[pulseaudio-discuss] rewind and underrun issues on start of playback

[Tanu Kaskinen]

On Sat, 2011-04-30 at 15:38 -0700, Baek Chang wrote:
> Hi,
> 
> I'm seeing some issue with underruns/rewinds occurring on the beginning of
> every sink input playback.
> I see rewind requests on alsa sink of 9600 bytes.  The alsa driver is
> configured with the following buffer sizes
> 
> I: sink.c:     device.buffering.buffer_size = "9600"
> I: sink.c:     device.buffering.fragment_size = "4800"
> 
> So it seems like one buffer size is always being rewinded on the beginning
> of playback.
> Is there a way to prevent these rewinds/underruns when starting playback?

Not to my knowledge, except by changing the code.

>  after the stream has started, there is no issue with audio dropouts or
> underruns, just on the beginning.
> 
> If i log the data that gets sent to alsa, from pulseaudio or in the alsa
> driver, i do see the beginning being dropped as well.
> 
> please see attached logs using both tshed=0 and tsched=1.
> 
> any help with this?
> thanks!

Are there any real problems with this rewinding, like the beginning of
the stream disappearing, or an audible drop-out in the audio? The sink
buffer has to be always rewound when a new stream is created, because
initially the sink buffer contains silence. That silence has to be
overwritten with the stream data, so that there's no unnecessary latency
due to waiting for the silence to be played.

That said, the underrun seems strange, because it happens after the
"Requesting rewind due to end of underrun" message. I don't know the
code well enough to be sure that it indicates any error, though. If the
messages would be ordered the other way around, then it would make more
sense: first when the stream is created, the buffer doesn't have any
data, but when the prebuffering phase ends, then a rewind is requested
on the sink to allow the stream playback to start immediately.

I don't know if this was helpful at all...

-- 
Tanu