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be acted on as an entity whilst most people will immediately think of a pair of earbuds or
hearing aids th at set could equally be a pair of speakers or a set of surround sound speakers
a coordinated set of ea rbuds should be represented as a single device so that anything that
happens to one happens to the other although how that happens is down to the
implementation when you adjust the volume for a pair of earbuds the volume of both should
change at the same time and if you decide to listen to a different audio source that change
should occur simultaneously on both left and right earbuds you do not want a user
experience where your left earbud is listening to your tv whilst the right earbud is stre aming
music from your phone
coordination is handled by the coordinated set identification profile and service csip and
csis which are referenced from within cap the main feature of csis and csip aside
this is a document that describes generic features used within bluetooth low energy
the volume control profile has the flexibility to allow these to be changed independently if the
user prefers
chapter new concepts in bluetooth le audio
from identifying devices as members of a coordinated set is to provide a lock function this
ensures that when an initiator interacts with one of the members the others can be locked
preventing any other initiator from interact ing with other members of that set
the need for such a lock is that ear worn devices such as hearing aids and earbuds have a
problem with communicating directly with each other using bluetooth technology thats
because the human head is remarkably goo d at attenuating ghz signals if an earbud is
small which means that its antenna will also be small it is unlikely that a transmission from a
device in one ear would be received by its partner in the other ear many current earbuds get
around this l imitation by including a different lower frequency radio within the earbud for
eartoear communication which is not significantly attenuated by the head typically using
near field magnetic induction nfmi this s econd radio adds cost and takes up space but
removing it and relying on the ghz bluetooth link between the earbuds raises the risk that
different initiators could send conflicting commands to left and right earbuds
with csip and csis if you accept a phone call on your left earbud the initiator would set
the lock on both earbuds and transition your right earbud to the same stream before releasing
the lock the lock feature allows these interactions to be managed without the need for the
members of the coordinated set to talk to each other
if members of a coordinated set do have another radio connection which can penetrate the
head the hearing access service has a feature which will signal that this radio can be used to
convey information to the other hearing aid at the moment this feature is limited to
information on preset settings but may be used for other features in the future
generally members of a coordinated set are configured at manufacture and shipped as a pair
but membership can be set to be written as well as read allowing for later configuration or
the replacement of faulty or lost units
presentation delay and seria lisation of audio data
supporting two earbuds brings us to another issue that bluetooth le audio had to solve
which is ensuring that the sound at both the left and the right ear is rendered at exactly the
same time in the past audio data was sent to a single device which knew how to extract the
left and right signals and render them at the same time with bluetooth le audio cises send
data to different destinations serially using different transmission slots a lthough the
incoming audio channels present data to the initiator at the same point in time the encoded
packets arrive at the accepto rs one after the other they may be further delayed by
retransmissions figure illustrates this by adding examples of left and right packets going
to two acceptors onto the cig diagram of figure
section presentation delay and serialisation of audio data
figure the serial t ransmission of audio data
this serialisation causes a problem the human head is remarkably good at detecting a
difference in the arrival time of sound between your left and right ears and uses that difference
to estimate the direction from which the sou nd is coming
figure effect of head rotation on sound arrival
chapter new concepts in bluetooth le audio
figure illustrates that if youre two metres away from an audio source and you rotate your
head by just degrees that equates to just over a µs difference in the arrival time of the
sound if theres a variation in the rendering time between your left and right ear your brain
interprets this as the sound source moving if that difference is much more than
microseconds and changes regularly you start to get an unpleasant effect where it feels as if
the sound is moving around within your head to prevent that its important to have a
synchronisation technique to ensure that the left and right earbuds always render their
respective audio data at exactly the same time
we cant rely on any bluetooth communication between two earbuds as weve said before
the human head is very efficient at attenuating a ghz signal as it contains a lot of water
if you have small earbuds which fit neatly in the ear canal there is no guarantee that they will
be able to communicate with each other
there are two parts to t he bluetooth le audio solution first both earbuds need to know a
common synchroni sation point which is the point in time at which every acceptor can
guarantee that every other acceptor has had every possible chance to receive a transmitted
packet whet her its a unicast or a broadcast stream this point in time has to be provided by
the initiator as it is the only device which knows how many attempts it will take to send
packets to all of the acceptors remember that the acceptors generally cant ta lk to each
other and are probably unaware of each others existence
in most cases the acceptor s will have received their audio data packet s earlier than that
common synchronisation point as in audio applications data packets are scheduled to be
retransmitted multiple times to maximise the chance of reception that is because of the
problem of drop outs in an audio stream as a result of missing packets these are particularly
annoying artefacts for the listener so retransmissions are used to help i mprove the robustness
of the signal this means that every acceptor needs to include enough buffering to store
packets from the earliest possible arrival time the time when their packet is first transmitted
until the common synchronisation point well learn more about the synchronisation point
in chapter
however you cant render the audio at the synchronisation point as its still encoded
between the synchronisation point and the final rendering point the data need s to be decoded
and any additional audio processing such as packet loss concealment plc active noise
cancellation anc or hearing aid audio adjustments performed before it can finally be
rendered
packet loss concealment is a technique that attempts to recreate missing or corrupted packets of
audio data generally based on what the previous packets contained it is an attempt to avoid audible
artefacts when an audio stream is disrupted
section presentation delay and serialisation of audio data
the time needed for that may vary between different acceptors whilst you expect a pair of