[alsa-devel] Recording devices opened mono still sending stereo causing corrupt audio

Clemens Ladisch clemens at ladisch.de
Fri Oct 11 14:33:33 CEST 2013

Sean M. Pappalardo - D.J. Pegasus wrote:
> the problem occurs when PA is asked to open just one channel of
> a sound input device yet the device seems to stream two anyway.

Many devices do not support mono streams.

> I'm suspecting there's a miscommunication between PA and ALSA

The communication between ALSA and PA works just fine; ALSA reports the
minimum number of channels, and PA saves this value.

What does not work is the communication between PA and the application;
PA has no mechanism to report the minimum number of channels.

Even worse, if some application tries to use a channel count that is too
low, PA cleverly replaces it with the minimum support channel count:

  self->numHostChannels = PA_MAX( params->channelCount, StreamDirection_In == streamDir ? devInfo->minInputChannels
          : devInfo->minOutputChannels );

and then does not bother to tell anybody about this.

This bug is especially egregious because PA does later check whether the
number of channels is actually supported by the device; the code above
ensures that this checks uses the wrong channel number.


More information about the Alsa-devel mailing list