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

David Henningsson david.henningsson at canonical.com
Wed Feb 29 10:21:35 CET 2012

On 02/28/2012 08:42 PM, Takashi Iwai wrote:
> 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...

Avoiding summing only for known bad devices and only when mixer is set 
to capture Internal Mic, is a quite complex condition that would have to 
implemented in not only PulseAudio, but every application using ALSA 
directly. (Well, and wants to either sum, or to avoid loss of bass and 
strange stereo effects.)

The lower volume problem is also an argument only if you want to sum the 
signal; so in this case it's lower volume against a cancelled signal 
altogether, in which case lower volume is better.

That leaves the lack of right signal in stereo streams, as a 
disadvantage with the proposed solution. In which use cases do you think 
this is a problem?

David Henningsson, Canonical Ltd.

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