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broadcast audio source enters the configured state at this stage it is not yet transmitting |
any audio data |
updating a broadcast audio stream |
at any point a broadcast source can update the ltv structures in the base containing the |
metadata this is normally done to reflect changes in the audio channel content such as new |
programinfo or context type s this is defined in the bap modifying broadcast source s |
procedure bap the broadcaster does not need to stop the audio stream but can |
make the change dynamically when in the configured or streaming states |
there is no guarantee that an acceptor which is receiving the audio stream will see such a |
change once an acceptor has synchronised to a stream using the information in the biginfo |
it has no further need to track the periodic advertisement s or read the base unless required |
to do so by another profile |
establishing a broadcast audio stream |
once the extended and periodic advertising is running the broadcast source needs to |
establish its audio streams and start transmitting it does this in three steps defined by the |
bap broadcast audio stream establishment procedure bap |
first it enters the broadcast iso chronous broadcasting mode core vol part c sect |
which prepares it to send pdus in the bises of its big it then starts the broadcast |
isochronous synchroni zability mode core vol part c sect which starts sending the |
biginfo in the acad fields of the periodic advertisement alerting any devices that are |
scanning for broadcast audio stream to its presence |
this should not be confused with the base its unfortuna te it has the same initials |
chapter setting up and using broadcast audio streams |
finally the broadcast source needs to set up the audio data path in the same way as for unicast |
using the le setup iso data path hci command core vol part e sect |
at this point the broadcast source is fully operational transmitting its big and constituent |
bises if it has no connections it will never know whether any device detects or synchronises |
to those broadcasts |
stopping a broadcast audio stream |
to stop broadcasting and return to the configured state an initiator needs to use the |
broadcast isochronous terminate procedure core vol part c sect this stops the |
transmission of bis pdus and the biginfo synchronised acceptors will receive a |
bigterminateind pdu to alert them to the end of transmission if they fail to |
receive this they get no further informati on other than the fact that both the bis and biginfo |
have disappeared |
at the end of the broadcast isochronous terminate procedure the broadcast source will |
return to the configured state it can then be released or re enabled |
releasing a broadcast aud io stream |
to return a broadcast source to its idle state it needs to exit the periodic advertising mode |
which returns it to the idle state |
receiving broadcast audio streams |
if a broadcast sink wants to receive a broadcast audio stream it needs to scan to find it the |
same procedures for finding it are used whether this is being done by a broadcast sink itself |
or a broadcast assistant well just look at the broadcast sink doing it in this section then |
see how the task can be delegated when a broadcast assistant is available |
because acceptors act as separate entities cap doesnt really come into play for the reception |
of broadcast audio streams cap defines a broadcast audio reception start procedure cap |
but unless the acceptors have a way of communicating with each other they dont |
know that the other is present they know theyre a member of a coordinated set but not |
whether the other set members are arou nd a commander can do that coordination task |
which well come to later or there could be an out of band mechanism such as a sub ghz |
radio which allows the acceptors to talk to each other but an acceptor scanning on its own |
is essentially operating a t the bap level |
audio announcement discovery |
the first step in receiving a broadcast stream is to find it which is performed using the |
observation procedure from the core core vol part c sect this is a standard sca n |
to find advertisement s here the ones of interest are extended advertisement s which contain |
the broadcast audio announcement service uuid with a broadcastid the extended |
section receiving broadcast audio streams |
advertisement s may also include ot her service uuids such as those defined in the public |
broadcast profile which the scanner can use to filter the streams which it wants to investigate |
having found an appropriate extended advertisement the scanner will look for the syncinfo |
data and use this to synchronise to the periodic advertisement s associated with the |
broadcastid once synchronised the scanner can find and parse the data in the base |
which provides information about the set of bises |
at this point the acceptor is acting quite differently from previous bluetooth peripherals |
instead of being told what to do by a central device it is collecting information about the |
available broadcast source s within range which have audio st reams of interest and decides |
for itself which one to receive this is entirely driven by the implementation it may look for |
a known broadcastid which it has connected to before or information in the base |
metadata it could connect to the first stre am it finds and then step through any other streams |
which are available or it could collect data from every broadcast source within range and |
present this to the user in some form to allow them to make a manual choice all of that is |
implementation spec ific |
the acceptor can use additional information to inform its choice it would not normally |
bother with streams if their context type did not match any of the acceptors available |
context type s nor would it want to receive a stream for a n audio locatio n that i t does not |
support but the decision is the acceptors without a commander or a proprietary link |
between two acceptors a user might have to choose a stream manually on both their left and |
right earbuds it is unlikely that this will ever be t he case as its such a bad user experience |
manufacturers will either provide a separate commander or a commander application or |
implement a proprietary link between left and right earbuds however designers should be |
aware of the need to allow pairs of devices to work together when they do not have a bluetooth |
link between them but thats where the remote control commander comes in |
synchronising to a broadcast audio stream |
once it has decided on a broadcast source to receive an acceptor starts the broadcast audio |
stream synchronization procedure bap this involves it reading the base information |
from the auxsyncind and any supplementary auxchain ind pdus to |
determine the codec configuration and the index number o f the bis which corresponds to the |
acceptors audio location the bisindex for each bis defines the order of the bises within |
the big knowing this the acceptor can determine which bis es in the big it wants to |
receive |
the acceptor then retrieves the biginfo which contains all of the information of the big |
structure the hopping sequence and where the bis anchor points are located once it has |
this it can invoke the broadcast isochronous synchronization establishment procedure from |
the core vol part c sect to acquire the appropriate bis if it has not already done |
so it needs to set up its audio data path having received the incoming audio packets it then |
chapter setting up and using broadcast audio streams |
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