[pulseaudio-discuss] About extra A2DP codecs support in bluetooth module

[Arun Raghavan]

On Fri, 14 Jul 2017, at 06:50 AM, Qu Wenruo wrote:
> 
> 
> On 2017年07月12日 17:57, Arun Raghavan wrote:
> > On Fri, 7 Jul 2017, at 08:35 PM, Tanu Kaskinen wrote:
> >> On Fri, 2017-07-07 at 20:33 +0800, Qu Wenruo wrote:
> >>> After a quick glance into the code (without much knowledge about
> >>> pulseaudio), I found that pulseaudio is just using sbc library to do the
> >>> encode.
> >>>
> >>>   From a2dp_process_render():
> >>> ---
> >>>       while (PA_LIKELY(to_encode > 0 && to_write > 0)) {
> >>>           ssize_t written;
> >>>           ssize_t encoded;
> >>>
> >>>           encoded = sbc_encode(&sbc_info->sbc,
> >>>                                p, to_encode,
> >>>                                d, to_write,
> >>>                                &written);
> >>> ---
> >>>
> >>> So there is really nothing blocking us to implement other codec.
> >>> For AAC codec, just (well, without tons of preparation and setup) call
> >>> faacEncEncode() will be the core part.
> >>> Copyright sh*t will only restrict the related library, not the PA module.
> >>> (So if we could create a aptX codec library, then it will be possible to
> >>> support)
> >>>
> >>> While the really hard part would be the preparation part, including
> >>> creating a structure for faac encoder to contain a faacEncHandle and
> >>> other needed info from sample rate to profile, just like sbc_info_t.
> >>>
> >>> Although I have a basic idea of what to do, I'm still figuring out how
> >>> to handle all the details.
> >>> Like how to create an endpoint for AAC codec (codec 0 is registered at
> >>> register_endpoint, but shouldn't it be A2DP_CODEC_SBC instead of
> >>> intermediate number 0?)
> >>
> >> I don't know bluetooth details enough to answer. I'll add Luiz to Cc in
> >> case he knows. You could try asking on the bluez mailing list too.
> > 
> > I would suggest just hiding away the entire RTP payloading and encoding
> > using GStreamer here. That neatly sidesteps the issue of
> > hardware/software codecs, IP-sensitive codec libraries, and so forth. We
> > probably want to keep the SBC path available the way it is right now to
> > avoid GStreamer as a hard-dep, but otherwise, I think that's the more
> > sensible approach to this.
> 
> I'm still fighting against dbus and bluez5 things, so GStreamer is not 
> my primary goal.
> But the idea to let a framework to handle everything indeed looks neat 
> and clean.
> 
> However I'm more concerned about how the final A2DP profile is
> determined.
> 
>  From what I can see, pulseaudio bluetooth modules just register 
> endpoint for bluez, with Codec, UUID and codec specified capabilities.
> However I didn't see how codec selection is done.
> 
> Is it done by bluez5 or pulseaudio?
> 
> My currect understanding to implement a new codec will need:
> 1) Register a new codec in register_endpoint() of bluez5-util.c of PA.
>     Instead of codec 0 (shouldn't it be A2DP_CODEC_SBC?), we must also 
> register codec
>     A2DP_CODEC_MPEG24 with a2dp_aac_t as capability.
> 
>    Well, updated a2dp-codec.h header will be needed, as there is no 
> a2dp_aac_t in PA.

Correct.

> 2) Handle codec capabilities negotiation
>     endpoint_set_configuration() seems to be responsible to send out the 
> final
>     SetConfiguration AVDTP packet.
>     But which codec will bluez5 choose? Just highest codec number of 
> both SINK and SOURCE
>     device?
> 
>     endpoint_select_configuration() seems to be related to handle the 
> response from the SINK
>     device.

Right again.

Maybe get this working, and then we can worry about how to select
codecs. It'll probably have to be based on some module configuration
(which in turn will be guided by the h/w that this is running on).
Default could be something we select based on order of quality.

>     But for multi-codec support, how do we distinguish one codec 
> capability from another?
> 
> 3) Record final codec profile into some structure of userdata in 
> module-bluez5-devices.c
> 
> 4) Call codec encode function in thread_func()
> 
> Any comment is welcomed.

With GStreamer, either you'd probably do something like appsrc !
<encoder> ! <rtp payloader> ! appsink to run the encode, in
a2dp_process_render().

-- Arun