[alsa-devel] [Audacity-devel] distortion with alsa I/O - no distortion with oss emulation

Takashi Iwai tiwai at suse.de
Tue Apr 10 11:51:48 CEST 2007

At Sun, 8 Apr 2007 20:35:32 -0400,
Lee Revell wrote:
> On 4/7/07, Simon Lewis <simon.lewis at mnet-online.de> wrote:
> > Dear Alsa developers
> >
> > Audacity seems to work perfectly with AOSS and Jack but not directly
> > with ALSA on some linux PCs. Trying to use ALSA directly leads to severe
> > distortion due to lack of sync with Alsa. (I hope that is the correct
> > terminology.)
> >
> > I received this reply from the Audacity developers, what tools are
> > available from ALSA to investigate this problem? How can I determine
> > what the real "Alsa configuration" is that portaudio has used setting up
> > communication with Alsa.
> Please try to narrow it down some.  What hardware/kernel/distro
> versions work OK and which don't?  Especially important is the
> motherboard chipset, ALSA version and exact make/model of soundcard.
> My guess is that different soundcards default to different
> period/buffer config values, and that Audacity wrongly assumes the
> buffer/period size it gets from ALSA by default is sane on all
> hardware.

Yes, and note that one of the most complicated things is the use of
dmix/dsnoop plugin.  Many devices are set up to use dmix/dsnoop as
default with ALSA, so when you open the PCM via "default" name, the
dmix/dsnoop will be invoked automatically.  When dmix plugin is used,
many parameters are restricted via the plugin setting rather than the
hardware constraints.

I believe the recent dmix/dsnoop code is stable enough, but it's worth
to try once "hw" PCM instead of "default" PCM to reduce the possible
problems around plugins.


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