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

[Takashi Iwai]

At Tue, 28 Feb 2012 19:11:15 +0100,
David Henningsson wrote:
> 
> On 02/28/2012 04:20 PM, Takashi Iwai wrote:
> >> I'm talking about recording an internal mic in *stereo*, as I just wrote
> >> below. Or don't you agree that is a valid and probably fairly common use
> >> case?
> >
> > Well, when you record it in stereo, and play it back, then you hear
> > the sound without problem.
> 
> That could definitely be questioned: depending on the distance between 
> speakers when you're finally playing it back, you might lose bass 
> frequencies [1]. (That said, I'm not sure how much bass these mics pick 
> up anyway.)

Well, it might be, in the worst case.

> > The problem happens only when you sum the
> > left and right signals into mono.  Thus, as long as the stream is
> > handled as stereo, it could be passed as is, although it's not
> > optimal.
> 
> So the official recommendation is that summing left and right to make a 
> mono signal, is to be considered an invalid operation?

It's not invalid in general but invalid for this digital mic.  That's
the only point.  Thus, avoiding summing only for known bad devices is
also a way to go, IMO.  It'd work more or less stably.
OTOH, muting the right reduces the risk but it also has a problem of
the lower volume and the lack of right signal in stereo streams, both
of which aren't easily avoided.

So we need to find some point of compromise...


Takashi