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

David Henningsson david.henningsson at canonical.com
Wed Feb 29 11:45:40 CET 2012

On 02/29/2012 10:56 AM, Takashi Iwai wrote:
> At Wed, 29 Feb 2012 10:21:35 +0100,
> David Henningsson wrote:
>> 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.)
> As mentioned, ALSA-native "default" doesn't sum for mono signals.
> It's not optimal for stereo, yeah, but better than summing blindly.
>> 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.
> Of course.  But my comparison is "pick up only left" vs "sum but
> right-mute".  In the latter case, the lower volume happens also in
> stereo streams (as a total volume), too.
>> 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?
> Honestly, I don't know.  It sounds really like a user's preference to
> me.


> BTW, it'd be possible to give some offsets to the internal mic capture
> volume to compensate the lack of a stream.

Hmm...could you elaborate on this? What type of offsets are you 
referring to?

> Or, to make this behavior
> selective via a mixer control.  In either way (especially the latter)
> will make the code more complex.  So the question still remains: how
> much compromise.

David Henningsson, Canonical Ltd.

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