[alsa-devel] snd-hda-intel+dmix: Disabling power save doesn't work

Takashi Iwai tiwai at suse.de
Tue May 8 12:31:35 CEST 2012

At Tue, 08 May 2012 20:19:56 +1000,
Adam Nielsen wrote:
> >> This works as expected when stopping playback, but if I only *pause* playback
> >> then the signal drops almost exactly one second later, no matter what
> >> "powersave" is set to.  So of course then resuming playback means I lose a
> >> couple of seconds of audio while the amplifier re-syncs to the SPDIF signal.
> >
> > Check the powersave option value when this happens.  The system such as
> > power-utils script may change the parameter by itself.
> Thanks for the quick reply!  I checked this and there was no change.  Before, 
> during and after playback, the module options haven't changed.
> >> It doesn't happen if I use hw:0,1 as the output device (the SPDIF signal keeps
> >> going, even when paused) but as soon as I change the output device to dmix
> >> then the signal drops whenever the audio is paused.
> >
> > It's weird becauase the pause function isn't implemented in dmix at
> > all.  Thus it's application's own behavior how to pause the stream.
> > Usually the application stops the stream like a normal stop then
> > restarts from the position manually.
> Upon more experimentation I have noticed that the SPDIF signal also drops out 
> during silent sections of audio.  If there's a 100% quiet part of a film or 
> movie I'm watching with mplayer or another program, during the silent sections 
> the signal drops out and I again miss a few seconds when they start speaking 
> again.
> I now realise this has been happening for a long time, as I created a .wav 
> file containing 60 seconds of very quiet white noise a few years ago.  I play 
> this in a loop in the background whenever I want to prevent the powersave from 
> kicking in in the silent parts when I'm watching a movie.
> It seems that this happens regardless of the output device - hw:0,1 or dmix 
> both drop out during silent sections.  I have double-checked during pausing 
> though, and as before hw:0,1 keeps going during a pause while dmix drops out.
> Is dmix closing its output PCM stream when all input streams are paused or 
> silent?  I'm not sure what else would cause this behaviour.

Well, maybe the difference is the support of PAUSE operation.
The hardware driver itself supports the pause operation.  Thus, if you
access via hw:0,1, the app just uses the hardware pause feature.  In
this case, the device is still opened, thus no power-saving is done.

OTOH, dmix provides no pause feature, thus the app stops (and often
closes) the stream once.  Thus the pause is not pause in this case.


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