[alsa-devel] Choppy audio with non-blocking writes

[Fernando Rodriguez]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA256

Hello,

I'm using non-blocking writes to play an audio stream. When snd_pcm_writei()
return -EGAIN I put the thread to sleep for the time it would take to play 
the packet that I'm trying to write and the packet size it's capped at 1/4 the 
ALSA buffer so that should ensure that the buffer never gets more than half 
empty. And it works as I don't get underruns. But sometimes the audio gets 
choppy and it sounds like it's playing out of time. Like periodically for a 
fraction of a second it plays a bit of audio that should've been played about 
a second ago.

Triggering an underrun or a call to snd_pcm_drain(), either by pausing the stream
or overloading the CPU corrects the problem so I think the internal ALSA ring buffer
pointers are getting out of sync somehow. Also it only happens with non-blocking 
writes. If I use blocking writes it works fine but there are other problems with that.
So I think either  I'm handling -EAGAIN incorrectly, or not configuring it properly?

You can look at the code here:
https://bitbucket.org/frodzdev/mediabox/src/84dced5b84eba4e6e639615dd3d30fccfb0c805b/src/audio.c?at=staging&fileviewer=file-view-default#audio.c-503

Any help will be appreciated.

- -- 

Fernando Rodriguez
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v2
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=RfWT
-----END PGP SIGNATURE-----