[alsa-devel] Help wanted: emachines em350 internal mic

[Raymond Yau]

2010/12/7 Eliot Blennerhassett <linux at 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=59c774ed5ee00e9623a204c3234191d6a6d8cf7a
>
> 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.
>
> 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?
>

The easy way is to mute one of the channel using capture switch while you
recording and the result signal will be L/2 or R/2

Simple mixer control 'Capture',0
  Capabilities: cvolume cswitch penum
  Capture channels: Front Left - Front Right
  Limits: Capture 0 - 31
  Front Left: Capture 31 [100%] [30.00dB] [on]
  Front Right: Capture 31 [100%] [30.00dB] [on]