[pulseaudio-discuss] Gap, silence and latency

Tanu Kaskinen tanuk at iki.fi
Tue Mar 20 21:31:55 PDT 2012

On Fri, 2012-03-16 at 17:34 +0000, James Lee wrote:
> A/V sync is handled in the decoders based on the current PCR and PTS values for 
> each audio/video frame.  Our concern is audio latency through pulse, not A/V 
> synchronization.  The latency jumps right after the gap.

I'd still say that your problem is A/V sync, not latency, unless the
latency information from Pulseaudio is incorrect. Doing A/V sync
requires you to know how long it will take for the audio to come out of
the speakers and for the video frames to be visible if you write data
now, and make timing decisions based on that. If you do your timing
decisions only based on the PCR and PTS values, it sounds like you
expect to hear the audio immediately when you issue a write command in
your code.

That said, it sounds like the assumption works well enough for you when
you play the first movie. I don't really see a reason why continuing
after a stream underrun should cause any longer delays than starting a
new stream, unless there's prebuffering involved in the underrun case.
If there's prebuffering in the underrun case, I'd expect there to be
similar prebuffering also in the stream creation case, though...

I guess you have checked that the A/V sync is actually still ok when the
previous movie ends, and it only goes wrong when the next one starts? If
you don't resample to compensate the drift between the server clock and
the sound card clock, data can accumulate slowly in the stream buffer.
Then again, stopping writing data during the gap between the movies
should drain any accumulated data and reset the situation...

I guess this was not very helpful in finding a solution. It sounds like
there may be a bug in Pulseaudio or the alsa plugin. You can file a bug
if you wish. A test program that shows that the latency is misbehaving
would be great.

Assuming that Pulseaudio reports the latency correctly, it should be
also possible for you to fix your application to monitor the latency and
adapt to jumps as needed. Also recreating the stream after the gap could
be a simple workaround?


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