[alsa-devel] Help wanted: emachines em350 internal mic
superquad.vortex2 at gmail.com
Wed Dec 8 05:13:06 CET 2010
2010/12/8 Takashi Iwai <tiwai at suse.de>
> At Tue, 07 Dec 2010 16:11:59 +1300,
> Eliot Blennerhassett wrote:
> > 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.
> > >
> > >
> > 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".
In theory , this should be (L+R)/2 for those stereo mic without phase
inversion to prevent clipping occur when sent to a mono output
> > So I'm back to wondering how to force an app (primarily PulseAudio) to
> > see the mic as mono?
> Via pulse configuration?
PA server use SND_PCM_NO_AUTO_CHANNELS , any plugin which defined in the
device string in the profile has no effect if your hardware does not support
so you need PA to provide downmixing option for recording mono from stereo
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