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

[Takashi Iwai]

At Wed, 29 Feb 2012 11:45:40 +0100,
David Henningsson wrote:
> 
> 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.
> 
> Ok.
> 
> > 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?

cx5051 has separate capture volumes and I thought of adding a base dB
offset (+6dB) only to digital-mic so that it'll be higher than other
inputs.


Takashi