2010/12/7 Eliot Blennerhassett linux@audioscience.com
Thanks Raymond,
On 03/12/10 15:33, Raymond Yau wrote:
Greetings,
There is only a single mic on this netbook. However, the alsa device shows up as stereo, and the right channel carries an inverted copy of the left channel.
http://git.alsa-project.org/?p=alsa-lib.git;a=commit;h=59c774ed5ee00e9623a20...
Commitlog was "Add route_policy copy to HDA-Intel.conf for capture Since some digital mics have the phase-inversion problem in one channel, adding both channels for mono stream results in the noise. Use route_policy copy to avoid that situation."
As far as I can guess, the commit helps when an application asks to record mono from the stereo device, by copying L rather than summing L+R
My machine already has this, however it doesn't really fix the root of the problem.
https://bugs.launchpad.net/ubuntu/+source/alsa-driver/+bug/639846
The route plugin is only used when the application are alsa 's default pcm/ctl device for hda-intel if you fall back to use dmix/dsnoop
pulseaudio is using front device for capturing (i.e. changing PCM Playback volume affect pulseaudio recording )
Because the internal mic appears as a stereo device, rather than a mono, applications can open it as stereo.
Only later when the resulting signal L+R is sent to a mono output does the signal "disappear".
So I'm back to wondering how to force an app (primarily PulseAudio) to see the mic as mono?
you have to ask PA developer