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broadcast application however the y are designed to be as generic as possible which means
that there are multiple different ways of putting the pieces together cap the common
audio profile defines a set of procedures to establish common ways to perform a ll of the
everyday actions that are needed to set up audio streams between an initiator and one or
more acceptors for most developers cap provides the interface on which they build their
applications
cap ties in the content control use case and render ing control concepts which we came across
in chapter it defines when these need to be used during the stream configuration and
establishment process it is also key to prevent ing multi profile issues when a bluetooth le
audio device transitions betwee n different use cases within a connection or makes
connections with different devices
the other thing that cap brings to the process is coordination the biggest market for
bluetooth audio today is in earbuds two separate devices which need to work as if they were
one the baps specifications deal with individual streams cap lays down rules for managing
streams wh en an initiator connects to multiple acceptors using the coordinated set
identification profile and service to bind them together
finally cap defines the commander role which is what elevates broadcast from a simple
telecoil replacement to a highly flexi ble audio distribution solution
in this chapter ill provide an overview of the cap procedures and how theyre used
essentially you can think of cap as a recipe book the baps specifications define all of the
ingredients for audio streams and cap tells you which ones to use for each procedure and
what order to use them in most of the detail will come out in the foll owing chapters on
setting up unicast and broadcast audio streams where well see how cap augments and
utilises the stream control and management procedures that are already in baps but before
we start on cap its important to understand csip and csis
csips the coordinated set identification profile and service
because adp was designed for streaming to a single device every bluetooth solution on the
market today that sends audio to multiple devices uses some proprietary method of making
them work together whether theyre a pair of earbuds hearing aids or speakers bluetooth
le audio needed to define a standard method of doing this so that any combination of
products could be used together the issue is not just making sure that they can have their
volume controlled together but also to make sure that if you change the device providing the
section csips the coordinated set identification profile and service
audio stream such as when a phone call interrupts you when youre watching tv then both
left and right earbuds change their connection to your phone at the same time that
requirement led to the concept of coordinated set s
the coordinated set identification service is instantiated on devices which form a group
called a coordinated set where the group members fulfil a specific use case act ing in a
concerted manner a typical example is a pair of earbuds the specifications are not limited
to audio devices they could equally be used for sets of medical sensors such as ecg patches
where data is being collected from separate sensors nor do all of the functions of those
devices need to be exactly the same but one feature should be for example in a pair of
earbuds the common feature is that they both would render audio streams but only one need
have a microphone
for a pair of earbuds coordination performs four main functions
it identifies the members of the coordinated set ie a left and right earbud
it is used to apply volume controls and mute to both devices
it is used to ensure they both receive audio streams from the same audio source
the streams themselves are generally different being left and right but that is
irrele vant to csips and
it allows a device to lock access to the earbuds preventing other devices from
accessing them
csip defines two roles
a device which is a member of a coordinated set is a set member and
a device which discovers a nd manages a coordinated set is a set coordinator
the set member role is essentially a passive role it simply states that it is part of a set the
set coordinator not only finds the members but then passes that knowledge on to other
procedures to ensure that those procedures act on all members of the set
csis requires that every member of a coordinated set includes a resolvable set identity rsi
ad type which allows a client device to recognize that it is a member of a coordinated set
this means that there must be two or more acceptors that need to be found the rsi is a
random six octet identifier which changes over time this reduces the risk of someone
tracking your b luetooth products
once a client device which can be an initiator or commander makes a connection to a device
exposing the rsi ad type it pairs and bonds with the set member it has discovered and reads
its set identity resolving key sirk characteristic the sirk is a bit random number
which is common to all members of the coordinated set which a client can use to decode
chapter cap and csips
the resolvable set identity it does not change for the lifetime of the device the sir k is
normally programmed into all of the devices which comprise the coordinated set during
manufacture the bluetooth le audio specifications do not specify a way to set it or change
it so if products need to be added to a coordinated set during their li fetime such as when a
lost or broken earbud is replaced manufacturers will need to determine a method to
accomplish this
the client also needs to read the coordinated set size characteristic which tells it how many
devices there are in that coordinated set it then proceeds to find the other members of the
set by using the set members discovery procedure defined in csip this involves the client
device looking for other acceptors exposing the rsi ad type connecting and pairing to
them and reading the ir sirk characteristic if the sirk has the same value as the first
coordinated device then it is a member of the same set if it is different the client device
should discard the pairing and look for the next device with an rsi ad type and check its
sirk characteri stic value the set members discovery process ends when all of the set
members are found the process reaches an implementation specific timeout usually around
ten seconds or it is terminated by the application if the initiator or comma nder cant find
them all they can proceed with those they have found but should continue to try to find the
missing members
if you are shipping acceptors which are part of a coordinated set then cap demands that
all four of the characteristics defined in csis are implemented in each device these are
shown in table
characteristic name mandatory properties optional properties
set identity resolving key sirk read notify
coordinated set size read notify
set member lock read write notify none
set member rank read notify
table coordinate d set characteristic properties for use with cap
currently no cap procedure uses the set member lock or rank characteristics although it
mandates that they are implemented on the acceptor
the reason for the set member lock is that when a client writes that characteristic the lock
gives that client exclusive access to features on that acceptor which features the lock
controls is defined by the higher layer application implementors should take care in using