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procedures in cap allow an acceptor to receive concurren t unicast and broadcast audio
streams from the initiator with the intention that the handover could be seamless with no
noticeable break in the stream in most cases that will not be the case as an acceptor will
not have the resources to receive both t ypes of streams at the same time especially if they are
using one of the higher sampli ng rate qos configurations to provide a sm ooth listening
experi ence i mplementations should attempt to conceal any breaks in transmission for the
original acceptor although that may be accomplished by an audible tone or message friends
joining the broadcast audio stream will not experience any break as they would not have been
receiving the original audio stream
the procedures cap and also ma ndate that any streaming context type s
and ccidlist used for the original stream are carried over to the reconfigured stream
although the user of the original stream will have transitioned it from unicast to broadcast
they would still maintain the acl connection with their audio source and expect any control
functionality to continue such as fast forward or volume control hence the need to maintain
chapter setting up and using broadcast aud io streams
the ccid association others who receive the broadcast will only have access to the audio
presentatio n delay setting value s for broadcast
with unicast acceptors let the initiator know about their ability to support presentation delay
by exposing their maximum and minimum presentation delay capabilities as well as their
preferred value range in broadcast there is no information transfer between initiator and
acceptor this means that the value which an initiator sets may not be supported by all of
the acceptors which decide to receive it
bap requires that all broadca st receivers must support a value of ms in their range so this
is probably a default value which many initiators will use however it starts to introduce
latency which m ay be excessive for live audio
tmap and hap place tighter constraints on the val ue of presentation delay mandating that
an acceptor must support ms as most acceptors are expected to be qualified to tmap
or hap most will probably support both setting ms for a broadcast source seems a
reasonable compromise however a bap onl y compliant device may not support this
this raises the question of what an acceptor should do if it does not support the value of
presentation delay exposed by a broadcast source the specification is silent on this but the
following guideline offer a pragmatic approach
if the presentation delay is lower than an acceptor can accommodate it should use
a value of ms which all devices must support
the specification give s a broadcast source three octets to carry the presentation delay value
that gets over the limitation of ms which would be the maximum if only two octets were
used the units a re µsec but it means that the value could be as high as seconds most
acceptors will h ave limited memory for buffering the audio stream so there may be occasions
where an inappropriately high va lue of presentation delay is outside their capability whilst
they could decide not to render the audio stream the pragmatic approach is to rever t to the
bap value of ms
in both of these cases this will ensure that both left and right devices render at the same time
if they are able to communicate with each other they may use a proprietary method to
determine the most appropriate value which will generally select the lowest common value
they support
section presentation delay setting values for broadcast
chapter telephony and media control
chapter telephony and media control
after the complexity of setting up unicast and broadcast streams the four specifications
covering telephony and media control are refreshingly simple albei t comprehensive when
bluetooth technology was first being developed most of our mobile products were fairly
straightforward and just did one thing mobile phones made phone calls using the cellular
network s and music players played music based on whate ver physical media they supported
typically pre recorded cassettes or cds since then both telephony and media by which we
mean music and any other audio which we can stop and start have become a lot more
complex
for telephony we no longer constrai n our phones to a cellular network the move to voice
over ip voip has seen an explosion in internet based telephony services either as over the
top ott applications on our phones or as programs on our laptops and pcs the
pandemic and the associ ated growth in home working have accelerated their use whilst we
still make cellular calls we also use skype zoom teams and a growing host of other telephony
services sometimes using more than one at the same time
media has also changed as we saw in chapter the growth of bluetooth audio has paralleled
the growth of music streaming services we no longer own most of what we listen to but
borrow it on demand that means that the traditional controls of stop play fast forward
and fast reverse need to be supplemented with new controls which move beyond the physical
device to the source of the audio
the control mechanisms of bluetooth classic audio profiles have struggled to keep up with
this evolution particularly in telephony where they are still based on the old at command
set which was first designed for landline modems back in before being integrated into
the early gsm standards in the s and bluetooth technologys headset and hands free
profiles in the early s the develo pment of bluetooth le audio has given us the
opportunity to go back to first principles for both telephony and media control with universal
state machines which are equally applicable to cellular and voip telephony and all kinds of
local and remote media sources
content control is currently defined using two sets of profiles and services in the case of
telephony they are
tbs the telephone bearer service which defines the state of the device handling
the call and
ccp the call control profile which is the client acting on tbs
section terminology and generic tbs and mcs features
for media the respective pair of specifications are
mcs the media control service defining the state of the source of the media
and
mcp the media control profile which is the client acting on mcp
terminology and generic tbs and mcs features
up until this point ive mostly been using the initiator and acceptor terminology from cap
as the easiest way of describing what happens to an audio stream now that were looking at
control which is orthogonal to the audio streams thats no longer appropriate as the basic
architect ure of bluetooth le audio separates the audio data plane and the control plane it
means that control features can be implemented on devices which dont take part in audio
streams because of that we need to revert to client server terminology in this cha pter where
the server is the instance of the service defining the state of the media player or the phone
for the telephon e bearer and media control services the server is located on the initiator
the client c an be on the acceptor but is equally likely to be in a remote control simply
because thats easier to use particularly for small devices like earbuds and hearing aids weve
got used to the ease of use of remote controls its why they were invented it doesnt need