[pulseaudio-discuss] Bluetooth A2DP AAC passthrough?

[Tanu Kaskinen]

On Wed, 2016-04-27 at 16:53 +0200, Nicole Færber wrote:
> Hello,
> 
> I know this has been a topic several times now. I searched the
> mailinglist archives, FAQ, current GIT and other internet resources for
> a practical way with current versions of BlueZ5 and Pulse Audio but it
> seems that most proposed patches have been dropped, am I correct with
> this assessment?
> 
> My personal goal would be to have a mode to playback AAC content to a
> paired and connected A2DP device capable of A2DP-AAC - in my case a
> Parrot Zik2.0. If such a way already exists I would be really happy for
> an advice on how to actually use it, e.g. using paplay?
> 
> If such a mode does not yet exist are there plans to implement other
> codecs? At least as pass-through? What is needed? Is it already being
> worked on? Can I give a hand? As usual time is limited and my knowledge
> on A2DP and especially Pulse Audio is limited but I am willing to help.

Compressed audio passthrough with bluetooth is not supported. I think
the feature would be welcome, though. I'm not aware of anyone working
on it.

We already support compressed passthrough with alsa, so it's not
necessary to start from absolute scratch. Alsa wants compressed audio
wrapped in "IEC 61937" encapsulation, and that format also makes it
easier for pulseaudio to deal with the data, because the encapsulation
makes it possible to convert between number of bytes and time (that is,
a certain number of bytes always corresponds to the same amount of
time, which isn't true with plain AAC audio). For that reason we
require applications to do the encapsulation, so pulseaudio doesn't
have to do any changes to the data.

I suppose bluetooth doesn't use the IEC 61937 encapsulation, so the
question arises if we should use it in pulseaudio when doing
passthrough with bluetooth. I think it would make sense to use
encapsulation at least in the initial implementation, because that
would then retain the nice property that we don't have to understand
anything about the AAC format as such. The bluetooth module would then
have to do the unpacking of the AAC data from the IEC 61937
encapsulation, which hopefully is reasonably straightforward.

-- 
Tanu