2010/12/8 Takashi Iwai <tiwai@xxxxxxx> > 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. > > > > > > > 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". > > > 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 1 channel so you need PA to provide downmixing option for recording mono from stereo mic 1) (Left+Right)/2 2) Left 3) Right _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
