[alsa-devel] a11y: howto reduce latency with alsa's dmix plugin?

Halim Sahin halim.sahin at freenet.de
Tue Sep 1 12:32:27 CEST 2009

On Mo, Aug 31 2009, Takashi Iwai wrote:

> At Sun, 23 Aug 2009 09:55:18 +0000 (UTC),
> Halim Sahin wrote:
>> Hello,
>> Is there a way to reduce latency on audiocard if the dmix plugin is in
>> use?
>> I am asking because there is a strange problem with asistive
>> technologies which are outputing through alsa.
>> E. G. speech-dispatcher takes almost a half sec to stop the
>> speech-synthesizer.
>> If the user navigates quickly through menus, there are simultane outputs
>> at a time.
>> (tested with an usb logitech headset, snd-intel8x0 based card,
>> snd-hda-intel).
>> The same problem can be reproduced by running mplayer with alsa output
>> driver and pressing
>> quickly left/right arrow.
>> You should hear several samples at a time.
>> Using an sblive card which doesn't use dmix doesn't have such problems.
>> Using the dmix plugin should audio much faster to avoid this problem.
> It's a bit hard to guess.  This could be the difference of PCM drain
> implementation between hw and dmix, but a half second is still too
> long.
> Could you prepare a small test case to reproduce the buggy behavior?

Yes that's easy:
The following should be tested with active dmix.
1. login to a textconsole
2. mplayer -ao alsa your_favourite_file
3. Press space during playback.

Result: it takes about an half sec to stop the playback.

The second test would be:
1. login to a console
2. mplayer -ao alsa:device=hw=0,0 your_file
3. press space during playback
The playback stops without delay

Hope you can help.
Thanks for your response
Kind regards

> thanks,
> Takashi

Halim Sahin
halim.sahin (at) t-online.de

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