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and broadcast isochronous stream s which are used for the broadcast use cases but the
underlying principles are very similar were no w ready to see how theyre all put together
which means diving into the core
isochronous stream s and roles
the isochronous stream s feature in the core release is a fundamentally new concept within
bluetooth low e nergy if youre familiar with hands free profile or adp youll know that
they have quite a constrained topology hfp has a bidirectional onetoone link typically
between a phone and a headset or hands free device it has two roles an audio gatewa y
and a hands free device adp is an even simpler unicast link specifying a source device
that generates audio and a sink device which can be your headphone speakers amplifier or a
recording device which receives that audio
figure bluetooth classic audio topologies
bluetooth le audio is built on the fundamental asymmetry that exists within the bluetooth
le specification where one device the central device is responsible for se tting up and
controlling the isochronous stream s the central can connect to a number of peripheral
devices which use those isochronous stream s to send and receive audio data the asymmetry
means that the peripheral devices can be much lower power for cises they have a say in
how the isochronous stream s are configured which will affect the audio quality latency and
their battery life giving them more control over the audio streams than with bluetooth classic
audio profiles for b ises the central makes all of the decisions with the peripheral deciding
section isochronous streams and roles
which broadcast isochronous stream s it wants to receive
repeating what we said in the terminology overview in section as we move up the stack
of the bluetooth le audio specifications well come across a number of different names for
the roles which devices perform in the core they are defined as central and peripheral
devices in the baps set of specif ications theyre called clients and servers and in cap they
become initiators and acceptors the initiator role always exists in a central device which is
responsible for scheduling the isochronous stream s acceptors are the devices that participate
in these streams there is always one initiator but there can be multiple acceptors
figure bluetooth le audio roles
when we move up into the top level profiles theres an avalanche of new role names with
senders broadcasters and receivers im going to ignore all of those and use the initiator and
acceptor names for most of time e ven though theyre technically roles because i think they
best explain the way that bluetooth le audio streams work when theres no audio stream
involved which is the case with the control specifications ill drop back to using client and
server
one thing id like to point out at this stage is that other than for broadcast either device can
act as an audio source generating audio data or as an audio sink receiving that data both
initiators and acceptors can be sources and sinks at the same time and they can each contain
multi ple bl uetooth le audio sinks and sources the concept of who generates the audio and
who receives and renders it is orthogonal to the concept of the initiator and the acceptor
the important thing to remember is that the i nitiator is the device that is responsible for
working out the timing of every transmission of audio data that i s sent that task is called the
scheduling the acceptor is the device that accepts those streams that concept applies both
in unicast and br oadcast an acceptor can also generat e audio data such as capturing your
voice from your headsets microphone but the initiator is responsible for telling it when it
needs to send that data back
as the initiator role is far more complex than the acceptor role initiators are normally devices
like phones tvs and tablets which have bigger batteries and more resources the scheduling
has to take account of other demands on the initiators radio whic h may include other
bluetooth connections and often wi fi as well thats a complex operation which is handled
by the chip designers but a s well see later on the bluetooth le audio profiles give
applications a fair amount of scope to influence that scheduling which is why developers need
to have a clear idea about how isochronous stream s work
chapter isochronous streams
for unicast bluetooth le audio we have a lot of flexibility in terms of topologies which are
depicted in figure we can replicate the same topologies that we had in hfp or adp
where we have a single device typically your phone and a single peripheral device such as
your headset with an audio l ink between them moving up from that bluetooth technology
can now support an initiator that talks to two or more acceptors the main application for
that is to allow your phone to talk to a left and a right pair of earbuds or hearing aids they
no lon ger need to be from the same manufacturer as bluetooth le audio is an interoperable
standard
we can extend this by adding additional unicast streams to support more than one pair of
earbuds or to connect multiple speakers to support surround sound sys tems with the example
of figure showing the addition of a central woofer unit
figure unicast audio topologies
in theory the specification can support up to separate unicast isochronous streams which
could connect different devices thats not actually realistic for audio as we start to run
out of bandwidth after three or four stream s the reason for the limit of streams in the
core specification is that the isochronous stream s feature was designed to support many
different time critical applications not just audio some of those need far les s bandwidth
than audio does when we look at the lc codec well discover some of the trade offs we
have to make between latency audio quality and robustness which place a limit on the number
of audio streams we can actually support
that airtime lim itation on the number of streams that unicast can support is one of the reasons
to use broadcast as figure shows a single broadcaster can talk to multiple accepto rs
which will often be configured as pairs of devices receiving either a single mono channel or
separate left and right audio channels depending on the audio quality which typically sets
the sampling rate and hence the airtime usage and maximum number of streams a
broadcaster may be able to offer greater functionality such as transmit ting simultaneous audio
streams in different languages these are the trade offs that need to be understood when
designing a bluetooth le audio application which well cover in more detail in the following
chapters
section connected isochronous streams
figure broadcast audio topologies
connected isochronous stream s
to understand the core isochronous features well start with unicast and connected
isochronous stream s which are known as cises their structure is quite complex but its
built on some very simple principles ill start by describing how connected isochronous
stream s were designed to explain the component parts and how they work then look at how
broadcast is different that gives you the foundations to move up into the generic audio