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LeAudio

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in this case the audio sink is the general term for what you have in your ear to listen to audio
streams it sidest eps the fact that i t can be an audio source as well the point is that the device in
your ear can be more involved in decisions
chapter new concepts in bluetooth le audio
the core refer s to central and peripheral devices where central devices send commands and
peripherals respond in bap theyre called clients and server roles moving up a la yer cap
defines them as initiator and acceptor roles the initiator role exists in a central device
which is responsible for setting up scheduling and managing the isochronous stream s
acceptors are the devices that participate in these streams typically hearing aids speakers
and earbuds there is always one initiator but there can be multiple acceptors the different
nomenclature is shown is figure
figure bluetooth le audio roles
im going to use the initiator and acceptor names most of time even though theyre
technically roles because i think they best explain the way that bluetooth le audio works
both initiators and acceptors can transmit and receive audio streams at the same time and
they can both contain multip le bluetooth le audio sinks and sources the important thing
to remember is that the initiator is the device that performs the configuration and scheduling
of the isochronous streams and the acceptor is the device that accepts those streams that
concept applies both in unicast and broadcast theres one other abbreviation ill make as
only initiators can broadcast audio streams ill save a few words and refer to initiators actin g
as broadcast sources as broadcasters unless theres a need to be explicit
at this point its worth having a slight diversion to look at some of the terminology which is
used within bluetooth le audio this will introduce some features we havent disc ussed yet
which well cover in detail in chapter when we get to isochronous stream s
throughout the specifications there are a lot of different names and phrases describing
channels and streams they have some very distinct and sometimes slightly d ifferent meanings
depending on which of the specifications youre looking at but ive tried to capture the main
ones in figure
figure bluetooth le audio terminology
section terminology
the diagram shows the conceptual transmi ssion of audio data from left to right at the very
left we have audio coming in which is typically an analogue signal its shown as audio in
at the other end we see audio being rendered at audio out which may be on a speaker or
an earbud the audio at these two points is called the a udio channel both ends are the same
audio channel which is equivalent to what would be carried if it were a wired connection
between an mp player and a speaker an audio channel is defined in bap as a unidirectional
flow of audio into the input of your bluetooth device and then out of the other end in
practice th at out will normally be directly into your headphone transducer or a speaker
the details of the input and output of the audio channel are implementation specific and
outside the bluetooth specifications what the audio is and how the audio is handled after the
wireless transmission and decoding is outside the scope of the specification
how that audio input is carried to the audio output is what is defined inside the bluetooth
specifications they are represented by the dotted box in figure the audio is encod ed
and decod ed using the new lc codec unless an implementation needs to use a specific
additional codec other codecs can be used but all devices must support some basic lc
configurations which are defined in bap to ensure interoperab ility the encoder produces
encoded audio data which goes into the payloads for the isochronous stream s
once the audio data is encoded it is placed into sdus service data units which the core
translate s into pdus protocol data units which are transmitted to the receiving device
once a pdu is received it is reconstructed as an sdu for delivery to the decoder the
isochronous stream is the term used to describe the transport of sdus encapsulated in
pdus from encoder output to the decoder input it includes retransmissions and any
buffering required to synchronise multiple isochronous stream s the flow of encoded audio
data over a n isochronous stream is defined in bap as an audio stream and like an audio
channel is always unidirectional the relationship of the different terms are illustrated in
figure
figure representation of streaming terms
with the exception of the presentation delay which states when it is rendered this is explained in
section
chapter new concepts in bluetooth le audio
the core places isochronous stream s serving the same application together into an
isochronous group for unicast streams its called a connec ted isochronous group cig
which contains one or more connected isochronous stream s cis for broadcast its a
broadcast isochronous group big where more than one isochronous stream is present
in an isochronous group carrying audio data in the same direction the isochronous streams
are expected to have a time relationship to each other at the application layer by time
relationship we mean audio channels which are expected to be rendered or captured at the
same time a typical application is rendering a left and a right stereo stream into two separate
earbuds with one or two return streams from their microphones which highlights the fact
that a cig or big can contain isochronous streams which go to multiple devices
bluetooth le audio has been designed to be very flexible which means that sometimes there
are different ways of doing things taking one example if we look at a simple connection
from a phone to a headset we need one audio stream going in each direction one way to do
that is to establish separate cises for each direction
figure two separate cises in a cig
in figure we can see two cises an outgoing one cis and an incoming one cis
both contained within the same cig we can optimise that by using a single cis which is
whats shown in figure which shows a single bidirectio nal cis carrying audio data in both
directions in the same cig well see exactly how th at works in the next chapter
figure a bidirectional cis in a cig
both of these options are allowed it is up t o the application to decide which is the most
appropriate for its use in most cases the optimization of bidirectionality would be the option
of choice as it saves airtime
section context types
figure a unidirectional and bidirectional cis for two acceptors eg a phone supporting a telephone call with two
hearing aids
figure shows a typical application with a cig containing two cises which might connect
a phone to a pair of earbuds but where the return microphone is only implemented in one of
the earbuds it shows cis which is a bidirectional cis carrying audio from a phone