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LeAudio

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bap the basic audio profile and
cap the common audio profile
cap sit s above the other three and defines procedures which use bap ascs a nd pacs to
configure and manage unicast audio s treams introducing coordination context type s and
the content control id to associate audio streams with use cases most of the time cap is
just a set of rules which tells you the correct order to run bap procedures and how to use
context type s and the content control id with them top level bluetooth le audio
profiles suc h as hap and tmap are built on top of cap mostly extending mandatory
requirements in this chapter ill concentrate on the underlying bap procedures as they do
the heavy lifting but will reference cap and higher layer profiles where necessary
all of the bluetooth le audio specifications are based on the gatt structure of bluetooth
le which has a client server relationship higher layer specifications provide context and
add control to unicast audio streams but bap ascs and pacs are the foundations they
build on
pacs the published audio capabilities service
pacs is the simplest of the specifications and is essentially a statement of what an acceptor
can do acceptors use pacs to exposes these capabilities in two characteristics a sink pac
characteristic if it supports the audio sink role and a source pac characteristic if it supports
the audio source role these contain published audio compatibility records called pac
records which include information about the codec and t he configurations which it supports
along with other optional features between them they expose the full range of capabilities
of a device
pacs can be thought of as a database of the audio capabilities of an acceptor which is
populated at the point of manufacture and is static for the life of the product although it may
be updated through a firmware update pacs specifies information which an initiator obtains
when it starts the process of configuring and establish ing an audio stream pacs can also be
used by a broadcast assistant to allow it to filter the broadcast streams which it detects so
that a broadcast sink is only presented with compati ble audio streams but well cover that in
the broadcast chapter
section pacs the published audio capabilities service
an important concept about the information specified by pacs is that it is a statement of fact
about the total capabilities of an acceptor pacs describes what an acceptor is capable of
doing it is the first step in devices getting to know each other after an initiator has read
these capabilities an acceptor will normally expose a more limited set of preferred capabilities
during the process of establishing a stream a nd the initiator should use those values
however an acceptor should not reject any configuration an initiator requests which is based
on the pacs information it has exposed
the intent is that this should result in an efficient configuration process especially when
multiple acceptors are involved particularly if they come from different manufacturers and
have differing capabilities its an evolution from the classic bluetooth audio profiles which
allow central and peripheral devices to go through n egotiation loops where the two devices
try to ascertain what the optimum settings are for an audio connection in some
implementations that resulted in a deadlock with a connection never being made or a poor
codec configuration which affected the qual ity of the audio its an issue which resulted in the
inclusion of the s and d coding settings as recommendations for cvsd and the t
esco parameter sets for msbc the aim of pacs is to prevent those problems arising
sink pac and source pac chara cteristics
both the sink pac and source pac characteristics contains an array of i pac records
taking the form shown in table
pac record descr iption
number ofpacrecords number of pac records i for this
characteristic
codecid i
octets octet codec defined in the bluetooth
assigned numbers
lc x
vendor specific xnn
octets company id if vendor specific
otherwise x
octets vendor specific codec id
otherwise x
codecspecificcapabilitieslength i
octet length of the codec specific capabilities for the
codec in th e ith pac record
the s safe set parameters for cvsd were introduced in the codec interoperability section
of hfp v in followed by the d and t parameters for the msbc in version in to
help ensure compatibility when using these codecs
chapter setting up unicast audio streams
pac record descr iption
codecspecificcapabilities i
length varies identification of the capabilities of the codec
implementation in the ith pac record
normally expressed as a bitfield
metadata length i
octet length of the metadata associated with th e ith
pac record
x if there is none
metadata i ltv formatted metadata for th e ith pac record
table format of a pac record
codec specific capabilities
every bluetooth le audio specification includes at least one pac record using the lc
codec as this is mandated in bap which is the lowest layer of the generic audio framework
the assigned number for lc is x so every bluetooth le audio acceptor will have at
least one pac record with the codecid of x note that the codecid is the
coding format described in the host controller interface assigned number list and not the
audio codec id d efined in the audiovideo assigned numbers list
the codecspecificcapabilities requirements for lc are defined in bap section and
consist of five ltv structures each ltv has a type code defined in the generic audio
assigned numbers document so that it can be identified by the device reading it
as three of these ltvs are bitfields this means that there are typically multiple combinations
described within the codecspecificcapabilities structure
the supportedsamplingfrequencies ltv
the first of these five ltv structures is the supportedsamplingfrequencies type x
shown in table which is a bitfield of sampling frequencies covering the range from khz
to khz support for each value is indicated by setting the corresponding bit to one this
ltv is mandatory note that this structure is used for any codec lc only specifie s
and khz sampling frequencies shown as lightly shaded