[alsa-devel] [RFC PATCH] Inverted internal mic

[Takashi Iwai]

At Wed, 20 Jun 2012 22:53:08 +0100,
James Courtier-Dutton wrote:
> 
> On 28 February 2012 08:57, David Henningsson
> <david.henningsson at canonical.com> wrote:
> > Hi,
> >
> > One of the more common problems on laptop machines, is that the internal mic
> > provides a stereo signal but with one channel phase inverted, or
> > differential output.
> >
> > This means problems for applications recording two channels but later
> > merging them into one, leaving them with zero or near-zero output.
> >
> 
> If the mic capture looks phase inverted, this might actually be a way
> to reduce common mode noise on the pickup.
> It might be a differential input for improved dynamic range.
> In which case, just dropping one of the channels is not the best,
> quality wise, option.
> To get the highest quality output, it might be best to do a
> differential calculation.
> I.e. Channel A - Channel B.

Yes.  And this is just like B is phase inverted in the end.
(So usually divided by 2, too).

The bigger problem is that the change of input sources may happen on
the fly during recording.  And, usually such a PDM source is only the
digital built-in mic.  If you switch to another source, "A - B"
results again in a wrong output.


Takashi