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LeAudio

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applies the presentation delay value to determine the renderin g point
stopping synchronisation to a broadcast audio stream
if an acceptor decides it wants to stop receiving a broadcast audio stream it acts
autonomously to disconnect the audio data path and stops synchronisation with the pa and
the bis
the broadca st reception user experience
although an acceptor is allowed to acquire a broadcast stream by itself its not a very good
user experience at its most basic i ts an exact analogue of the telecoil experience where
someone wearing a hearing aid presses its telecoil button to acquire a telecoil stream that
user experience works because telecoils are inductive loops and you are normally only within
range of one loop which means theres no issue of whether or not youre connecting to the
right one if there is only one bluetooth le audio transmitter within range that experience
will be the same but as soon as broadcast applications become popular thats no lo nger going
to be true in many cases youre likely to be within range of many broadcast transmitters its
exactly the same situation as we have with wi fi if you scan for wi fi access points with
your phone or laptop youll frequently find a dozen or more the user experience for
bluetooth le audio is potentially a lot more difficult because in most cases youll be wearing
a pair of earbuds or hearing aids which have a minimal user interface theres also the added
complication of needing to start and stop streaming together and always connecting both
earbuds to the same broadcast source
so although the explanations above are valid and indicate how to receive a broadcast stream
theyre largely academic to make this work in the real world we need to involve another
device which does a better job of the hard work of finding the broadcast streams and telling
one or more acceptors what to do with them we also need a way to distribute the keys
needed to decode encrypted audio streams which bri ngs us to commanders and broadcast
assistant s
bass the broadcast audio scan service
first a few words about bass the broadcast audio scan service bass is a service which
is instantiated in an accept or to expose the details of which broadcast audio streams it knows
about which ones its connected to and whether it wants a broadcast assistant to help it
manage that information and help it to make connections bass works with broadcast
assistant s which are defined in bap and which well get to in a minute to manage broadcast
connections that they select and manage its a key part of expanding the broadcast ecosystem
using acl connections to assist the distribution and control of broadcast info rmation
the key elements of bass are two characteristics both of which are mandatory when ever it
is used
section commanders
characteristic properties quantity
broadcast audio scan control point write write wo response only one
broadcast receive state read notify one or more
table bass characteristics
the broadcast audio scan control point characteristic allows one or more broadcast
assistant s to inform an acceptor of whether theyre actively working on its behalf to look for
broadcast source s they can tell the acceptor how to find a big connect to a big
disconnect from a big and provide the broadcastcode to decrypt an encrypted audio
stream as well see there is one really important parameter within the broadcast audio scan
control point characteristic which is the bissync when a broadcast a ssistant sets this to
b it tells the acceptor to start reception of a stream when it sets it to b the acceptor
should stop receiving it
the one or more broadcast assistant s statement on the first line of the previous paragraph
is an important one an acceptor can use as many broadcast assistant s as it likes all that is
required is that each has a n acl link to that acceptor once theyre connected and have
registered the fact that theyre helping the acceptor to find a broadcast stream the broadcast
assistants operate on a first come first served basis limited only by any locks which are
invoked by csip if theyre operating on a coordinated set of acceptors each broadcast
assistant needs to register with the broadcast receive states for notifications which means
that all of them will be updated whenever a change is made to the broadcast state either by a
broadcast assistant writing to it or as a result of a local action on the acceptor
the broadcast receive state characteristic expose s what the acceptor is doing which big
its connected to which bises within that big its currently receiving whether it can decrypt
them and the current metadata it has for each of them an acceptor has to have at least one
broadcast receive state ch aracteristic for each big it can simultaneously synchronise to in
many cases that will be just one now on to the commander
commanders
although most people see broadcast as the big new feature of bluetooth le audio the most
important innovation is probably the concept of the commander which provides remote
control and management of broadcast audio s treams in a new world of multiple br oadcast
streams commanders provide a simple way for users to select what they hear although the
concept of broadcast was inspired by the telecoil experience of picking up inductive loops in
hearing aids bluetooth le audio broadcast applications go a lo ng long way beyond what
telecoil can do
looking back at the inductive loop paradigm this was straightforward you would only
receive an audio stream if you were standing within the boundary of the loop there might
be a problem if you were in the roo m directly above it but that was an edge case with
chapter setting up and using broadcast audio streams
bluetooth broadcasts they can and will overlap and go through walls so users need a simple
way to select the broadcast they want to listen to weve already seen how the metadata
information in base and service information from profiles like the public broadcast profile
pbp help inform that choice however that is just for public broadcasts which are open
for anyone to hear for personal broadcasts the broadcas t audio streams are encrypted
requiring methods to acquire the encryption keys once again that is enabled by
commanders
figure the major elements of a commander and broadcast acceptor
figure illustrates the major components of a commander and the main broadcast related
parts of an acceptor both are roles defined in cap but im using them more generally in
this book its useful to visualise a commander as a device in its own right like a tv remote
control because for a user what it does is very similar commanders usually contain four
main elements
a csip set coordinator which ensures that any commands are se nt to all of the
members of a coordinated set wheth er thats information about a broadcast
source or volume control
a broadcast assistant which is used to scan for broadcast source s and lets the user
select which one they want to li sten to
a vcp volume controller which well cover in chapter to control the volume
of each member of the coordinated set and
the ability to obtain and distribute a broadcastcode to decrypt broadcast audio
streams
witho ut the flexibility that comes with commanders and the capabilities they provide
broadcast encryption becomes cumbersome making use cases around private broadcast