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wwwbluetoothcomb loga technical overview oflc which includes a link to an audio
demo
multiple stimuli with hidden reference and anchor methodology for testing perceived quality
defined in itu bs
european broadcasting union tech sound quality asses sment material recordings for
subjective tests
section multi channel lc audio
sampling rate description
khz suitable for voice of telephony quality
khz higher quality voice adequate for voice recognition applications
khz adequate for music where there are imperfect listening conditions
such as background noise or where a listener has any hearing
impairment listeners are likely to detect a difference from higher
sampling rates if they are concentra ting on the audio stream in
noise free surroundings
khz most users will not detect a difference from the original when
listening with any background noise
khz users cannot detect a difference to the original
table a subjective description of lc audio quality for different sampling rates
what is important is that audio application designers understand that there are compromises
in designing a wireless audio experience and that some qos choices may exclude the
development of new use cases some sections of the audio industry will jump onto the
higher quality that lc offers and promote the highest sampling rates despite the fact that
limitations in re production and the listening environment will probably mean that few if any
listener s will appreciate them in addition the resulting latency is unsuitable for live
applications and some devices may not be able to decode them others will look at the n ew
features and use cases that are enabled by bluetooth le audio and choose khz or
khz as an acceptable compromise which allows new use cases to develop only time will
determine what users value most it may be more flexibility in their applications or a desire
to see a bigger quality number in the marketing literature
in the past there have been two occasions when the audio industry took the decision to
reduce audio quality the first was the introduction of cds the second was the use of
mp which enabled audio streaming each time there was a balance between greater ease
of use versus maintaining the current audio quality on both occasions consumers
expressed a major preference for ease of use
multi channel lc audio
in chapter we introduced the concept of audio channels it means that there are
multiple different ways of transmitting the same audio data between devices to understand
how th ey work its best to look at a couple of examples in each of the se the following
nomenclature is used to describe how many audio channels are multiplexed into a cis or
bis the number of arrowheads on each cis or bis corresponds to the number of audio
channels that it is carrying
chapter lc latency and qos
figure representation of the number of audio channels in a cis or bis
figure shows the two possible options for transmitting a stereo stream between an
initiator and a single acceptor this is the simple unicast use case of stream ing music from a
phone to headphones o r from a tv to a soundbar
figure multiplexing options for a stereo stream from an initiator to one acceptor
at the top of figure option a uses two separate cises one to carry the left audio
channel and the other to carry the right channel below that option b sets the channel
allocation to so that both of the lc encoder outputs are concatenated into a single
payload by the controller the link layer ll which is sent in a single cis at the
acceptor the individual left and right payloads are separated in the controller decoded and
rendered as individual channels after the presentation delay the controller is shown as
the ll block link layer in the diagrams where it is not multiplexing a stream but simply
transmitting or receiving a single channel codec frame it is omitted to simplify the diagrams
sending the same unicast stereo input to a mono acceptor is a little more interesting as at
some point between the input and output the stereo signal needs to be downmixed to a single
mono stream there are three possib le approaches to do that which are shown in figure
section multi channel lc audio
figure transmitting unicast stereo to a mono acceptor
for all three options the acceptor would set its audio sink locations as front l eft plus front
right so its supported audio location would be x note that there is
no mono audio location as mono is a property of a stream not a physical audio location
in option a the initiator can deduce that the acceptor will render a mono signal as it has
set the channel allocation to which means it will only render the stream to one location if
it had wanted both the left and the right channel it would have set channel allocation to
setting both audio location bit s but stating that it only supports a single non multiplexed
cis signifies that it requires the initiator to downmix the input audio channel to mono before
it is sent
in option b and option c the channel allocation and audio sink location informat ion
the initiator receives after the codec configuration procedure is identical to what it would
have received if the acceptor were a stereo device see figure it means that t he initiator
has no way of knowing whether it is a mono or a stereo device therefore it supplies it with
stereo information either as two separate cises in option b or a multiplexed stream on a
single cis in option c in this case the acceptor is responsible for downmix ing the resultant
streams
bap requires all devices which set multi ple bl uetooth le audio locations to be able to
support at least the same number of streams so in option a the acceptor would also need
to be able to support the two cis configuration of option b if it requires the initiator to
the initiator could look for an instance of the volume offset control service and infer from its
presence that its a stereo device but that may be trying to be too clever
chapter lc latency and qos
perform the downmixing it would specify the single channel allocation and left plus right
audio locations in a preferr ed pac record which well cover in chapter to signal its desire
for a single mono stream
the popular use case of separate earbuds also has different possible configurations figure
shows the two options for transmitting a stereo audio stream to a pair of acceptors
figure stream options for a pair of stereo earbuds
option a illustrates the configuration which will generally be used where the earbuds set
their audio location to just left or right not left and right as in the previous examples
the initiator will send each earbud a single cis corresponding to their audio location
in option b both earbuds have set their channel allocation to so they will receive a