changes usually take place in definite
steps, known as musical intervals. The pitch of the tone is deter-
mined by the fundamental frequency. This frequency, however,
need not be present in the musical tone as the overtones which are
multiples of the fundamental may cause the correct pitch sensation.

The frequencies that are present in music depend upon the type
of instrument and the character of the music. The composition
of musical tones can be obtained by considering the construction
of various types of instruments, and by wave form studies.

Musical instruments may be divided into two general classes,
string instruments and wind instruments. With strings, the tones
may be produced by plucking, striking, or bowing, and are usually
reenforced by resonating air cavities and sounding boards. With
wind instruments, the tone may be produced with the aid of reeds
as in the clarinet, or without reeds asin the flute. In the playing of
horns the lips act as reeds.

Each class may be further subdivided into melody and harmony
instruments. In the former only one note is usually produced at
a time; in the latter several notes are usually produced simul-
taneously. In general, harmony instruments are capable of pro-
ducing notes over a much wider frequency range than melody
instruments. A given type of instrument of the latter class may,
therefore, include several instruments covering different frequency
ranges such as the bass, tenor and alto trombone.

Experiments have indicated that notes of different frequency
or pitch as produced by a musical instrument, appear about equally
loud to the ear. This seems reasonable when we consider that the
ear has played an important part in the design of such instruments.
Since the sound pressures that are necessary for equality of loud-
ness of various tones are known, it is possible to approximately

determine the range of pressures that are met with in covering the
frequency range of music.

In Fig. 5 contour lines of equal loudness are shown for fre-
quencies from 32 to 4000 cycles. The frequency range has been
divided into three parts, bass, tenor and alto, and soprano registers,
corresponding to the notes produced by various instruments. The
contour lines indicate that the notes of the lower registers have
greater sound pressures than those of the higher. The range of
pressures for various instruments and for various musical notations
of loudness are smaller for low notes. Direct measurements of the
pressures that are produced by various instruments when played
by musicians show this tendency.

The contour lines for loud tones show a smaller change in
pressure in going from low to high notes than do the contour lines
for faint tones. If instruments are played so that the notes are of
approximate equal loudness, an orchestra playing in a resonant
room where the sound reaching the players’ ears is loud would
produce a smaller range of pressures than if it were playing in a
very well damped room.

Percussion instruments such as drums and the various ac-
cessory traps produce the greatest pressures that are used in music.
Although the fundamental frequency of the notes which they emit
is fairly. low, the notes are particularly rich in tones of higher fre-
quency, which may extend as high as 10,000 cycles. Although these
higher tones die out rather rapidly, they are essential to good
definition.

The organ, the piano, and the harp have the greatest compass
and cover a frequency range from about 16 to 4000 cycles. All
three of these instruments are characterized by a rather prominent
first overtone, so that their effective range extends as high as 8000
or 9000 cycles.

Owing to their limited compass, the melody instruments are
among the easiest to reproduce. In any given register these instru-
ments may be arranged in the following intensity order: wind
instruments, and string instruments, the violin producing the
faintest sounds. Asa class these instruments produce notes covering
the frequency range of 32 to 4000 cycles.

From the auditory sensation area, we have seen that the ear
is able to perceive a large number of tones which differ in sound
pressure and frequency. We have also seen that the voice and
various musical instruments produce tones which cover a large
portion of the auditory sensation area. In order to obtain informa-
tion as to the relative importance of various parts of this area to
the sensory characteristics of speech and music, experiments have
been performed in which the tones falling in various parts have
been eliminated from the sounds by means of filters.

When frequencies below 100 cycles, 200 cycles, 300 cycles and
on up to 1000 cycles are progressively eliminated from speech, the
character of the speech changes markedly. The terms “‘timbre’”’
or ‘‘tone color” best describe the characteristic which is lost. This
characteristic appears to be associated with the fundamental and
the first few overtones of the voiced sounds and their presence is
necessary in order to convey the sensation of timbre. Frequencies
below 300 cycles, however, do not appear essential to the correct
interpretation of the speech sounds.

When frequencies above 8000 cycles, 7000 cycles, and on down
to 3000 cycles are eliminated, the character of the speech again
changes markedly. The term “gibilance’”’ appears to best describe
the characteristic which is lost. It refers to the prominence of the
hissing or frictional character of speech. If attention is directed to
such sounds as s, f, th and z, the elimination of frequencies above
6000 or 7000 cycles is readily detectable. It requires rather close
attention to detect the elimination of frequencies above 8000 cycles.
Elimination of frequencies above 7000 cycles slightly impairs the
interpretation of the s and z sounds of woman’s voice. Elimination
of frequencies above 6000 cycles impairs the interpretation of the

f and th sounds of man’s voice, and the interpretation of the f, th,
sand zsounds of woman’s voice. The impairment due to eliminating
higher frequencies is usually greater in the case of female voices.

As in the case of speech the tone color or timbre of musical
tones appears to be associated with the fundamental and the first
few overtones of the note produced. ‘Timbre is probably more
important in music than it is in speech as it is one of the things
which distinguish the tones of various instruments. It appears
that, in general, the fundamental and the first three or four over-
tones are necessary in order to distinguish the tones of various
instruments. When overtones higher than these are eliminated
the tones lose a characteristic which can best be described by the
terms ‘“‘brilliance’”’ or ‘‘definition.”” The tones seem to lose life and
become dull. The prominence of these characteristics varies with
the type of instrument, the composition of the music and the
personality of the musician.

The notes which are used most in music are contained in the
octave below middle C and the octave above middle C, that is, ©
from 128 to 512 cycles. The fourth overtone of 512 cycles has a
vibration frequency of 8192 cycles, so that tones of this frequency
and below occur frequently in music. A trained ear could no doubt
detect the elimination of frequencies above this range from the
ordinary run of music, but it is probable that the average individual
would have difficulty in detecting the elimination of frequencies
above even 6000 or 7000 cycles, unless he gave particularly close
attention to the percussion instruments.

Owing to the masking effect, the character of both speech and
musical sounds changes as the loudness of the sound is increased.
The higher notes are masked more and more which has the effect
of accentuating the bass notes. The best simulation is obtained
therefore when the reproduced sounds are about as loud as the |
original sounds. rs :

The effects discussed in the above paragraphs were illustrated by means
of speech and music reproduced from phonograph records. In recording
the compositions, various frequency ranges were removed by means of
electrical filters, and different recording levels were used. Hence, the effects
could be shown by reproducing the records from an ordinary phonograph.

The records were prepared by Bell Telephone Laboratories and infor-
mation concerning them can be secured from its Bureau of Publication,
463 West Street, New York City.

lin Inst., June (1922).
2. The Frequency Sensitivity of Normal Ears by H. Fletcher and R. L. We-

in the air was known to the ancient Greeks, and that objects
are set in vibration by intense sounds must have been observed by
primitive man, but it was not until 1857, or less than a century ago,
that the first instrument was constructed for making a graphical
record of sound waves. In that year Léon Scott patented in France
an instrument which he called the phonautograph. In Fig. 1 is
shown a picture of this instrument. A piece of smoked paper was

attached to the cylindrical surface of a drum, which could be
rotated by hand and moved forward by a screw. The center of a
diaphragm was attached to a stylus through a system of levers in
such a manner that the stylus was moved laterally along the surface
of the cylinder when the diaphragm vibrated. Over the diaphragm
was placed a barrel-shaped mouthpiece. When the drum was
rotated, words spoken into the mouthpiece caused the stylus to
trace a wavy line upon the smoked paper. This wavy line was the
first known record of sound vibrations.

epoch-making invention. Edison constructed a machine very
similar to the phonautograph but differing in two important details.
The smoked paper was replaced by a sheet of tinfoil and the stylus
was attached directly to the diaphragm, so that it traced an im-
pression of variable depth when the diaphragm vibrated, instead
of a wavy line as in the case of the phonautograph. After such a
record had been made the drum was set to the starting point and
with the stylus in place was again rotated at the same speed as
before. The recorded sound was then intelligibly reproduced. Thus
Edison gave us the first phonograph.

In subsequent models the tinfoil was replaced by a wax cylinder. —
For many years the wax record, either in cylinder or disc form, was
used almost exclusively for the recording and reproducing of sound.
Although many other methods of recording have been suggested, it
is only in the last few years that records made photographically
have come into the commercial field as competitors. Both the wax
and the photographic records are now being used in conjunction with
motion pictures.

Photographic records are now being made by many different
types of apparatus. But they may be divided into two general
classes. In one of these classes the record is a trace of constant
photographic density but of variable width, while in the other it is
a trace of constant width but of variable density. An illustration
of each of these is shown in Fig. 2. In one or two proposed methods
the record is a combination of both types.

In almost all systems experimented with today there is at
least one element which they have in common with the phonauto-
graph, viz.: a diaphragm which is set in vibration by the sound to
be recorded. As in the phonautograph the diaphragm may be
mechanically connected to the engraving mechanism or recorder;
er, again, it may be connected electrically as in most modern
systems. But in practically all of them the diaphragm forms an
essential element.

Unfortunately a diaphragm does not in general have the same
response at all frequencies. A favorite experiment in lectures on
elementary physics is to sound a tuning fork and with it, through
the air, set in vibration a second tuning fork. In this experiment it
is important that the pitch, or the resonant frequency, of the two
forks be very nearly the same, otherwise the motion set up in the
second fork will be too small to be observable. Diaphragms, and in
fact almost any other type of mechanical system, will have at
least one resonant frequency, which means that, under the action of
sound waves, the response will in general be much greater in this
region than at other frequencies.

In the older methods of recording, resonance was purposely
introduced in order to obtain records of sufficient amplitude. The
frequencies lying in the resonance region were then much over-
emphasized. The sound reproduced from such records had a blasting
and metallic quality, and well deserved the title ‘“‘canned music.”

Because of the complex nature of speech and music and of the
ereat amount of distortion introduced into the early recorders and
reproducers it is not surprising that the quality of reproduction was
poor, but it is really astonishing that the reproduced sounds were
at all intelligible. In fact, it has been suggested that, had the
complex nature of speech sounds been generally known at the time,
the invention of the telephone, which preceded the phonograph,
might have been delayed for many years since its inventor probably
would have dismissed his ideas as altogether impracticable.

Although considerable distortion may permissibly be intro-
duced by the recording and reproducing systems before the char-
acter of the sounds is so changed that they can no longer be recog-
nized, it is equally true that, if all classes of sounds are to be
reproduced to a degree of fidelity where the ear cannot distinguish
them from the original, the amount of distortion must be kept

extremely small. It is therefore necessary to diminish the distortion
by the diaphragm to a negligible value.

Primarily that a diaphragm giving a uniform response may be
used and that a record of sufficient amplitude may still be obtained,
the electrical method of recording has been developed which is
today widely used in the production of commercial sound records.
In this method the pick-up diaphragm is made a component part
of the recording microphone. Here we can content ourselves with
a small amplitude of motion and amplify the voltage generated to
an amount sufficient for operating a rugged and distortionless
recorder. It may be of interest here to compare the amplitude of
motion of the diaphragm in the Edison recorder with that of the
microphone used in the majority of present recording systems. In
the former the maximum amplitude required for the loudest sounds
is about 0.001 inch, whereas in the latter under ordinary recording
conditions it is only about one-tenth as great and the weight is
only one-twentieth as great. It can thus be seen how the problem
of. design of a pick-up diaphragm is greatly simplified in the electrical
method.

It is of course important that the rest of the recording system
shall also be free from distortion. However, if a microphone of
uniform response is available, the design of a distortionless recorder
is made comparatively easy, for its sensitivity may to a large extent
be disregarded, inasmuch as the required power can in general be
obtained by the use of vacuum tube amplifiers.

In the electrical method, extraordinary improvements have
been made over the older systems in the elimination of distortion.

The problem of developing recording apparatus is in many
respects identical with that of developing a high quality radio
transmitter. In the former, however, there is the additional problem
of distortion introduced by the record itself. If, for instance, a
record is run at a speed of ten inches per second, and a tone having
a frequency of 5000 cycles per second is recorded, the length of one
cycle on the record will cover a distance of only 0.002 inch. In the
case of wax records the needle must have a very fine point; and in
the photographic record the width of the light beam as measured
along the direction of motion of the film must be extremely small.
At whatever speed the record may be driven, there will always be
some frequency beyond which all tones will become more and more
attenuated. While the loss of the higher frequencies does not in

general impair the tone quality to the same extent as does the
presence of sharp resonance regions, yet it reduces the intelligibility
of speech and the richness and brilliancy of musical sounds.

There is another type of distortion commonly present in
reproduced sound, which is frequently designated as non-linear
distortion. This type of distortion is introduced when the excursion
of any element of the system is not proportional to the stimulus.
For example, a pure tone, which is of sine wave form, such as is
shown in Fig. 3, a, may be reproduced so as to have a wave form
similar to that shown in b. Physically, distortion of the wave form
in this manner is equivalent to the introduction of extraneous
frequencies. If the magnitude of these extraneous frequencies is
too great, the tone quality will be very disagreeable. However, a
small amount of distortion of this kind is not noticeable, for the

reason that the primary will mask the extraneous tone. It is a well
known fact that a tone must be much more intense to be heard
if another tone is sounded simultaneously.

Another type of distortion peculiar to recording is that intro-
duced by a non-uniform speed of the medium on which the record
is engraved. This may not always be serious, but, in certain cases
of sustained tones, speed variations cause a disagreeable flutter
and in some types of music a decided harshness of tone.

One of the most serious problems with which the radio engineer
has to contend is static interference. This also has its counterpart
in sound reproduction from records. As the ether through which
the radio waves are sent is non-homogeneous because of extraneous
electrical disturbances, so the sound record is non-homogeneous
on account of the non-uniformity of the material on which it is
engraved. The noise resulting from these irregularities is often
designated as surface noise. In the case of the wax record, an
appreciable part of this noise has its origin in the minute irregular-

ities of the material and, in the case of the photographic record, in
the finite size of the grains forming the photographic image. The
difficulties of eliminating this noise arise from the fact that the
physical intensity of audible sounds covers an exceedingly wide
range. Fig. 4 shows curves published by Wegel! on the sensitivity
of the ear. The lower curve gives the threshold of audibility and
the upper curve the feeling level, i.e. the level of intensity of sound
which becomes painfully loud. The ratio of pressures of the maxi-
mum of these curves is about ten million. If a record of this extreme
range of volume were to be recorded the amplitude of the loudest

tone would have to be ten million times as great as for the faintest
tone. There is, in general, a maximum amplitude that a record can
accommodate, which, for instance, in the case of the wax record is
about 0.002 inch. If a tone having an intensity near the feeling
level is recorded at this amplitude, then the amplitude of a tone
just audible would be only 0.000,000,000,2 inch. It is difficult to get
a material having a degree of homogeneity corresponding to this
value. A similar condition obtains in the case of photographic
records, where the pattern is formed by grains in the emulsion
which have a magnitude somewhat less than 0.001 mm. depending
upon the type of emulsion used. The range of volume considered
here is extreme. Practically it is not necessary to record a range of
this extent, but it serves to illustrate the extraordinary requirements

1 The Physical Examination of Hearing and Binaural Aids for the Deaf,
by R. L. Wegel. Proceedings of the National Academy of Sciences, Vol. 8,
No. 7, July (1922).

placed upon the recording medium. When the range of frequencies
that are tobe reproduced is increased the surface noise effect becomes
greater. As in the case of the different types of distortion discussed
above, the difficulties to be met are increased as the quality of
reproduction is improved.

Synopsis: This paper considers chiefly the frequency-response charac-
teristics and limitations of the lateral cut ‘‘wax’’ record. It shows that the
frequency range from 30 to 8,000 cycles can be recorded and reproduced
from the record with practically negligible deviation from a flat frequency-
response characteristic. The paper brings out the ease with which the
record can immediately be replayed from the ‘‘wax”’ as an aid in assisting the
artist to obtain the best results. A brief description is given of commercial
processing methods including both plating and pressing. ‘These methods
give essentially a perfect copy of the original ‘‘wax.’’ The time required
for this work has been considerably reduced of late so that a test pressing can
be obtained within three hours of the cutting of the original ‘‘wax.”’

he the recording and reproducing of sound by the so-called “electric”’
method with the ‘‘wax’’ disc, the process may be considered as
consisting of eleven steps. In order, these are: (1) studio, with
its acoustic conditions, (2) microphone, (3) amplifier, (4) electro-
mechanical reeorder, (5) ‘‘wax”’ record, (6) copying or reproducing
apparatus, (7) hard record or ‘‘pressing,” (8) electric pickup, (9)
amplifier, (10) loud speaker, (11) auditorium.

With this chain of apparatus the chief problem is that of making
the reproduced sound in the auditorium a perfect copy of that in the
studio. This isa matter of quality or fidelity of reproduction. There
are other problems of cost, reliability, time required, etc., which are
important but secondary to that of fidelity. While it may be necessary
or convenient to introduce distortion in one of these links to com-
pensate for such unavoidable distortion as may occur in other links,
experience shows that it is desirable for the sake of simplicity, reliability
and flexibility to reduce such corrective warping to a minimum and
to make each step in the process as nearly perfect as possible. Per-
fection of a complete system may be judged by the practical method
of listening to the overall result. It is necessary, however, to analyze
each element of the complete system. To do this, other more
analytical methods of test and standards of performance must be
used. One of the most useful of these is the response-frequency curve.
In order that all frequencies be reproduced equally and that the
ordinary faults of resonance be avoided, this must be flat and free
from sharp peaks. Good reproduction requires that frequencies from
50 to 5,000 cycles be included without discrimination. If, however,

1 Presented before Society of Motion Picture Engineers at Lake Placid, New
York, September 26, 1928.

the low frequency range be lowered to 25 or 30 cycles, a noticeable
improvement will be obtained with some classes of music, whereas
if the upper limit be increased to 8,000 or even 10,000 cycles, the
naturalness and smoothness of practically all classes of reproduction
will be noticeably improved.

A second important requirement in the judgment or analysis of
any such system is that the ratio of output to input shall not vary
over the range of currents or loudnesses (as well as frequencies) from
the minimum up to the maximum used. If this requirement is not
met, sounds or frequencies not present in the original reproduction
will be introduced. This type of distortion has probably been heard
by all of us in listening to an overloaded vacuum tube amplifier and
is often referred to as ‘‘non-linear’’ distortion.

A third requirement not entirely disassociated from the first two
is that any shifts in the phase relations shall be proportional to
frequency.

Our judgment of the degree of perfection needed in sound repro-
duction systems is changing and growing more critical, so that what
seemed excellent yesterday may be only fair today and tomorrow
may seem intolerable. It is therefore necessary that our consideration
and analysis be continually more searching and fundamental.

wax’’ recording and reproduction, only five are peculiar to the
‘‘wax’’ method. These are the electromechanical recorder, ‘‘wax’’
record, the copying apparatus, the “‘pressing’’ and the pickup or
reproducer. The extent to which the ‘‘wax’”’ method is capable of
the highest quality of reproduction will be disclosed by an examination
of these five links. Any consideration of the practical advantages or
disadvantages of the method can logically follow this examination
into the quality possibilities.

The consideration which follows refers to the so-called “‘lateral’’
cut record; that is, a record in which the groove is of constant depth
and oscillates or undulates laterally about a smooth spiral. This is
the type used in the Western Electric Company disc record type of
synchronized motion picture system. Some, but not all of the con-
siderations and conclusions might apply to the “hill and dale” type
record. It is not the purpose of this paper to consider the relative
characteristics of ‘‘hill and dale” and ‘‘lateral’’ ‘“‘wax”’ records.

It is the task of the electromechanical recorder to take power from
the amplifier and drive a mechanical recording stylus. The present-

day recorder is a highly developed apparatus based on extensive
experimental as well as theoretical studies. A diagrammatic view is
given in Fig. 1.2. Recorders which have been supplied by the Western
Electric Company have been designed to operate over a range of
frequencies from 30 to 5,500 cycles. A typical frequency characteristic

is shown in Fig. 2. The device operates in linear fashion over the
range of amplitudes involved in speech and music. | As is seen, the
response falls off below about 250 cycles. This falling characteristic
is necessary in order that the maximum loudness be obtained from a
record for a given spacing between grooves without cutting over

2 High Quality Recording and Reproducing of Music and Speech,” by J. P.
Maxfield and H. C. Harrison, presented at 14th Midwinter Convention of the
American Institute of Electrical Engineers, New York, N. Y., Feb., 1926.

from groove to groove. In order that a lateral oscillation in a groove
may represent constant intensity of sound or a constant energy over
a range of frequencies, not the amplitude of the oscillation but the
velocity, which is proportional to the product of the amplitude and
the frequency, must be maintained constant. With the characteristic
shown with these recorders, constant velocity is obtained from about

250 cycles to 5,500 cycles. Below 250 cycles an approximately
constant amplitude is obtained. If, therefore, sounds of constant
absolute intensity are to be recorded over this range of 30 to 250
cycles, there is equal tendency for sounds of the different frequencies
in this range to over-cut the record groove. It may be corrected in
reproduction by a suitable electric network. Such a network will
increase the subsequent amplification required but, as this additional

amplification occurs in the first stages, it is not expensive. Practically
it has not been found necessary or desirable to introduce such a
corrective network since the correction has been largely cared for by
the characteristics of the pickups used.

Recent development studies have established the possibility of
flattening the response at the low frequency end and of raising the

high frequency cut-off of the recorder. Fig. 3 shows a characteristic
obtained with such a laboratory model. This shows uniform per-
formance within +1 TU from 250 to 7,500 cycles and within
+4 TU from 30 to 8,000 cycles. Although its immediate practical
value might be limited by other portions of the system, this device
is of great interest in that it establishes beyond question the fact
that an extremely broad range of frequencies can be successfully
recorded in the ‘‘wax.”’

The broad, flat characteristic obtained with electric recorders has
been made possible by so designing their elements that they constitute
correctly designed transmission systems. In such a transmission
system, whether it be an electrical recorder or a long telephone line,
a correct terminating impedance is required. ‘The load imposed by
the ‘‘wax’’ is somewhat variable but fortunately is rather small.
It has been found desirable to make the other impedances in the
recorder relatively large so as to dominate the system and thus
minimize the effects of any changes in the impedance imposed on the
stylus by the ‘“‘wax.”” The mechanical load used as a terminating
impedance and to control the device has consisted of a rod of gum
rubber 25 cm. long. ‘Torsional vibrations are transmitted along this
rod. The rate of propagation is about 3,000 centimeters per second
so that its length is equivalent to an ideal electrical line of about
1,500 miles. The dissipation along this rubber rod is such that a
vibration is substantially dissipated by the time it has travelled down
the line and back. It thus constitutes substantially a pure mechanical
resistance, the magnitude of this resistance being approximately 2,500
mechanical abohms, referred to the stylus point as its point of
application.

In recording the usual procedure is to use a disc from 1 in. to 2 in.
thick and from 13 in. to 17 in. in diameter, composed of a metallic
soap with small amounts of various addition agents to improve the
texture. This is shaved to a highly polished surface on a lathe.
This polished disc or so-called ‘‘wax”’ is placed in a recording machine.
In Fig. 4 is shown what is essentially a high grade lathe arranged to
rotate the ‘‘wax”’ in a horizontal plane at a very uniform speed in a
definite relation to the film with which it is synchronized. The
recorder with its cutting tool or stylus is moved relative to the surface
of the disc, common phonograph procedure being to record from the
outer edge of the disc towards the center, whereas in the Western

3“‘Some Technical Aspects of the Vitaphone,” by P. M. Rainey, presented at
the meeting of the Society of Motion Picture Engineers at Norfolk, Va., April,
1927.

Electric Company theater system the direction of cutting is reversed.
After a record has been cut into the “wax,” the “wax’’ may be
handled and with proper precautions readily shipped from place to
place.

The shape of the groove varies somewhat in commercial practice.
The groove and stylus most commonly used with Western Electric
apparatus are shown in Fig. 5. The groove is approximately .006 in.
wide and .0025 in. deep. The pitch of the groove is between .010 in.
and .011 in. so that the space between grooves is about .004 in. Thus

the maximum safe amplitude is about .002 in. If this occurs at 250
cycles the corresponding amplitude at 5,000 cycles, assuming constant
absolute intensity of sound over this range, would be .0001 in.

It is important that a smooth groove be cut as any roughness in
the walls introduces extraneous noise in the reproduced sound. To
insure a truly smooth groove the surface of the ‘‘wax’’ must be shaved
to a high polish. The texture of the ‘“‘wax’’ must be fine and homo-
geneous which requires not only that the ‘“‘wax’’ composition be
correct, but that it be operated at the proper temperature. ‘‘Waxes”’
may be obtained commercially which will operate satisfactorily over

the ordinary range of room temperature. The ‘‘wax’’ must be
levelled in the recording machine with reasonable care. The stylus
must be sharp and so ground that the cut will be very clean. The
““wax’’ shaving is removed as cut by air suction. The operator is
aided in maintaining the correct depth of cut by the use of a so-
called ‘‘advance”’ ball which rides lightly on the ‘‘wax’’ and serves to
maintain uniform depth of cut in spite of small inaccuracies of leveling
of the ‘‘wax”’ or deviations from planeness. The ‘‘advance’”’ ball is
adjusted relative to the stylus by observing the cut with a calibrated
microscope. A satisfactory operation of the recording machine re-
quires an ordinarily skilled mechanic with reasonable experience.

The rate of rotation is dependent upon the diameter of the record
groove which is determined primarily by the length of time which it is
desired to have covered by a single disc. The controlling element is
the linear speed of the groove past the recorder or reproducer. In the
Western Electric system the speed varies from 70 ft. to 140 ft. per
minute, in other words, of the same order of magnitude as with the
film record. The wave-lengths also are about the same as for a
sound record on a film. At the minimum linear speed the half wave-
length for a 5,000 cycle wave is .0014 in. If the minimum linear
speed is fixed at 70 ft. per minute and the groove spacing is fixed,
there is an optimum relation between the size of the record, the rate
of rotation and the playing time. This is illustrated graphically in
Fig. 6.

After a record has been cut, the sound may be reproduced directly
from the ‘‘wax”’ by using a suitable pickup or reproducer. Ordinary
reproducers or pickups rest much too heavily on the records to be

used on ordinary ‘“‘wax.’’ That this would be so is obvious from the
fact that the vertical pressures between the point of the needle and
the record in an ordinary phonograph are of the order of 50,000 pounds
per square inch. Obviously any such pressures would destroy a
groove cut in soft ‘“‘wax.’’ These high pressures have been necessary
in order that the groove might properly drive the needle point of
the reproducer. Reduction of this pressure requires reduction of the
impedance offered by the needle point to transverse vibration.

24Vo
Tm = MAX. PLAYING TIME IN MINUTES
N = GROOVES PER INCH
R = OUTSIDE GROOVE RADIUS-INCHES
V, = MIN. LINEAR SPEED-FT. PER MINUTE
r = INSIDE GROOVE RADIUS-INCHES

Fig. 6—Relation between playing time and rate of rotation of disc for various
values of R(R = 2r).

The design of a suitable ‘“‘wax”’ “playback”’ requires reduction of
both the mass and the stiffness of the reproducing system to a
minimum. In the past such “‘playbacks”’ have failed to reproduce
the higher and lower frequencies with much satisfaction. The device
shown in Fig. 7 represents a large advance toward ideal reproduction.

The response of such a device when driven by a ‘‘wax”’ record recorded
at constant velocity over the frequency range is shown in Fig. 8.
This reproduction is not widely dissimilar from that obtained from
finished records with the best electric pickups now commercially
available and is sufficiently good to serve as a very valuable criterion
in judging the quality of the record. The record may be played a
number of times without great injury. The extent of the injury is
indicated by the frequency characteristics obtained on successive

playings shown in Fig.9. They show little change in the low frequency
response and a loss of about 2 TU per playing at high frequencies.
The practical value in studio work of being able to let an artist immedi-
ately hear and criticize the results of his own efforts can hardly be
overestimated.

Fig. 9 Loss in response on successive playings of a wax playback driven by constant
velocity wax record.

After a groove has been satisfactorily cut into the ‘“‘wax’’ record,
the usual procedure is to render the surface of the ‘‘wax’’ conducting
by brushing into it an extremely find conducting powder. It is then
electroplated. The technique in this step varies somewhat with the

different companies doing such work, although not in any fundamental
manner. ‘The negative electroplate thus made may be used to hot-
press a molding compound such as shellac containing a finely ground
filler. This first electroplate is called a ‘‘master.’’ From it two
test pressings are usually made. If satisfactory the matter is then
electroplated with a positive, being first treated so that this positive
plate may be easily removed. This positive is sometimes called an
“original.’’ From this in turn is plated a metal mold or “‘stamper.”’
From these, duplicate “‘originals’’ may be plated and from them,
duplicate ‘‘molds’”’ or “stampers.’’ These processes involve no
measurable injury to the quality of the record and are comparatively
simple and extremely safe in practice. By this practice of making a
number of duplicates it is possible to safeguard the ‘“‘master’’ and
insure against any accident which might destroy a valuable record.
From a single ‘‘stamper”’ it is not unusual to make a thousand finished
pressings. The time required for these operations is such that test
pressings are commonly obtained from the ‘“‘wax’”’ in 12 hours. Recent
refinements in the art have reduced the time required so that finished
records may, if necessary, be obtained in 3 hours after delivery of
the ‘wax.”’

Various materials have been used in making the hard record or
‘‘pressing.’’ In some cases the material has been made homogeneous
and in others the surface is of a different material from that used in
the body of the record. Some have used a laminated structure.
There has not, however, been much latitude allowed the experimenter
concerned with materials for the hard record. ‘The material has had
to be quite hard and, in order to show a reasonable life, it has had to
contain sufficient abrasive to grind the needle quickly to a good fit.
At the beginning of the run of a new needle due to the small bearing
surface, the pressures are very high. They rapidly decrease so that
with an ordinary loud steel needle after one minute’s wear in the
ordinary phonograph, the bearing area is increased to such an extent
that the pressure is only about 50,000 pounds per square inch. As
the needle continues to wear to a larger bearing surface, the pressure
obviously continues to decrease. ‘These high pressures and necessary
abrasive characteristics of the record have introduced irregularities
which are responsible for most of the extraneous noise commonly
known as “‘surface”’ or ‘needle scratch.”’ }

The “pressing’’ copies the ‘‘wax”’ record with a very high degree
of accuracy so that if our attention be confined to frequency charac-
teristics alone, the ‘“‘pressing’”’ shows almost complete perfection.

Moreover, it is cheap and durable, and reproduction of the sounds
from this record calls for no fine adjustments or intricate apparatus
as has been long evidenced by the broad use of the ordinary phono-
graph.

The major part of the extraneous or ‘‘surface’’ noise found with
this method of reproduction comes from the material of the finished
record. Recent progress has been made in reducing this noise. Asa
result of this, together with refinement in the plating processes,
records used with Western Electric Company theater equipment
during the last two years have shown a reduction of 3 to 6 TU in
“surface’’ noise. ‘This corresponds to eliminating 50 per cent to
75 per cent of that previously present. It is not necessary to reduce
the level of ‘‘surface’’ noise to the zero point but merely to the
threshold of audibility under the conditions of minimum auditorium
noise which are of interest. This noise masks the surface. More-
over, it is not the absolute amplitude of the imperfection giving rise
to ‘“‘surface’’ noise but the relative magnitude in comparison with
the useful sound amplitudes which counts. Thus, an effective re-
duction in ‘‘surface’’ could be made if we were willing to use larger
records or if we were willing to reduce the playing time of the present
records by increasing the spacing of the grooves and the amplitude at
which the grooves are cut. Any large reduction in “surface”’ noise
made by a reduction in the irregularities in the record material would
open the door to increasing the playing time of a record of given size.
There is no known absolute or fundamental reason why further
improvements in record materials may not be expected to reduce
further the amount of ‘“‘surface’’ noise. Moreover, large advances
in pickup design open distinctly new possibilities as to reductions in
suittacer

It has sometimes been thought that in order to reproduce high fre-
quencies properly, the linear record speed would have to be increased
or the size of the needle point reduced. At present the diameter of
the bearing portion of a representative needle is about .003 in. whereas,
as mentioned before, the half wave-length for a 5,000 cycle wave is
0014 in. The factor determining whether a needle will follow the
undulation of the groove is not any consideration of the relative
diameter of the needle point and the undulation of the groove but
rather the radius of curvature of the needle and the bend of the groove.
As indicated before, the amplitude at 5,000 cycles would be only
about .0001 in. if sounds of that frequency were as intense as those of
lower frequencies (.002 in. at 250 cycles). Asa matter of fact, sounds
of 5,000 cycles or more in speech or music are characterized by lower

intensity than those of lower frequency. If, however, we assume an
amplitude of .0001 in. at 5,000 cycles and assume a linear record
speed of 70 ft. per minute, then the minimum radius of curvature
of the undulation of the groove is .00193 in.t With the foregoing
assumption, the radius of curvature of the undulation of the groove
becomes equal to that of the needle point at about 7,000 cycles.
Taking into account the lower intensities of sounds encountered at
these high frequencies, it is obvious that present commercial needle
points are quite capable of following the high frequency undulations
of the groove up to frequencies of at least 10,000 cycles. The limita-
tions of high frequency reproduction commonly found in the past are
associated with limitations in the design of the pickup or reproducer
and relate either to inability of the record groove to drive the needle
point, with resultant chatter, or inability of the pickup structure to
transmit high frequency motions from the needle point to the armature.

Large advances have been made within the last two or three years
in designing electric reproducing structures. The mechanical im-

pedance at the needle point has been reduced so that the needle
point truthfully follows the undulations in the groove without necessi-
tating excessive and somewhat destructive bearing pressures. At the

same time the transmitting structure has been so designed that a
very broad range of frequencies is properly transmitted from the
needle point to the armature. Moreover, proper mechanical loads
have been provided so that the motions after transmission are absorbed
and hence not reflected back. This is another way of stating the fact

that resonance as ordinarily considered has been eliminated from
these structures.

The curves shown in Figs. 10 to 12 illustrate the steps which have
been taken. The pickup shown in Fig. 11 is free from the resonances
shown in that of Fig. 10. The resonances present in the earlier

Fig. 12—Response of experimental pickup driven by constant velocity pressings.

pickup involved high needle point impedances in the region of these
resonances. These high impedances involved large driving forces
destructive both to needle and record. Certain records were injured
after only a few playings with this reproducer. The later reproducer
is characterized not only by considerably reduced average needle
point impedance, but, as shown by the curves, the resonance is

practically eliminated and hence there is an even greater reduction
from the maximum impedance which occurred in the earlier reproducer
at resonance. Both needles and records have a relatively long life
with the later type pickup, which has been in commercial use for
some months. As is seen, the higher frequencies are reproduced in
considerably better fashion. A third curve is given in Fig. 12. This
was obtained with a more recent experimental model in which a
further large reduction in the needle point impedance had been
effected and in which, in addition, a very much more rigid, though a
lighter structure served to connect the needle point with the armature.
This model shows further reduction in wear and tear on the record
and very greatly improved reproduction at the high frequency end
of the scale.

The application of the processes of sound recording on ‘‘wax’’ to
the synchronized film has involved meeting a number of conditions
not previously encountered in the phonograph field. One of the
most important of these relates to editing, cutting and rearranging
of the picture. Various methods have long been used to copy or
‘“‘dub”’ a disc record. The prime requirement is that there be no
sacrifice in quality. To attain this end records have sometimes
been copied at very low speed. ‘This method appears unnecessarily
laborious and slow and the results obtained are not altogether satis-
factory in the light of possibilities presented by pickups and recorders
of the characteristics shown above. Rearrangement of material on
records is entirely practicable, portions may be deleted or new portions
added either as a whole or the new sounds added to those already on a
record—in fact any changes of this type may be made which can be
made in the picture.

The detailed technique of ‘‘dubbing”’ appears to offer no serious
technical difficulties. The refinement reached and the extent of its
future use may be expected to be governed by the demand in the
synchronized motion picture field.

Synopsis: The light valve developed by Bell Telephone Laboratories is an
electromagnetic shutter consisting of afloop of duralumin tape formed into a
slit at right angles to a magnetic field. Sound currents from the microphone
and amplifier flow in this loop causing it to open and close in accordance
with the current variations.

The slit is focussed by a lens on the sound negative film. An incandescent

ribbon filament is focussed on the light valve, and the light passed by the
undisturbed slit appears on the film as a line at right angles to the direction
of the film travel. As the valve aperture is modulated by sound currents,
the film receives a varying exposure and a sound record of the variable
density type is obtained.
. For talking pictures such a sound film is made on a separate recording
machine synchronized with the camera and is printed alongside the picture
on the finished positive. The prints are displaced so that the sound is
advanced over the corresponding picture. This is in order that the sound
may be projected at a point of continuous film motion below the picture
gate.

It is not difficult to specify the requirements of this type of sound
film. So far as possible the exposure of the negative must be kept
within the straight line portion of the Hurter and Driffield curve for
the emulsion chosen, and the print must be timed with the same
restriction. The development of the negative and of the print must
result in a positive where the transmission of each element of length
is proportional to the exposure of the corresponding element of the
negative. The light modulator must be supplied with undistorted
power from the recording microphone and amplifier. When the
positive is projected, the striations of the sound track must be enabled
to modulate the illumination of a photo-sensitive cell to retranslate
the photographic effect into electrical current which shall be a fair
copy of the microphone current generated by the original sound.
From this point on the problem is the familiar one of sound re-
enforcement, the film and cell having taken-the places of the sound
source and microphone.

Fig. 1 shows a photograph of the light valve, invented in 1922 by
E. C. Wente of the Bell Telephone Laboratories. Essentially, it
consists of a loop of duralumin tape suspended in a plane at right

1 Presented before Society of Motion Picture Engineers at Lake Placid, New
York, September 25, 1928. =F ;

angles to a magnetic field. The tape, 6 mils wide and 0.3 mil thick,
is secured to windlasses A and A’ and stretched tight by the spring
held pulley B. At points C and C’ insulated pincers confine the
central portions of the tape between windlasses and pulley to form a
slit 2 mils wide. Supporting this loop and adjusting devices is a
slab of metal with central elevation D, which constitutes the armature
of an electromagnet. The central portions of the loop are supported
on insulating bridges to lie 3 mils above the face of D; here the sides
of the loop are centered over a tapered slot, 8 mils wide by 256 mils
long in this plane, opening to 204 mils by 256 mils at the outside face
of the armature. Viewed against the light, the valve appears as a
slit 2 mils by 256 mils.

The electromagnet core has a similar elevation opposing D across
an air gap of 8 mils which closes to 7 mils when the magnet is energized
from a 12 volt battery. A tapered slot in the magnet core begins
8 mils wide by 256 mils long and opens with the same taper as the
slot in the armature. When the assembly of magnet and armature is
complete, the valve constitutes a slit 2 mils by 256 mils, its sides
lying in a plane at right angles to the lines of force and approximately
centered in the air gap. The windlasses A and A’, one of which is
grounded, are connected to the output terminals of the recording
amplifier. If the magnet is energized and the amplifier supplies a
sine wave current from an oscillator, the duralumin loop opens and
closes in accordance with the current alternations.

When one side of the wave opens the valve to 4 mils and the other
side closes it completely, full modulation of the aperture is accom-

plished. The natural frequency of the valve is set by adjusting the
tension applied by the pulley B; for reasons which involve many
considerations the valve is tuned to 7,000 cycles per second. Under
these circumstances about 10 milliwatts of A.C. power are required
for full modulation at a frequency remote from resonance; about one
one-hundredth of this power at the resonant frequency. The im-
pedance of the valve with protecting fuse is about 12 ohms.

PLANE OF _ PLANE OF PLAN
RIBBON OF VALVE IMAGE ON
LIGHT RIB an FIL
X 0.256” ath p
( x ) K or ‘x a 128")

If this appliance is interposed between a light source and a photo- .
graphic film we have a camera shutter of unconventional design.
Fig. 2 shows a diagram of the optical system for studio recording.
At the left is a light source, a ribbon filament 18 ampere projection
lamp, which is focussed on the plane of the valve. The light passed
by the valve is then focussed with a 2 to 1 reduction on the photo-
graphic film at the right. A simple achromat is used to form the
image of the filament at the valve plane, but a more complicated
lens, designed to exacting specifications by Bausch and Lomb, is
required for focussing the valve on the film. The undisturbed valve
opening appears on the film as a line 1 mil by 128 mils, its length at
right angles to the direction of film travel. The width of this line
varies with the sound currents supplied to the valve, so that the film
receives a varying exposure: light of fixed specific intensity through a
varying slit.

Fig. 3 shows a studio recording machine with the door of the exposure
chamber open. In this machine the film travels at 90 feet per minute,
and the sound track is made at the edge away from the observer.
The line of light, the image of the valve, overruns the perforations
by 6 mils, extending toward the center of the film 122 mils inside the
perforation line. The right-hand sprocket serves to draw film from
the feed magazine above and to feed it to the take-up magazine below;
this sprocket is driven from the motor shaft through a worm and
worm-wheel. The left-hand sprocket engages 20 perforations and is
driven through a mechanical filter from a worm and worm-wheel

similar to that driving the feed sprocket. The mechanical filter
enforces uniform angular velocity of the left-hand sprocket which
carries the film past the line of exposure: the focussed image of the
valve: balancing of the flywheel which forms part of this mechanical
filter holds the angular velocity constant to one-tenth of one per cent,
despite the imperfections of the driving gears.

So far we have provided a means for driving the film and a means
for modulating the light thereon, but we have not chosen the average
illumination about which the modulation is to take place. The
maximum exposure corresponds to the maximum opening of the
valve and is therefore double the average.

be developed and draw the Hurter and Driffield curve for this contrast
for the emulsion chosen for the negative sound record. The maximum
exposure should correspond to the beginning of over-exposure, the
average should be half this. The Hurter and Driffield curve will give
the density of the over-exposure point for the chosen contrast and the
density for half this exposure. Let the machine be run to expose
film to light through the unmodulated valve for several values of
the lamp current. Develop the film and measure the densities due
to the various values of lamp current. Select, by interpolation if
necessary, the lamp current which corresponds to half over-exposure.
With this current in the lamp the machine is ready to make a sound
record, since the focussing of the valve has already been. done and
manufacturing specifications insure that the line of illumination shall
lie, within 3 minutes of arc, at right angles to the direction of film
travel.

Consider at this point the procedure in the recording studio. Adding
sound to the picture introduces no complication of technique other
than to require sufficient rehearsing to make sure of satisfactory
pick-up of the sound: microphone placement must be established and’
amplifiers adjusted to feed the light valve currents which just drive
it to the edge of overload in the fortissimo passages of music or the
loudest utterances of speakers.

In Fig. 3 the photograph shows a photoelectric cell mounted
inside the left-hand sprocket, which carries the film past the line of
exposure. Fresh film transmits some 4 per cent of the light falling
on it, and modulation of this light during the record is appreciated by
the cell inside the sprocket. This cell is connected to a preliminary
amplifier mounted below the exposure chamber, and with suitable
further amplification the operator may hear from the loud speaker the
record as it is actually being shot on the film. Full modulation of
the valve implies complete closing of the slit by one side of the wave
of current; this modulation should not be exceeded or photographic
overload will abound.

Fig. 4 is a schematic diagram of the studio equipment for sound
recording. Provision is made for combining if desired the contri-
butions of several microphones on the set. This combination is

under the control of the mixer operator in the monitoring room,
viewing the set through a double window in the studio wall. The
mixer controls also the gain of the amplifiers for the recording machines.

The diagram shows relays which permit the mixer to connect the
horn circuit either directly to the recording amplifier or to one or the
other of the monitoring photoelectric cells in the film recorders.
The direct connection is used in preparing the sound pick-up in the
studio: the program is rehearsed until satisfactory arrangement of
microphones and of amplifier gain is effected. The electrical charac-

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teristic of this direct monitoring circuit is so designed that the sound
quality heard in the horns shall be the same as the quality to be
expected in the reproduction of the positive print in the theater.
Acoustic treatment of the walls of the monitoring room secures the
reverberation characteristic of the theater, and the monitoring level
is so adjusted that the mixer operator hears the same loudness that
he would wish to hear from the theater horns. It is capitally im-
portant that the operator judge his pick-up on the basis of sound
closely identical in loudness and quality with that to be heard later in
theater reproduction.

circuit, the output of the recording amplifier is applied to the light
valves and the monitoring horns are connected to the photo cell
amplifiers on the recording machines. With no film in the machine
and at a convenient lamp current a complete rehearsal is made to
verify the operation of the valves at the proper level. Film is then
loaded, cameras and sound recorders are interlocked and starting
marks made on all films by punches or light flashes.

A light signal from the recording room warns the studio, which
after lighting up signals back its readiness to start. "The machine
operator starts the cameras and sound recorders, brings up the lamp
current to the proper value, and when the machines are up to speed
signals the studio to start. During the recording, the mixer operator
monitors the record through the light valves, thereby assuring himself
that no record is lost.

In the choice of emulsion for the sound negative, the usual designa-
tion of speed may be disregarded, because it is desired to make the
exposure of the unmodulated track many times the under-exposure
of the emulsion used. The advantages of positive emulsion for the
sound negative have come to be generally recognized; positive has
been used by Bell Telephone Laboratories since 1924. The scale of
Eastman positive film is about 20 to 1; we adjust the recording lamp
current to give an illumination on the film for the unmodulated track
of 10 times the under-exposure. After one lamp has been calibrated
as described before it may be replaced when necessary by another
in which the wattage in the ribbon filament is the same; the light
emission is very closely correlated with the wattage. Where the
unmodulated or average exposure is ten times the under-exposure
minimum, 90 per cent modulation of the light can be permitted with-
out running into under-exposure on the faint side of the wave. For
sound currents reaching 100 per cent modulation of the light, 90
per cent of the wave is free from distortion; if the average light were
halved, still 80 per cent would be free from distortion. There is
therefore considerable latitude in the average exposure, and the
negative is satisfactory if the transmission of the unmodulated track
lies between fairly wide limits.

The choice of the negative sound gamma is determined by the
practice of the laboratory in regard to picture development. It is
usual to see on the screen pictures whose overall gamma considerably
exceeds unity. On the sound track the overall gamma should equal
unity, and the development of sound negatives should be uniform,
though that of picture negatives is left to the judgment of the finisher.

Theoretically, it should be immaterial what combination of reciprocal
values is chosen for the negative and positive sound gammas. Prac-

tically, we have to recognize the existence of ground noise in all
records and take precaution to minimize it. No matter how ex-
cellently we reproduce the fortissimo passages, our record is unsatis-
factory unless the ground noise is low enough for a wide volume range,
that is, a wide range in level between fortissimo and pianissimo.
Whether our negative sound record is made on negative or positive
emulsion, there is always the danger that in reproduction we shall
encounter variations in transmission from point to point due to local
variations in the celluloid base, to local action of the developing agent,
or to a developer excessively granular in action. The photoelectric
cell is able to recognize variations of 1/10 of 1 per cent, whereas the
eye ignores contrasts under 2 per cent. These local variations in
transmission, continued to the positive print, constitute the ground
noise.

The remedy is, in part, to choose a developer as little granular in
its effect as possible. In part, to insist on machine development of
the sound film with thoroughly agitated developer. Further, to
carry the sound development to a high gamma; this obviates to a
large extent flow marks of the developer, and goes a long way to
escape local variations in the base by developing the negative striations
to be conspicuous in comparison.

In 1924 we concluded that the optimum choice was positive emulsion
developed to unit gamma for both sound negative and sound print.
This is feasible for sound records separate from pictures, but a compro-
mise must be made for the combination of sound and picture in a
single positive print. Here the positive development required for a
satisfactory picture is always to a gamma far above unity.

It is customary to develop picture negatives by inspection, having
in mind the uniform positive development to be undergone by the
prints from these negatives. The gamma of these positives need
never exceed 1.8; the sound negative then should be developed to
0.55. In order not to disturb the practice of the film laboratory, we
ask that the positive development be standardized and its gamma
ascertained, the reciprocal of this gamma then arranged for in the
standardized negative development. A negative gamma above 0.5,
together with the precautions of careful handling, permits the realiza-
tion of an adequate volume range.

It is beyond the scope of this paper to discuss the details of manipu-
lation and of choice of developer, but I wish to acknowledge the
cooperation of Mr. J. W. Coffman in the solution of such problems.
The problem is the reduction of ground noise, and its seriousness is
not to be diminished by choosing a different recording method.

In printing the sound negative, a uniform density for the print of
the unmodulated track is desired. The volume of reproduced sound

for a given reproducing light source, varies directly with the average
transmission and the per cent modulation of this average. This
average density should be on the straight line portion of the positive
Hurter and Driffield curve, far enough to keep the denser negative
portions from reaching the under-exposure region. For Eastman
positive film a suitable transmission of the unmodulated portion of
the sound print is 35 per cent, referred to air, for the usual values of
positive gamma: 1.4 to 1.8. At this average transmission only the
peaks of the recorded sound will encroach on the region of under-
exposure. For the reciprocally developed negative track the region
of under-exposure will have been reached by occasional peaks on the
other side of the wave, and such photographic distortion as exists will
be balanced between positive and negative.

Here we appropriately consider the photographic distortion as it
occurs in variable density records. If the entire negative exposure
has been confined to the under-exposure region of the emulsion chosen,
a huskiness will result in the reproduction which can not be corrected
by any known technique. But if the unmodulated negative trans-
mission, for a gamma of 0.55, is about 16 per cent referred to air,
90 per cent of the wave will be clear of under-exposure, and experience
shows that the ear detects no distortion. In telephonic terms,
everything at a level 1 TU below full modulation will be free from
distortion, and the peaks will be substantially perfect. The same
may be said of the positive printed to an average transmission of
35 per cent, provided the overall gamma approximates unity.

It has been calculated that if the overall gamma departs from
unity by 0.2 in either direction, a harmonic of 5 per cent amplitude
of the fundamental will be introduced. Experimentation has shown
that a 5 per cent harmonic is the least detectible. We state then the
tolerance on the overall gamma for the sound track as 0.8 to 1.2.
Variation of corresponding amount in the contrast of a picture print
is intolerable; therefore greater latitude in contrast is permissible in
the sound record than could be tolerated in the accompanying picture.

In printing these sound negatives in combination with pictures for
projection in the theater, it is customary at the present time to print
one negative, masking the space needed for the other, then run the
positive again through the printer with the other negative, masking
now the space already printed. In printing the picture negative,
light changes are made as usual; for the sound negative the light is
regulated to result in 35 per cent transmission of the unmodulated
track after positive development. Provision of suitable masks in the
camera has been made to show in the finder and expose on the film
only the portion which will be available for picture projection.

In the theater projector, the sound gate is located 14.5 inches
below the picture gate, in order to project the sound record at a
point where the film is in continuous motion. Therefore in the
printing it is arranged to print the sound negative displaced along
the length of the positive enough to bring the sound 14.5 inches ahead

NOTE:
IN PRINTING, THE SOUND
IS DISPLACED 14.5" AHEAD
OF THE PICTURE WITH WHICH
IT IS SYNCHRONIZED.

Fig. 5—Picture and sound track dimensions of synchronized sound film for standard
35 mm. positive stock.

of the corresponding frame. The printer apertures are chosen to
give a dark no man’s land 17 mils wide between picture and sound
track; the latter at the outside is separated 4 mils from the inner
perforation edge. 7

Fig. 5 exhibits the present practice for the finished positive. » It will
be seen that the sound track covers 100 mils clear, and is illuminated

in the projector by a line of light 80 mils long, 1 mil wide, centered on
the striations. This gives a margin of 10 mils at each end of the
reproducing line, an allowance for lateral shifting of the film on the
sprocket teeth.

In conclusion, let me estimate the quality of the sound record to
be expected. Assume that the recording lamp current has been set
to within 5 per cent of the theoretical optimum, the overall gamma
held between 0.8 and 1.2, and the final average positive transmission
is between 32 per cent and 38 per cent. ‘Then the distortion of wave
form due to photographic handling is so small that the ear can not
distinguish the record from a theoretically perfect one. The fre-
quency-amplitude characteristic of the reproduced sound remains to
be stated.

Due to the fact that the element of illumination, both in recording
and in reproducing, is 1 mil wide instead of infinitely narrow, the
final print will not reproduce the higher frequencies as efficiently as
the lower. For example, at the standard speed of 90 feet per minute,
the line of illumination covers on the film an entire cycle length of the
frequency of 18,000 cycles. This frequency is therefore extinguished
completely. The drooping characteristic resulting from this effect,
called the film transfer loss, may be largely offset by judicious choice
of electrical characteristics and by taking advantage of the mechanical
tuning of the light valve.

In Fig. 6 is shown in curve A the light modulation by the valve in
recording for constant sound pressure of various frequencies at the
transmitter; in curve B the overall characteristic of the reproduction
in terms of electrical power delivered to the loud speaker for constant
sound pressure at the transmitter in the studio. Experience shows
that curve B is close enough to flat; the success of the record, as of
the picture, depends on the director.

Synopsis: The reproduction of the synchronized sound picture of today
presents no serious problem of synchronization, for this factor has been
practically eliminated by the perfection of electrical means for reproducing
sound with equipment which may be coupled mechanically to the picture
projector.

The important problem of the present day, in connection with the
reproduction of synchronized sound pictures, is the provision of suitable
means for maintaining a constant speed of the sound reproducing mechanism
in order that the pitch of the sound being reproduced may not suffer any
sudden change which would be sensed by a good musical ear. Control
circuits using vacuum tubes with a frequency bridge as a speed standard
with provision for manual variable speed control are described and explained
for use with both A.C. and D.C. motors. Remote synchronization permitting
the recording of pictures and sound simultaneously on equipment located
some distance apart is obtained by a modification of the Michalke electric
gear system.

HEN Thomas A. Edison gave a demonstration of his talking
motion pictures nearly sixteen years ago one of his chief
problems was proper synchronization between his acoustic phonograph
and the motion picture projector. It was then necessary to locate
the phonograph behind the screen in order to make the sound appear
to come from the picture. A system of belts and pulleys running
from one end of the theater to the other was used to secure synchro-
nization with the projector in the booth.

The development of the electrical reproducer has made it possible
to locate the turntable and reproducing mechanism in the projection
booth permitting a direct mechanical coupling between it and the
projector. The horns are located behind the screen and electrically
connected by wires with the electrical reproducer.

However, such mechanical coupling between the projector and sound
recorder (either of the disc or film type) makes it necessary to provide
very close speed regulation on the projector motor, since variations in
speed produce proportional changes in the pitch of the sound.

1 Presented before Society of Motion Picture Engineers at Lake Placid, New
York, September 24, 1928.

A good musical ear while having a sense of absolute pitch of only
about 3 per cent is extremely sensitive to sudden changes in pitch.
It has been found that a sudden change in pitch as small as one half
of 1 per cent may be noticed if made abruptly. In order to properly
take care of this requirement, therefore, the speed regulation or
change in speed of the motor drive over normal variations in line
voltage and load should be held within 2/10 of 1 per cent.

The absolute speed must also be held near these limits. since at
the end of a film it is necessary to switch from one projector to another
with minimum change in the pitch of the sound reproduction.

A study of the voltage variations in power supply systems indicated
a range from 100 to 125 volts. At a particular location the normal
variation of voltage was found to be 5 per cent above or below the
mean value with occasional momentary variations of as much as
10 per cent above and below mean value.

An investigation of variations in frequency of the supply voltage
showed that in the large cities the frequency was held very accurately
at 60 cycles. In New York City for example the frequency stays
within one quarter of 1 cycle and does not change rapidly. However,
in some small power systems the frequency varied as much as 5 cycles
and in some cases was subject to rapid changes in frequency.

The load of the motor is due mainly to mechanical friction in the
projector and take-up mechanism. This load was found to be on
the average 1/10 of a horse power but subject to wide variations.
In the case of a new machine with a stiff adjustment of the take-up
mechanism, the load was found to be as high as one-fifth of a horse

A consideration of the variables just discussed imposes rather
severe requirements of speed control, the two extremes being (1) the
combination of low line voltage, low frequency and heavy load,
(2) the combination of high line voltage, high frequency and light load.
Ordinarily it might be possible to compromise and not provide for
such an extremely unfavorable combination of requirements. How-
ever, it must be borne in mind that in the case of a musical program
the failure of the speed regulating system for even as short a time as a
fraction of a second would be a very serious matter causing the music
to sound off pitch similar to a phonograph which has run down while
in operation.

An examination of the standard commercial types of speed control
indicated that there was nothing exactly suitable. The nearest
approach to a suitable governor is the standard type of phonograph
governor but this friction brake type of governor has serious objections
if applied to a motor of considerable power output. In order, there-
fore, to have a control system which would be free from maintenance,
it was necessary to develop a special form of control system for the
purpose. Fig. 1 shows a photograph of the A.C. motor and its control
cabinet.

Fig. 2 shows the A.C. motor circuit which consists of a repulsion
type of motor coupled to a small auxiliary alternator providing a
frequency of 720 cycles which through a control circuit is made to
operate a variable reactor across the armature terminals of the motor.
If the speed of the motor is too high, the control circuit produces a
maximum impedance in the reactor L,; thereby reducing the armature
current of the motor and causing it to slow down. While if the
speed is too low the control circuit causes the reactor Li to have a
minimum of impedance increasing the armature current and causing
the motor to speed up.

This reactor ZL; is of the D.C. saturating type having two outer
legs with A.C. windings and the middle leg with a D.C. winding.
The A.C. flux circulates around the two outer legs. The D.C. flux
flows from the middle leg and returns through the outer legs in
parallel. When D.C. flux is sent through the middle leg it saturates

reactance is old and was employed by Alexanderson as a magnetic
modulator in his early radio work.

Fig. 3 shows one element of the speed control circuit. This consists
of a bridge circuit having one variable arm and three fixed arms.
The variable arm comprises a tuned circuit consisting of the in-

ductance LZ and the capacity C which are designed to tune at exactly
the frequency corresponding to the desired motor speed. When this
circuit is in tune it has a resistive impedance which is balanced by the
fixed arm R of the bridge. The other two fixed arms on the left

are windings of a transformer with a mid tap. If a voltage 2F,
having a frequency of 720 cycles (which is the frequency corresponding
to the desired speed of 1,200 R.P.M.) is supplied to the bridge it will
be apparent that the output voltage E» will be zero. If, however,
the speed is low the tuned circuit will have a condensive reactance
while if the speed is high it will have an inductive reactance. The
output voltage HE», will, therefore, change abruptly 180 electrical
degrees from a speed below 1,200 to a speed above 1,200. This
characteristic is shown in Fig. 4.

the bridge. In this way an overall characteristic of the desired
sharpness is secured using a comparatively small and inexpensive
coil and condenser.

Fig. 5 shows the complete control circuit. The output from the
bridge circuit is supplied to the grid of tube V4 which is called the
detector tube. The plate voltage of this tube comes from the 720-
cycle generator through the step-up transformer 74. The phase of
this voltage, therefore, remains constant. The phase angle of the grid
voltage, however, comes from the bridge output circuit through the
step-up transformer 73 and as previously explained suffers a sudden
reversal of phase as the speed passes through 1,200 R.P.M. Fig. 6
shows the resulting current characteristic through tube V4. This
current flows through coupling resistance R; which drives the grids

of tubes V; and V2 negative. This in turn reduces the plate current
through tubes Vi and V2 and hence through the D.C. winding of
the inductance ZL; controlling the armature current of the motor.

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Fig. 7 shows the performance characteristics of the motor. It will
be noted that the actual speed characteristic is practically flat. This

flat characteristic is secured by a compensating network consisting
of the resistances Re, R; and R, and the condenser C,. This com-
pensating network feeds back on the grid of tube Via portion of the
voltage drop across the D.C. winding of inductance L; thereby cor-
recting for the “static fluctuation’? of the control circuit. By a
suitable adjustment of this compensating resistance the control circuit
may be arranged to give flat regulation, under regulation or even
over regulation if desired. Fig. 7 has been drawn with line voltage
as the variable. A similar characteristic is also obtained with load
as the variable instead of voltage.

An interesting point in connection with this compensation circuit
is the necessity for avoiding hunting or surging of the speed. It is a
well-known property of all forms of governors that if they are adjusted
to too great a sensitivity the speed instead of remaining constant will
fluctuate up and down about a mean value. The simplest method of
preventing such speed fluctuations is to decrease the sensitivity of
the governor allowing a bigger change in speed with load (or voltage)
and then compensating for this change of speed or ‘‘static fluctuation”’
by means of a delayed action compensator. This phenomenon is well-
known in the mechanical governor art and is described by Trinks in
his book ‘Governors and the Governing of Prime Movers.’ The
electrical equivalent of this mechanical system is obtained by intro-

ducing the condenser C2 in series with the high resistance R,. When a
change in current through the regulating reactance L; occurs the
corresponding change in voltage drop is not transmitted to the con-
denser C2 immediately, but C, changes its voltage after a certain
time lag (approximately 1 second), required to charge the condenser
through the resistance Ry. The introduction of this time lag restores
the precision of the circuit to the flat characteristic desired without
introducing hunting.

By throwing the switch S; to the right the operator can disconnect
the tuned circuit control and substitute a potentiometer P; as a

source of grid voltage for tube Vs. By means of this potentiometer
the operator can adjust the speed of the motor at any speed from
900 to 1,500 R.P.M. corresponding to 68 to 112 feet of film per minute.
This feature is employed for ordinary motion picture work where it is
unnecessary to synchronize the picture with the sound. The regu-
lation of the circuit under these conditions is sufficiently good for
ordinary motion pictures.

An interesting feature in this connection is that theaters in many
cases have preferred to use the regulated speed position for ordinary
motion pictures as well as synchronized pictures. The reason for
this being that with the speed of the projector precisely controlled
the orchestra leader is better able to keep his orchestra in step with
the picture indicating apparently that closer speed regulation than is

at present provided would be desirable for ordinary motion pictures
as well as synchronized pictures.

A.C, circuit. Since a strengthening of the field of the D.C. motor is
required in order to reduce the speed it is necessary to reverse the
phase relationship of the transformer 7,, so that the current in the
detector tube decreases at speeds above 1,200 instead of increasing
as in the case of the A.C. circuit.

The operation of the circuit is as follows: When the line switch is
first thrown the motor acts as an ordinary D.C. shunt motor and
accelerates. At low speeds the output from the 720-cycle generator
is low and consequently there is no plate voltage supplied to the tubes
and no current through the auxiliary field winding. The field is,
therefore, weak and the motor speeds up. This condition is main-
tained until the equilibrium speed of 1,200 R.P.M. is approached.
The phase angle of the voltage supplied to the grid of the tube V3 is
then in phase with the voltage supplied to the plate so that the grid
of the tube goes positive at the same time that the plate is positive.

This causes a current to flow through the coupling resistances R; and
R». which drives the grids of tubes V; and V2 negative, thereby keeping
down the current through these tubes and hence maintaining a weak
motor field. The motor, therefore, continues to accelerate until a
speed of 1,200 R.P.M.isreached. At this point as previously explained
under the description of the bridge circuit, the phase of the output
suddenly reverses whereupon the grid of the detector tube goes
negative at the same time that the plate goes positive, thereby cutting
off the current through the detector tube V3 and reducing the negative
C voltage on the grids of tubes V; and V2. This increases the plate
current through the regulating field thereby stiffening the field of
the motor and checking its rise in speed. In practice the current
through the detector tube is neither at one extreme nor the other but
reaches an equilibrium at the speed of 1,200 R.P.M. A feedback
network having the delay feature for prevention of hunting is included
in the same manner as previously described for the A.C. circuit. The
characteristic curves for the D.C. motor are similar to those shown in
Fig. 7 for the A.C. motor.

For the operation of ordinary motion pictures the motor is changed
to a simple shunt D.C. motor by the switch S,; and the speed varied
by means of the field rheostat.

It might appear that the simplest method of securing synchronization
in recording work would also be mechanical connection between the
recording machine and the camera. It has been found desirable,
however, from a practical standpoint to have the camera movable
with respect to the recording machine as the recorder has to be
accurately lined up and adjusted and is not essentially a portable
machine whereas the camera in ordinary motion picture work must be
a portable piece of equipment. It has been necessary, therefore, to
develop a motor drive equipment which will satisfactorily interlock
the camera and the recording machine but leave the camera unit
portable. It is essential that the interlock should hold not only
during normal conditions but during acceleration and deceleration.
In other words, the system must be the full equivalent of a mechan-
ically geared system. The principle employed is old being disclosed
in a patent issued to Michalke in 1901. In Fig. 10, A and B are two
units which it is desired to interlock. Each unit has a three phase
stator and a three phase rotor, the latter provided with slip rings.
Magnetizing current for the system is supplied from an independent
three phase, 60-cycle source. If the rotors of A and B are in exactly

the same positions with respect to the stators it is evident that the
e.m.f.’s produced in them by transformer action will be identical as
to voltage and phase. Consequently there will be no flow of current
over the rotor leads and hence no torque developed. If, however,
unit A is turned through a small angle then the phase of the e.m-.f.’s
produced in the rotor circuits will differ from that in B and a current
will flow in the rotor circuits producing a torque which will tend to
make unit B assume the same position as A. If A is rotated con-
tinuously B will follow it up to synchronous speed of the stator field
at which point the torque will drop to zero since no e.m.f. is induced
in the rotor of either machine.

The portion of the circuit shown in Fig. 10, is merely the equivalent
of a mechanical gear, neither unit tending to rotate as a motor by
itself. In order to produce such rotation, therefore, a distributor
set is added as shown in Fig. 11, the distributor acting, to Use a
mechanical analogy, as the driving gear of the system and each of the
individual units of the system as driven gears. The distributor is
itself driven by a D.C. motor provided with the speed control circuit
previously described and shown in Fig. 9. The speed of the system
is thus solely dependent on the D.C. driving motor and independent
of the 60-cycle excitation frequency.

In practice the system is controlled by an operator at the distributor
set and by means of switches any desired number of cameras, recording
machines, or projectors may be employed. The projecting machines
are used in case it is desired to make up a sound record to accompany
an ordinary motion picture film which has previously been recorded
without sound accompaniment. It has been found that the system
operates very satisfactorily and requires very little maintenance.

When starting up for the first time it is necessary that the various
units should line up properly as to phase otherwise there will be a
local flow of current in the rotor circuits, which will cause the motors
to operate as induction motors and run away. Under running con-
ditions the system is very stable showing no tendency to hunt or surge
between units, for the reason that being polyphase each phase as it
becomes inactive (when the induced e.m.f. passes through zero) acts
as a damping winding for the other two active phases.

If the load on a particular unit of this system is varied there will
be a variation in the phase angle between this unit and other parts
of the system in the same manner as in the case of a synchronous
motor of the ordinary type. The magnitude of this phase angle,
however, does not vary more than 30 electrical degrees or 15 mechanical
degrees and is sufficiently small so that it produces an inappreciable
effect on the synchronization.

In the design of this equipment, first consideration has been given '
to its precision and reliability in operation and the provision of
adequate margins to care for all variations in service conditions. Asa
result it has been possible to maintain a high standard of quality in
music and speech reproduction.

Synopsis: The general problem involved in the design of a system
suitable to be used to record and reproduce sounds such as are required for
‘‘talking’’ motion pictures is outlined. The general method of attack is
indicated. There follows a description of the several pieces of apparatus
which comprise the theatre equipment, including a discussion of some of
their salient features and of the part each plays in the sound projector
system.

N order to reproduce in a theater the pictorial record of events
accompanied by the sound associated with those events, it is, of

course, necessary to add equipment to that installed to produce only
the silent motion picture. It is the purpose of this paper to outline
and discuss briefly the major items of such equipment as developed
by Bell System engineers.

In the design of sound equipment a primary requisite is that there
shall be freedom from distortion. Distortion may be of the sort
which is independent of load and is evident in that the intensity of
some portion or portions of the sound spectrum is increased or de-
creased in comparison with the rest; or there may be the distortion
which is a function of the level at which the device is operated and is
characterized by the reduction of a pure tone into fundamental and
one or more harmonics. ‘This latter condition is most often the conse-
quence of operating a vacuum tube amplifier above its proper energy
handling capacity.

It is the resonances of vibrating strings or reeds or air columns or
vocal cords that give us the music we record but we are careful that a
minimum of the resonances of the recording system itself shall go
into the record, and that any resonances of the reproducing system
shall not appear in the output of the sound projectors. Aside from
the effects of overloading, the prevention of distortion is largely a
matter of getting away from resonance phenomena since it is the
characteristic of the resonant system to respond with disproportionate
amplitude to stimuli in the region of its own natural period. The
whole story of the passage from sound energy through the various
recording and reproducing devices back to sound energy again is
one of contest with this fundamental physical phenomenon.

1 Presented before Society of Motion Picture Engineers at Lake Placid, New
York, September, 1928.

There are in general two things one can do to avoid the harmful
effects of resonance in vibration transmitting or transforming appa-
ratus: (1) the period of resonance of each piece of equipment can be
moved outside the range of frequencies one wishes to transmit, at
the same time providing damping means to minimize free vibrations;
(2) the distortion produced by resonance in one piece of apparatus
can be compensated for or equalized by similar and opposite distortion

in some associated apparatus. The first is not always easy of practical
accomplishment in any particular device and generally results in an
instrument of very low inherent efficiency; the second usually involves
loss of energy. In both cases increased amplification is required.
The sound record comes to the theater either as a wavy groove in a
composition disc or as a striated track of varying density at one
side of the picture film. It is the function of the apparatus being
considered to derive from these records an electric current in which

all the variations in pitch and loudness are accurately represented, to
suitably amplify this current, to effect its conversion into sounds
approximating those from which the records were made, and to so
direct those sounds as to reasonably create the illusion that sound
and picture are cognate.

The disc records do not differ essentially from those used in the
ordinary phonograph except that they are considerably larger and
run at a much slower speed so that a single record will play throughout
an entire reel. The reproducer used is in some ways similar to that

Fig. 2—Diagram of motion picture projector equipped for reproducing sound from
film.

used on the acoustic phonograph, the needle holder being connected
to a clamped diaphragm. This diaphragm is of highly tempered
spring steel and to it there is fastened an armature made of a special
high permeability alloy and so arranged that as the diaphragm vibrates
the flux in the air-gap of a permanent magnet varies correspondingly,
thereby inducing in appropriately placed coils currents which are the
electric representation of the wavy groove which the needle travels.
This reproducer is shown in Fig. 1. Although the energy delivered
by this instrument is comparatively low it has a very uniform response
over a wide frequency range. This result is largely brought about

by moving all resonances out of the working range and by filling the
magnet chamber back of the diaphragm with a heavy damping oil.
The film used with the disc record, called a synchronized film, differs
from ordinary film only in that one frame at the beginning is specially
marked to give the starting point.

The film sound record, as has been said, consists of a track of
varying density running along one side of the picture. This sound
track is 1/10’ wide. Differences or changes in intensity of sound
are represented by differences in the density of the record, while pitch

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is represented by the number of changes from dark to light and back
again in a given length of track. This sound record is converted
into a corresponding electric current by arranging that a narrow high
intensity beam of light shall pass through it and fall upon a photo-
electric cell. The arrangement is shown in Fig. 2. The light from

the bright filament of the exciting lamp is focused as a very narrow
line upon the film by passing through a system of lenses and an
aperture plate. The lamp filament is focused upon a slit of dimensions
0015” x 3/16". The image of this slit is then brought to focus upon
the film as a .001” line whose length has been reduced in passing
through the aperture plate to .080’.. This reduction in length allows
010” on either side for variations in position of the .100” sound track.
The position and focus of the lens tube are fixed, but the carriage of

the exciting lamp is movable so that when replacing lamps the filament
may be properly brought on focus.

A photoelectric cell of the type used is shown in Fig. 3. The
characteristic of this device is that when it is polarized by a proper
voltage and is used within proper limits the current through it is pro-
portional to the incident light. The circuit is shown in Fig. 4. It is
to be noted that the polarizing voltage is supplied to the photo cell
through a very high resistance and there is, therefore, obtained across

this resistance a voltage which is proportional to the light falling upon
the cell and accordingly bears a direct relation to the varying density
of the sound track interposed between the exciting lamp and the
cell.

The photo cell circuit is inherently one of high impedance. In
such a circuit there are two matters which require attention. (1)
Local interference—‘‘static,” to use the radio expression, is most
readily picked up, and at this point where the energy level is low, é

may be appreciable in comparison with the sound currents themselves;
(2) also the shunting effect of capacity between the electrical con-
ductors becomes noticeable, particularly at the higher frequencies.

MACHINE #1
HORNS
NS
La oR a SWITCHING PANEL
BeOEre THEATRE
TURNTABLE To
OTHER
UPPER
TRANSVOX HORN i
FADER als ae noun CONTROL ae HORN
AMPLIFIER | JAMPLIFIER [] AMPLIFIER ape
BOX

Hence a vacuum tube amplifier, which serves both to increase the
energy and to make that energy available across a low impedance
circuit, is closely associated with the cell upon the projector itself.

The cell and amplifier are enclosed in a heavy metal box or shield
which is made fast to the frame of the projector and the projector
itself is carefully grounded. This amplifier is shown in Fig. 5. It is
designed to bring the level of the electric counterpart of the film sound

record up substantially to the same energy value as that obtained from
the magnet coils of the disc reproducer. The filaments are heated
from a 12-volt storage battery. Small dry batteries supply its plate

Fig. 9—Amplifier Panel.
current and also the polarizing potential for the photo cell. These
batteries and the battery leads are shielded.

Vibration of a vacuum tube often produces sufficient motion of its
elements with respect to each other to effect changes in the stream

of electrons which appear when sufficiently amplified as noise from a
loud speaker. In spite of all precautions there is a certain amount of
vibration of the projector when in operation and it has therefore been
necessary to design a rather elaborate shock-proof mounting for the
photo cell amplifier.

It is evident from the relative location of apparatus as shown in
Fig. 2 that it is not feasible to print the film sound record directly
beside the picture to which it applies. Asa matter of fact, there is a
spacing of 143’ between picture and corresponding sound record and
a certain amount of slack is allowed between the sprocket which
carries the picture with an intermittent motion before the picture
projection lens and the sprocket which must carry the sound record

with a uniform motion in front of the photoelectric cell. In this
connection it is noteworthy that special precautions are necessary in
order to prevent vibrations and speed fluctuations due to either
varying supply voltage or varying load from affecting the uniformity
of rotation of this sound sprocket. This is taken care of by the very
effective means of automatically controlling the speed of the driving
motor and by means of a mechanical device interposed between the
sound sprocket and the rest of the moving equipment of the projector
which effectively opposes the transmission of any abrupt change of
speed to this sprocket.

The control box which contains the apparatus for governing the
speed of the driving motor is arranged to hold the record speed the

same as that at which the records are made, i.e. 90 feet per minute
in the case of synchronized sound and picture productions. By
throwing a switch the automatic feature may be cut out and the speed
of the machine may then be manually controlled by the operator.

This completes the apparatus associated directly with the projector.
The general arrangement of the latest type of projection machine,
equipped with a Simplex head, is shown on Fig. 6. Incidentally this
projector is also arranged to be fitted with the Powers or the
Motiograph head. Fig. 7 shows a typical layout of a sound projector
system as installed for use with talking motion pictures,

As in ordinary pictures, in order to run a continuous program, it is
necessary to use two projectors alternately. As the picture from one
machine is faded imperceptibly into that on the other so the sound
record may be faded from one machine to the other without the
audience being aware that a change has been made. At the end of
each record or sound film the music overlaps the beginning of the next
and a device called a fader is employed in making the transition. All
that is necessary is to turn the fader knob when the incoming machine
is started. ‘This fader is in fact a double potentiometer. In the upper
or normal operating range the change in volume in moving from one
step to the next is hardly more than perceptible whereas in the lower
range used only in fading the steps are large and the volume decreases
to zero on one machine and builds up on the other very rapidly. By
choosing the proper step in the upper range one can obtain any
volume of sound desired within reasonable limits and thereby equalize
the level obtained from different sound records. The fader is ordi-
narily installed with one or more auxiliary dials and handles inter-
connected so that it may be operated from any projector position.
In connection with the fader there is provided a switch for changing
from the film to the disc input system and also a key for switching a
spare projector in place of either of the regular machines. Fig. 8
shows a fader with one auxiliary position.

Following the fader, we come to the main amplifier which raises
the energy of the feeble electric currents to a level adequate to supply
the loud speakers with sufficient volume to serve the particular
theater. Fig. 9 shows a typical amplifier panel. This combination is
capable of an energy amplification of about 100,000,000 times and is
so designed that all frequencies in the range from 40 to 10,000 cycles
are amplified practically equally. A potentiometer is provided on
the amplifier but while its handle is readily accessible it is ordinarily
not used after having once been set at the time of installation to give
proper results in the particular theater. Necessary adjustments are

made on the fader. The amplifier shown consists of three units.
The first consists of three low power tubes in tandem, resistance
coupled, and requiring a 12-volt battery delivering 1/4 ampere to heat
their filaments. The second consists of a single stage of two medium
power tubes, connected in push-pull arrangement with filaments

heated by low voltage alternating current. ‘Two similar tubes in this
unit operate as a full wave rectifier and supply rectified alternating
current for the plate circuits of the amplifier tubes of both the first
and second units. The third unit has a single stage of high power
push-pull amplifier tubes and push-pull rectifier tubes and also operates
entirely on alternating current.

These three types are capable of arrangement into combinations to
meet the particular need. For small theaters only No. 1 and No. 2
are required. In the larger houses the high power unit No. 3 is added,
while to meet exceptional conditions two or more of the high power
amplifiers may be operated in parallel from the output of No. 2.

Following the amplifier there is an output control panel. This
consists of an auto-transformer having a large number of taps, the
taps being multipled to a number of dial switches, to which the sound
projectors or loud speakers are connected. By means of this panel,
it is possible to match the impedance of the amplifier output to the
desired number of horns in order to obtain the most efficient use of
the power available and also to adjust the relative volume of the
individual horns.

The ordinary theater installation employs four horns, two mounted
at the line of the stage and pointed upward toward the balconies and
two mounted at the upper edge or above the screen and pointed
downward. This combination has been found to give good distri-
bution throughout the house.

The loud speaker unit used with the horns in theater equipments
is essentially that recently described by Messrs. Wente and Thuras.?
As brought out in this article, this unit shows extremely high efficiency;
about 30 per cent of the electrical power supplied is radiated in the
form of sound. This is important since the higher the loud speaker
efficiency, the smaller the power capacity of the amplifier needed in
the system. The frequency-response characteristic of a typical re-
ceiver and horn is given in Fig. 10. An individual horn may be
equipped with two, four or nine loud speakers by using the throats
shown in Fig. 11. The power capacity for continued safe operation
of the horn with one, four and nine throats is approximately 5, 20
and 45 watts, respectively (electrical input). The number of horns
used is dependent upon the particular installation and is related to
the directive characteristic of the horn. If it is necessary to disperse
the sound over a large angle, more horns are needed than when it is
desired to concentrate over a comparatively small angle. This
directive characteristic of the horn is important in talking motion
pictures as it is responsible for the illusion of the sound coming directly

from the mouth of the horn; that is, from the screen. If the horn
is replaced by a loud speaker of otherwise identical characteristics
but which radiates its sound over a very wide angle, there is a tendency
for.the sound to appear to come from a point some distance back of
the screen, thus tending to destroy the illusion.

The power supply equipment has been fairly completely covered in
discussing various parts of the system. Under ordinary conditions
the requisite power is obtained from the electric mains in the theater
except for the 12-volt battery required for some of the vacuum tube
filaments and for the electromagnets in the loud speakers and the
dry cells used with the photo cell and photo cell amplifier. Where
110-volt D.C. only is available there is a projector-driving equipment
which operates on this voltage, but a D.C. motor driving a 60-cycle
generator is required for supplying the amplifiers. Where 110-volt
A.C. is available, it is only necessary to connect the projector motor
and the amplifiers to this supply.

Large Power Capacity for Horn-type Loud Speakers,’’ by E. C. Wente and A. L.
Thuras.

WESTERN ELECTRIC SOUND PROJECTING SYSTEMS
FOR USE IN MOTION PICTURE THEATRES

To obtain a clear picture of the whole problem in all its com-
plexities, it will perhaps be best to consider an installation from its
very inception and follow it through to completion. This treatment,
while including mention of details which may possibly appear
superfluous, will nevertheless present a fair idea of the scope of the
work involved.

The Electrical Research Products, Inc. supplies to theatres
the equipment of the Western Electric Sound Projector Systems,
supervises its installation, instructs the local personnel in its opera-
tion, and provides inspection service during its subsequent use.

One of the most important factors in preparing a theatre for
sound projection is the initial engineering survey, for upon this
survey rests the decision of the type of system to be installed and
the determination of requirements for apparatus layout and of
possible special treatment. Each theatre is an individual problem
and must be considered as such.

The survey engineer gathers pertinent information and pre-
pares his recommendations. And while his recommendations are
given due consideration, a special staff at the home office studies
the survey and renders final decision. Survey information must
therefore be complete. |

In the projection room the engineer must investigate the power
supply not only as to voltage and kind but to determine if there is
ample capacity to carry the load to be added, and if the regulation
is suitable. The type and condition of the projection machines

* Theatre Systems Engineer, Electrical Research Products, Inc., New
York, Nes

must be noted and the angle of projection learned. Is the space
between machines ample for the attachment of the sound apparatus
and for convenient operation? Is there an acceptable location
in the room for the amplifying equipment and associated controls?
It is sometimes necessary, particularly in the older theatres, to
require rearrangement of the projection equipment or possibly an
enlargement of the projection room. Such changes are naturally
welcomed by the operating staff. Space, conveniently near but
separately enclosed, must also be found to house the storage
batteries.

The auditorium presents an item for careful study, for upon
its characteristics is based the size of the system to be installed.
The types of systems available, varying in power output, meet
conditions imposed by theatres from the largest to the smallest and
are so arranged and selected that, while sufficient sound volume
is at hand in each case, no theatre is overburdened with excess.

As blue prints of the auditorium are not always available it is
frequently necessary for the engineer to take the dimensions him-
self, for the cubical volume is an important factor. Seating capacity
and the distribution of seats also bear influence. The general
acoustics of the house must be considered. What degree of rever-
beration is present, bearing in mind the absorptive effect of the
audience. Are there areas of echo or interference or of otherwise
bad hearing? Are internal or external disturbing noises a factor?
A final and specific study of acoustic properties can best be made at
the time of system testing upon completion of the installation, but
general effects can be and are observed during the survey process.

The stage must receive consideration as to size and to layout
with reference to location of the horns and of the special screen
used with the system. If the theatre policy covers the showing of
moving pictures only, the horns may be located permanently on a
suitable structure. If vaudeville or stage presentations are in-
cluded, however, they are generally placed in movable horn towers
or else flown from battens.

Information sufficient to permit a knowledge of the conduit
runs for the complete installation is also obtained at the time of
survey.

The data collected, augmented by sketches prepared by
the engineer, in conjunction with blueprints of the building, if
available, thus provide the home office survey staff with a complete

picture of the theatre. The type of sound projector system is ac-
cordingly selected and the dates of shipment of equipment and
installations are placed on schedule. If, perchance, architectural
changes in the theatre are required the theatre management is so
notified in the anticipation that alterations will be completed
prior to the actual introduction of apparatus.

At the outset of an installation, it is important for the engineer
to establish satisfactory personal contacts with the local people
and organizations that will be related to the work. This is neces-
sary that the work may proceed smoothly and without delay, as
an opening date has by this time usually been fixed. While sufficient
time for the installation, testing and rehearsals is allowed, obviously
no large safety factor can be tolerated. The engineer first meets
the. house manager, and subsequently the projectionists, the house
electrican and stage hands, and familiarizes himself with house
conditions such as policy, house hours, labor conditions, best time
for work, etc. It is necessary for him to find a competent electrical
contractor, one, if possible, who already has knowledge of the
theatre. 4

The installation proceeds in accordance with a logical plan.
It is usual first to attach the driving equipment to the motion
picture projector, for, while the projector mechanism itself remains
practically intact, its driving motor is replaced by one with elec-
trically regulated speed control. A film speed of 90 feet per minute
is assured for sound picture work, but variable speeds may be
obtained, if desired, for silent film. As these motors require four
or five seconds to pick up speed, it is advantageous to have the pro-
jectionists become accustomed to such features in order to be able
to give full attention to the handling of film or disc record when
the time arrives.

The battery equipment is next installed with its switching
panel and charging apparatus. This permits an investigation of
the condition of the batteries and ample time for charging if
required. Furthermore, the projectionists may study this portion
of the apparatus at some leisure and have explained to them proper
schedules of charging. If a motor generator is required, it is set up
at this time.

The amplifiers and associated equipment are placed upon the
relay racks and the fader with its auxiliary positions is mounted
at the front of the projection room wall. The position of the monitor

horn is carefully selected so that the operating personnel will at
all times be able to hear clearly, for it is by following the program
through this horn that fader change-overs are made. .

While the apparatus is being placed in position the conduit
installation keeps apace. It should now be possible to connect the
storage batteries and A.C. supply to the amplifying equipment
and check its functioning. The fader can be electrically joined to the
film and disc reproducers associated with the production machines.
A film-dise transfer panel is mounted above the master fader
position to permit either the film or disc reproducers to be used at
will, simply by momentarily depressing the proper push button. A
signal light gives visible evidence as to which circuit is set up.
Interconnections between physically disassociated groups of equip-
ment are made through a junction box located in the projection
room. This method not only facilitates installation but provides a
ready means for circuit checking and trouble finding.

While equipment and installation methods are constantly
being simplified, it may be noted that a rather marked advance in
this direction is to be introduced within a few months. At the
present time the driving motor and disc turntable have their
separate mounting pedestals. The film reproducer and associated
amplifier are supplied as separate units to be assembled as an
appendage. The film-disc transfer panel is an apparatus separately
mounted on the wall. The new plan provides that these individual
pieces shall join into a single united whole. A pedestal is to be
supplied which will have space for placing the driving motor and
mounting the dise turntable. The film reproducer and amplifier
will form a definite part of the pedestal. A film-dise transfer will
be associated with each unit which will permit the projectionist
in making his machine ready for the succeeding selection to prepare
completely for the change-over. As any equalizers which may be
necessary will also be included in the pedestal, special wiring or
switching of such units will be eliminated. A mechanical brake will
provide ready stopping of the projector. A model of this complete
device has been installed in the booth in this auditorium and is
being used in the projection of sound pictures.

The installation engineer next turns his attention to the stage.
The horns are placed in the mountings which have been constructed
by now and are located in positions which experience has taught
the engineer may be roughly correct for this type of house. After

the loud speaking receivers are attached to the horns and con-
nected in circuit, a cursory test follows to assure that no major
errors are present. This is the first opportunity to produce sound
through the complete system. Any imperfections that are im-
mediately obvious are corrected. Comparative tests are then made
between the sound reproducers on the projectors equipped, so that
variations in volume can be compensated for. Each receiver and
horn is heard separately, and then in unison to check poling.
Receivers incorrectly poled give a most unpleasant effect of pressure
in the ears of the listener if he is at the junction of their sound
paths.

Before proceeding further it is desirable to have in place
the screen used with the sound system. An obvious requisite of
talking pictures is that the sound shall seem to emanate directly
from the action shown on the screen. If the sound source is placed
immedately behind the area of action in the picture, the illusion is
optimum. While it is true that the human mind tends to adjust
itself to disregard imperfections in this particular, extensive study
has shown that, where the sound originates from locations other
than at the visual source of action, an impression of unnaturalness
is often present. The auditor is probably not aware of the cause of
his uneasiness, but he is unconsciously attempting to convince
himself that he is hearing aright. Imperfections of location become
less annoying, of course, the farther one is away from the screen.
To present the best illusion a special semi-porous screen is used
which permits sound from the horns located directly behind it to
pass through readily. With the present type in use, a small per-
centage of light is lost from the picture.

With the screen in place and the horns temporarily set, the
acoustic tests can be made and the final check given to the system.

Acoustic conditions of theatres are responsible for widely
different results obtained. Some houses give a mellow effect,
others make reproduced sounds appear metallic. The shape of
the interior has less effect in general than has the nature of the sur-
face of the walls, ceiling and floor. If these surfaces are hard and
smooth like ordinary plaster, concrete, polished wood or glass,
they will reflect most of the sound striking them. Different fre-—
quencies may be reflected in varying proportion. Where surfaces are
soft, thick materials such as heavy drapes, carpets, upholstery, or
where the audience covers a relatively large space, a goodly portion

of the sound is absorbed. Pilasters, moldings, cornices and such
broken surfaces disperse sound waves and tend to damp them out
quickly.

The hard or reverberant type of theatre is especially annoying
for speech selections since the duration of audibility is so great
that a sound will remain to overlap the sound succeeding it. This
gives an effect of poor articulation. For music reproduction, how-
ever, a reasonable amount of reverberation is acceptable as it
tends to give the effect of fullness and roundness. Another varia-
ble factor which further complicates the situation is the changing
amount of audience which in itself is an excellent absorbent.
Theatres which may be entirely too reverberant empty will some-
times give pleasing results when filled. Unfortunately, it is not
possible to have a completely filled house at every performance. It
will therfore be seen that at best, a compromise must be tolerated.

Reverberation depends upon the rate of decay of sound and is
measured by its inverse, the duration of audibility of a sound of
known initial intensity. The introduction of highly absorbent
material into a room will have greater absorptive effect on the higher
frequencies. If the thickness of the material is increased considera-
bly the absorption of the lower tones will be increased while that
of the higher frequencies will remain practically intact. Very thin
fabrics such as cheese cloth or bunting absorb only extremely
high frequencies.

A highly damped auditorium will cause sound to appear to
Jack brilliance and definition and will have deadness which in
extreme cases may be almost depressing. Greater electrical power
will be required to fill the house with comfortable sound volume,
but the effect of the audience will be relatively less than it is in a
reverberant house.

Echoes, generally speaking, are due to too great differences
in the lengths of the sound paths of direct and reflected sound which
reach an auditor in a given position in a room. An interval of about
1/16 second is noticeable. Proper location of the source of sound,
or placing draping material over the offending reflecting surface
will ordinarily correct echo.

Interference is usually produced by the crossing of many
trains of sound waves reflected from various parts of the room.

It results in distortion and is overcome by proper placing of absor-
bent.

Resonance occurs with a surface consisting of some thin, hard
material which is free to vibrate as a diaphragm at its own natural
frequency. Thus sound is reinforced. Distortion will probably
result, since only that portion of complex sound whose frequency
is near the natural period of the resonator is affected.

Armed with a knowledge of these general acoustic considera-
tions and bearing in mind the three prime requisites to good
hearing, i.e., that sound should be sufficiently but not unnaturally
loud for all auditors, that successive sounds be clear and distinct,
and that the components of complex sound should retain their
relative intensities, the engineer makes his acoustic adjustments.
He usually tests first for sound distribution and arranges his horns
so that in so far as possible all portions of the house receive a like
amount. Good distribution can normally be obtained by proper
flaring and tilting of the horns. Any acoustic peculiarities are
next investigated and where possible corrected. Slight variation
in the location of the horns may result in quite marked differences
in the results obtained, for which reason this test may involve
many hours of labor. It may be necessary to convince the manage-
ment that acoustic treatment is essential if good reproduction is to
be obtained.

One standard practice which is followed in all cases requires
that the horns and the back of the screen not occupied by the
horns be completely enveloped by absorbent drapes. If the sheet is
set back from the proscenium arch, it is also usual to hang drapes
in shadow box effect. This treatment eliminates back stage reflec-
tions of sound.

After the horn locations have been definitely fixed and the
equipment given a final check, the engineer proceeds to calibrate
the theatre. He has available several test records embracing a
rather wide variety of character of selection to cover various
types of entertainment which will be shown in future in the theatre.
Each record has been marked with a proper fader setting deter-
mined in a standard theatre. By playing these records at the
marked value and adjusting the main volume control potentio-
meter on the amplifying equipment until the proper effects are
obtained, it should be possible from then on to obtain good results
by using normal fader volumes for any succeeding selections. In
other words, this potentiometer setting which then remains fixed
compensates for the varying sizes of theatres and permits like effects

of reproduction to be obtained universally. In addition, various
types of selection require that varying degrees of volume be fed to
the different horns. From the test records the engineer is able
definitely to select what these volumes should be and so establish
the horn setting values which are normally required. When these
potentiometer values and horn settings are determined the theatre
is considered calibrated. Such routine tends to standardize sound
reproduction in so far as that is possible in theatres using sound
projector systems.

While the installation has been progressing the operating
personnel have been instructed in regard to the functioning of the
equipment and the proper method of handling it. At the outset
each man has been given a copy of a complete operating instruction
bulletin for his personal keeping for study and reference. By the
time of the completion of the work, and as a result of the running
of the equipment during the acoustic tests the projectionist should
now be able to start up and adjust the amplifying equipment pre-
paratory to running the show. He should be able to thread the pro-
jector with film through the sound gate compartment with surety
and for film reproduction adjust his sound reproducer lamp. For
disc reproduction he should know how to place the starting mark
of the film in its aperture and fix the needle of the reproducer on
the starting mark of the record.

The rehearsal of the opening program is an important event.
It not only enables the projectionists to demonstrate their ability
in handling the equipment under real operating conditions, but
also gives them experience in actual change-overs of picture and
fader. The engineer can acquaint the manager with the technique
of running the show to obtain best results. While the physical
operation of the apparatus is the responsibility of the projectionist,
the manager in the last analysis is responsible for the general effects
in the theatre proper. Either he or someone assigned to the work
should be present in the audience during the showing of sound
pictures. Even though rehearsals of programs have indicated proper
operating points of the various selections certain conditions arise
which may require changes in volume. The observer is able to be
in touch with the projection room for transmitting his wishes by
means of a buzzer signal system and telephone circuit, which are
provided. It is, of course, essential that this observer not only be
entirely familiar with the results which the system may be expected

to give, but should also know how to originate requests for obtain-
ing the changes which he may want. Upon the ability and conscien-
tious effort of this observer depends in a large measure the success
of the sound picture programs.

The installation personnel remain at the theatre until it is
evident that the equipment can be handled in a commendable
manner. At such time the installation is transferred to the service
organization of the Electrical Research Products, Ine.

The function of the Service Department is evidenced bp its
name. The service engineer in whose district the theatre is located
is available for emergency calls at all times. In addition, he visits
the theatre periodically to check the equipment and recommend
what minor adjustments may be necessary. He listens to the selec-
tions on the program and makes any suggestion to the manager
which may occur to him. In this manner all of the theatre systems
are kept under frequent engineering supervision in an earnest
endeavor to maintain the class of service which they may be
expected to give.

Bell Telephone Laboratories Incorporated: Sound Pictures