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vcp one profile can operate on three different services the public broadcast profile pbp
is an anomaly as its a profile without a service but thats one of the consequences of
broadcast as you cannot have a traditional client server interaction when there is no
connection
figure overview of the bluetooth le audio specifications
if the acronym or initialism ends with a p its a profile if it ends with an s its a service if it
ends with ps it normally refers to a combination of separate profile and service documents
chapter the bluetooth le audio architecture
the generic audio framework
we can now look at the constituent parts of the gaf there is a significant amount of
interaction between the v arious specifications which makes it difficult to draw a clear
hierarchy or set of relationships between them but they can be broadly separated into four
functional groups arranged as in figure
figure functional grouping of specifications within the generic audio framework
this grouping is largely for the sake of explanation i n real implementations of bluetooth le
audio most of these specifications interact with each other to a greater or lesser degree its
perfectly possible to make working products with just a few of them but to design richly
featured interoperable produ cts the majority of them will be required
stream configuration and management baps
starting at the bottom of figure we have a group of four specifications whic h are
collectively known as the baps specifications these four specifications form the foundation
of the generic audio framework at their core is bap the basic audio profile which is
used to set up and manage unicast and broadcast audio streams as a profile it works with
three services
pacs the published audio capabilities service which exposes the capabilities of
a device
ascs the audio stream control service which defines the state machine for
setting up and maintaining unicast audio streams and
bass the broadcast audio scan service which defines the procedures for
discovering and connecting to broadcast audio streams and distributing the
broadcast encryption keys
section the bluetooth le audio architecture
between them they are responsible for the way we set up the underlying isochronous
chann els which carry the audio data they also define a standard set of codec configurations
for lc and a corresponding range of quality of service qos settings for use with broadcast
and unicast applications
state machines for each individual isochronous c hannel are defined for both unicast and
broadcast both of which move the audio stream through a configured state to a streaming
state as illustrated in the simplified state machine of figure
figure the simplified isochronous channel state machine
for unicast the state machine is defined in the ascs specification t he state resides within
individual audio endpoints in the server with the client control defined in bap for
broadcast where there is no connection between the transmitter and receiv er the concept of
a client server model becomes a little tenuous as a result a state machine is only defined for
the transmitter and is solely under the control of its local application with broadcast t he
receiver needs to detect the presence of a s tream and then receive it but has no way of
affecting its state
multiple unicast or broadcast isochronous channels are bound together in groups which
we explore in chapter bap defines how these groups and their constituent isochronous
channels are put together for both broadcast and unicast streams
you can make a bluetooth le audio product with just three of these specifications bap
ascs and pacs for unicast and bap alone for broadcast although you ll need to add pacs
and bass if you want to use a phone or remote control to help find the broadcasts it would
be quite a limited device in terms of functionality just setting up an audio stream using it to
transmit audio and stopping it however by being able to do this the baps set of
specifications provide a base level of interoperability for all bluetooth le audio devices if
two bluetooth le audio devices have different top level profiles they should still be able to
set up an audio stream using bap it may have restricted functionality but should provide an
acceptable level of performance removing the issue of multi profile incompatibility that is
present in bluetooth classic audio where devices with no common audio profile would not
work together
chapter the bluetooth le audio architecture
rendering and capture control
having set up a stream users want to control the volume both of the audio streams being
rendered in their ears and the pick up of microphones
volume is a surprisingly difficult topic as there are multiple pl aces where the volume can be
adjusted on the source device on the hearing aid earbud or speaker or on another remote
control device which could be a smartwatch or a separate controller in bluetooth le
audio the final gain of the volume is perfor med in the hearing aid earbud or speaker and
not on the incoming audio stream although top level profiles may require that as well with
that assumption the volume control profile vcp defines how a client manages the gain on
the audio sink device the state of that gain is defined in the volume control service vcs
with one instance of vcs on each audio sink the volume can be expressed as a bsolute or
relative and can also be muted
where there are multi ple audio streams as with earbuds and hearing aids a second service is
required vocs the volume offset control service effectively ac ts as a balance control
allowing the relative volume of multiple devices to be adjusted these may be rendered on
different devices such as separate left and right earbuds or speakers or on a single device like
a pair of headphones or a soundbar the audio input control service aics acknowledges
the fact that most devices have the ability to support a number of different audio streams as
illustrated in figure aics provides the ability to control multiple different inputs that
can be mixed together and rendered within your earbud or speaker illustrates how these three
services could be used in a soundbar which has a bluetooth hdmi and microphone input
figure audio input control service aics volume control service vcs and volume offset control
service vocs
balance is the usual consumer nomenclature audi o engineers would probably consider it to be a
mixing control
section the bluetooth le audio architecture
for a hearing aid the inputs might be a bluetooth stream a microphone providing an ambient
audio stream and a telecoil antenna receiving a stream from an audio loop at any point in