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

[A. C. Censi]

hi

This question of Pulseaudio using front device and not default device
where the quirk in ALSA for the array mics with inverted signals in
each channel works, is somewhat old, but still affects a lot of
netbook users. I know because I have to help several Ubuntu users of
my circle, to use Skype, Gtalk, etc.

I have tried to search if it is already addressed in PA, but not found
(not a very thru search).

Can you look after it?

Thanks

A C Censi

---------- Forwarded message ----------
From: Raymond Yau <superquad.vortex2 at gmail.com>
Date: Tue, Dec 7, 2010 at 1:43 AM
Subject: Re: [alsa-devel] Help wanted: emachines em350 internal mic
To: ALSA Development Mailing List <alsa-devel at alsa-project.org>


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.
>

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
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A. C. Censi
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