Re: [Bluez-devel] An explanation of a2dpd weird behaviour on high resolution timers enabled kernels

[Fabien Chevalier <fabchevalier@free.fr>]

Hi Marcel & Frederic,

> Hi Frederic,
> 
>>> you might wanna focus on the Bluetooth audio service in bluez-utils (the
>>> code got committed today) which now has A2DP support.
>> That's great news !!! I was thinking of this coming out soon, but didn't expect something so early.
> 
> the CSR based headphones seem to work pretty good so far. We have some
> minor issues on the AVDTP protocol level, but they are fixable. The
> current big issue are the Broadcom based headphones. While CSR seems to
> be quite forgiving if you send data too fast (actually I think they use
> flow control in the baseband to slow us down), the Broadcom ones are
> not. We have to send the next SBC frame at the exact time slot.
> Otherwise the audio sounds too fast or frames are skipped. If you have
> any ideas on how we can improve pcm_bluetooth.c (yes, that handling is
> inside the ALSA plugin), I would like to know.

The issue you plotted here is a well known issue. Basically a2dp/avdtp 
profile does not specify how flow control is supposed to be handled, 
which leaves it open to implementation-specific behaviours (play faster, 
drop samples, use l2cap flow control...). :-(
The way handset makers have solved the issue is to throttle the data 
sending, to avoid triggering moonlight specific headset behaviours.
What i would suggest is to do the same thing using usleep() calls inside 
the alsa plugin.

That will trigger another issue which is clock drifting, because the 
headset will supposedly consume data at master clock speed, while we 
will send data at host clock speed. I think for this one the best is to 
calculate and compensate for the drift using OCF_READ_CLOCK_OFFSET hci 
command: however it's just my wild guess ... ;-)

I hope that helps...

Fabien

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