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right audio locations an initiator would determine the number of streams an acceptor
supports along with the number of speakers available and make a decision of whether to send
a downmixed stereo stream ie mono or a stereo stream that it assumes the speaker could
downmix to mono broadcaster s would denote a mono stream by labelling it as front left
and front right well look further at exactly how a udio locatio ns are used in chapter
but it brings us on to channel allocation
channel allocation multiplexing
you always knew what was being transported in the bluetooth classic audio profiles hfp
carries a mono audio stream and adp uses the four channel modes of sbc codec to transmit
mono dual channel stereo or joint stereo encoded streams bluetooth le audio is a lot more
flexible allowing a cis or bis to contain one or more channels multiplexed in to a single
packet limited only by the available bandwidth
archiveorgstreamcta gc tagdjvutxt
section channel allocation multiplexing
the reason for this approach is that the lc which is the mandatory codec for all bluetooth
le audio implementations is a single channel codec this means that it encode s each audio
channel sep arately into discrete frames of a fixed lengt h with sbc joint stereo coding which
combines both left and right audio channels into a single encoded stream was popular because
it was more efficient than two separate left and right channels that was du e to its ability to
encode differences between the two input audio channels the more efficient design of lc
means that there is very little advantage in joint stereo coding compared to encoding each
channel separately and then concatenatin g the individu al encoded frames this approach
allows more than two channels to be encoded and group ed together
however this meant that a mechanism needed to be introduced to package together multiple
encoded frames for multiple channels which is performed using cha nnel allocation bap
section
figure example of multiplexing multi ple bluetooth le audio channels
figure shows an example of how this works for a five channel surround sound system
five audio input channels are separately encoded using lc and the encoded frames are then
arranged into a media p acket which is transmitted as a single isochronous pdu the lc
codec frames are always arranged in ascending order of the published audio capability audio
location associated with each audio channel using the assigned numbers which were
summarised in table so in this case they will be ordered as front left x
front right x front centre x back left x and
back right x
the media packet contains only encoded audio frames it can be expanded to include multiple
blocks of encoded audio frames each of which include one frame for each of the audio
locations as shown in figure cfn refers to frame s from the first sample of each of
the incoming audio channels cfn to those from the next samplings of those audio
channels
chapter new concepts in bluetooth le audio
figure media packet containing two blocks of five audio channels
using blocks may look efficient but it comes with some important caveats it results in larger
packets whic h are more vulnerable to interference it also increases the latency as the
controller needs to wait for multiple frames to be sampled before it can start transmission if
multiple blocks are added to the multiple isochronous channel features of burst n umber or
pretransmission offset which well come to in the next chapter it very quickly results in
latenc ies that can be hundreds of milliseconds there are some occasions where that may be
useful b ut not in the general audio applications we use today
there is no header information associated with the media packet instead the number of
audio channels that can be supported are specified in the audiochannellocation ltv
which is included in the codec specific capabilities ltv in the pac records with the value
for each isochronous stream being set by the initiator during the stream configuration
process the number of blocks being used is configured at the same time using the
codecframeblockspersdu ltv structure
well look at these in more detai l in chapter when we discuss the lc and q uality of service
call control id ccid
a consequence of separating the control and data planes in bluetooth le audio is that there
is no longer any direct relationship betw een a content control signal such as phone control or
media control and the audio stream that adds flexibility to bluetooth le audio but results
in a little more complexity an initiator might have multiple applications that are running
concurrently where a user may want to associate control with a specific one of those
applications think of the case of two separate telephony calls such as a cellular call and a
concurrent teams meeting where the user may want to put one call on hold or terminate
one whilst retaining the other to cope with this situation the content control id has been
introduced which associates a content control service instance with a specific unicast or
broadcast stream
the statement that ccids can be used for broadcast might seem contradictory but highlights
the fact that although broadcast streams dont need an acl connection there are many
applications where one might be present for example if you transition from using unicast to
listen to a music stream on your p hone to broadcast so that you can share it with your friends
you would still expect to be able to control the media player in this case you would keep the
acl link alive and associate the media controls with the broadcast stream
section coordinated sets
the content control id characteristic is defined in section of the gatt specification
supplement as having a value that uniquely identifies an instance of a service that either
controls or provides status information on an audio related feature it is a single octet integer
which provides a unique identifier across all instances of content control se rvices on a device
when an audio stream contains content which is controlled by a content control service it
includes the ccid in a list of such services in the audio streams metadata to tell both
acceptors and commanders where they can find the correct service well look at
commanders in a moment in section
ccids are currently only used for the telephone bearer service and the media control
service they d o not apply to rendering or capture control
coordinated set s
despite the fact that weve only had them for a few years w ere already so familiar with the
concept of tws earbuds that most people forget that the bluetooth classic audio
specifications dont cover the way they work as explained in chapter they all rely on
proprietary extensions from silicon chip companies the design of isochronous channels
rectifies that allowing an initiator to send separate audio streams to multiple acceptors along
with a common reference point at which all acceptors know they have received the audio data
and can start decoding it however the flexibility of bluetooth le audio including the new
use cases coming from broadcast raised the need for a way to link acceptors together as sets
of devices which can be treated as a single entity
the concept of a coordinated set addresses that need it allows devices to expose the fact
that they are part of a group of devices which tog ether support a common use case and should