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LeAudio

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trade off is tha t the quality is limited and because there is no compression it is effectively real
time with no opportunity for retransmission
later versions of hfp include msbc a modified version of the sbc codec specified in
adp to support wideband s peech msbc is effectively a cut down version of sbc with a
limited sampling frequency for a single monaural stream being a frame based codec it
increases the latency resulting in typical overall delays of around ms these put hfp in the
low late ncy low to medium quality quadrant of figure
in contrast adp was designed for high quality music it mandates the sbc sub band
coding codec which is a frame based codec with fairly basic psychoacoustic modelling it
can produce very good audio quality which is close to the limit of what an experienced listener
can detect compared to the original audio stream the adp specification also allows the
chapter lc latency and qos
use of alternative codecs which were developed by external companies or standards groups
a selection which includes aac which is used by apple in most of their bluetooth
products mp and atrac as well as an option for companies to use proprietary codecs
a number of these have become popular of which the best known is the aptx range from
qualcomm almost all of these codecs have longer latencies
in figure adp is in the top right quadrant of the diagram with a long latency that is
partly driven by the desire for using retransmission s to try to make the audio more robust
whereas listeners used to accept glitches like scratches on records they appear to be far les s
tolerant of the occasional pop or dropout in an audio stream the simplest solution is to
add more retransmissions and buffering but that means that wireless music streaming typically
has a latency of ms even if youre streaming from a fil e on your phone or computer
although the codec isnt involved in this delay the knowledge that it happens means that
codec designers havent generally concentrated on improving latency unless its for a specific
application like gaming
although a ms delay sounds excessive for most music applications its not a
problem when streaming music whether from a music player or an internet service the user
has nothing to indicate whether theyre listening in real time or not as long as the music
stream starts within a second of them pressing the play button and the music stream is
continuous without annoying interruptions theyre happy however when the audio is a
soundtrack for a video they may notice a lipsynch problem as a ms delay be tween seeing
someone talking and hearing their voice looks wrong phone and tv manufacturers can
address this by delaying the video to compensate for any audio delay the audiovideo
distribution transport protocol avdtp which underlies the adp profi le contains a
delay reporting feature which allows audio source devices to ask the receivers what the
latency will be in the audio path knowing this tvs and phones can delay the video so that
both sound and picture are synchronised however many tvs and earbuds have limited
memory for audio or video buffering so latencies above a few hundred milliseconds may be
problematic
even short audio delays can become a problem where a user can hear both the bluetooth
audio and also the original ambient sour ce of the sound this has long been recognised by
the hearing aid industry where users are listening to live sound via a telecoil system in a theatre
or cinema but can also hear the ambient sound the same problem occur s at home where a
family watching the tv includes some members who are wearing hearing aids with support
for wireless transmission and some who are not telecoil induction loops which are currently
used for these applications are analogue so exhibit virtually no delay moving to blue tooth
referred to as optional in older specifications and additional in more recent ones
aac is the advanced audio codec
atrac i s the adaptive transform acoustic coding codec developed by sony
section classic bluetooth codecs their strengths and limitatio ns
require s a codec that is able to cover much more of the quality latency spectrum than sbc
figure lc a more efficient codec
during the bluetooth le audio development it became apparent th at the current bluetooth
codecs would struggle to meet the requirements not only were they limited in their quality
and latency trade offs but sbc is not as efficient as earbud and hearing aid designers would
like it has a relatively low complexity but takes up too much airtime which has a major effect
on the battery life of an earbud thats a problem for hearing aids which run on small zinc
air batteries these are sensitive to both peak current and the length of a current burst for
reception or transmission bluetooth chips can often consume more current receiving than
transmitting if operating limits are exceeded for these batteries their life can be drastically
reduced t o address these limitations t he bluetooth sig went on a codec hunt which
resulted in the inclusion of lc
bluetooth le audio allows manufacturers to use other codecs but lc is mandatory for all
devices the reason for this is to ensure interoperability as every audio source and every
audio sink has to support it the full specification for the codec is published and falls under
the bluetooth randz license so anybody can write their own implementation and
incorporate it into their bluetooth product as long as those products pass the bluetooth
qualification proce ss given its quality thats a powerful incentive to use it
a randz licence is a reasonable and non discriminatory zero fee license which is how the
bluetooth ip is licensed that doesnt mean there are no conditions but they are not arduous
theyre explained at the bluetooth website
chapter lc latency and qos
the lc codec
the lc is one of the most advanced audio codecs available today providing enormous
flexibility and covering everything from voice to high quality audio anyone can develop
their own implementation of lc for a bluetooth le audio product although given the
specialised nature of writing and optimising a codec very few people are ever likely to do
that because of that ill only provide an overview of what it does and how it works
theres a more detailed introduction in the lc specification along with around two
hundred pages of specification detail for anyone who fancies doing their own
implementation
the lc specification is among the most successful attempt s so far to cover the full range
of audio quality and latency requirements for wireless audio in a single codec it is optimised
for a frame size of ms and it is expected that all new applications in particular public
broadcast applications will use the manda tory ms frames it also works with a ms
frame size to provide compatibility with bluetooth classic audio applications which run
with ms intervals corresponding to ev sco packets it also supports an extended
ms frame to provide legacy khz sampling this is reduced to ms for a ms
based system which needs to support khz however these are specific variants to
support legacy or combined bluetooth classic audio bluetooth le audio
implementations
feature supported range
frame duration ms khz sampling