[alsa-devel] [RFC PATCH] Inverted internal mic
tiwai at suse.de
Wed Feb 29 10:56:01 CET 2012
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 . (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
BTW, it'd be possible to give some offsets to the internal mic capture
volume to compensate the lack of a stream. 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
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