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LeAudio

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the original cig had never been disabled
as we saw in the cig state machine in chapter once the cig has been enabled the cis
configurations cannot be changed so any change to the codec configuration qos parameters
or presentation delay would require the cig to be terminated and the configuration process
restarted an individual cis can be disconnected or restored using the le create cis
command but only their metadata can be updated once they are in the enabling or streaming
state
section configuring an ase and a cig
audio data paths
the isochronous stream s are now up and running but we havent connected any audio data
the isochronous channels are just conveying empty packets the next step if it has not
already been done is to connect a data path for each ase which uses the audio data path
setup procedure bap which in turn uses the le setup iso data path hci
command
bluetooth le audio provides a lot of flexibility for the audio data path and where the codec
is located the codec can reside in the host or in the controller and the audio data can come
through hci or more typically via pcm or a vendor proprietary route
figure common audio data path configurations
figure shows three of the most common data path configurations s howing that the codec
can be implemented in the host or the controller audio is typically routed from the codec
via a pcm interface but if encoded in the host it can also be transported over hci to set
up each data path the initiator and acceptors will both use the le setup iso data path hci
command to bind the codec configurations that were written during the config codec state
to the connection handle of each cis specifying the direction depending on whether it is a
source or sink ase the data path configuration may be different in the initiator and
acceptor as the codec location and data path implementation will normally depend on the
specific chipset being used as the data path setup is internal to each device thats an
implementation choi ce this level of detail is normally hidden from the application developer
below a more general api but its useful to know what happens at each stage of the
configuration process
up to this point the process is common for both sink and source ases now the two
processes diverge theyre still on the same path within the state machines but there is a
difference in the order of commands
chapter setting up unicast audio streams
for a sink ase once the acceptor has successfully set up its data path it may autonomously
move the ase t o the streaming state then notify each sink ase characteristic to the initiator
with the ase state code set to x streaming as soon as it receives this notification point
the initiator can start streaming audio packets to that ase sink
figure establishment of a sink ase showing the ase states
figure shows a very simplified message sequence chart for e stablishing a sink ase
showing the overall sequence and the acceptors autonomous transition to the streaming
state more detailed mscs are provided in bap section
for source ases the acceptor needs to wait for the initiator to c onfirm that it is ready to
receive data from that ase once it has received the ase characteristic notification from the
acceptor and as soon as it is ready to transmit audio data the initiator will write the ase
control point characteristic for that as e with the opcode set to x receive start ready
note that sink ases do not need to be included in the array of ases in this command as
they will independently move to the streaming state at this point the acceptor will
transition the source ase to the streaming state notify its ase characteristic and commence
audio streaming
a corresponding msc for a source ase is shown in figure here the initiator needs to
issue the receiver start ready command to the acceptors ase control point characteristic
to initiate the start of audio data packet transfer
section configuring an ase and a cig
figure establishme nt of a source ase showing the ase states
updating unicast metadata
whilst in the enabling or streaming states an initiator can update the metadata for any ase
by writing to the ase control point with an opcode of x update metadata bap
this is a convenient way of reusing an ase for a different audio applica tion without having
to tear down and re establish any audio streams
this is described in the cap unicast audio update procedure cap and allows an
ase to be reused keeping its current configuration but changing the context type and
ccidlist m etadata a typical application would be to use the same stream to move from a
phone call to a music player on the same phone there are inherent limitations which is that
the codec configuration cannot be changed another issue is that a phone call is normally
bidirectional whereas music stream s are unidirectional however during the configuration
process enough ases could be configured to cope with this at the point of transition of the
use case the initiator could disable and enable ases until it has the number of configured
ases which it needs updating their metadata in the process note that in the case of a
bidirectional cis an ase might be disabled but the cis would remain enabled to support
the other direction and its ase it is a good example of how the bluetooth classic audio
multi profile issues have been designed out by adding flexibility for audio streams to be
repurposed and reused
chapter setting up unicast audio streams
ending a unicast stream
the process of ending a unicast stream is where the sink and source ase state machines
diverge well cover the source ase in the next section the difference is that a source ase
has a disabling state whereas the sink ase does not
to stop streaming to a sink ase the ase needs to transition back to the qos configured
state using the cap unicast audio stop procedure cap this calls the bap
disabling an ase procedure bap where the initiator writes to an ase with the x
disable command at thi s point the initiator stops transmitting audio data to the ase if
the cis is bidirectional and the source ase has not been disabled the initiator will continue
to transmit null pdus to allow the acceptor to retur n its audio data
the associated cis is not automatically disabled at this point if the initiator wishes to remove
it it should use the hcidisconnect command core vol part e as disabling the
sink ase only moves it to the qos configured state the ase can b e reestablished at a later
point by writing the enable opcode to the ase control point characteristic if the cis has
been disabled using the hci disconnect it can still be restored using the hci le create cis
command
if there is no intention of reusi ng the ase it can be moved to the releasing state by
performing the bap releasing an ase procedure bap once the ase is in the
releasing state the acceptor autonomously transitions it to the idle state or if it wants to
simplify the next ase c onfiguration cycle it can cache the codec configuration by returning
it to the codec configured state