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

[Takashi Iwai]

At Tue, 28 Feb 2012 15:19:16 +0100,
David Henningsson wrote:
> 
> On 02/28/2012 02:22 PM, Takashi Iwai wrote:
> > At Tue, 28 Feb 2012 14:07:59 +0100,
> > David Henningsson wrote:
> >> Ok. My question was more about the following: When I look at
> >> patch_realtek.c, I can find functions alc271_fixup_dmic and
> >> alc269_fixup_stereo_dmic. I have also seen machines having ALC268 and
> >> ALC272X that have this internal mic behaviour. Is there a way we can
> >> know the corresponding processing coefficients to set for ALC268 and
> >> ALC272X as well?
> >
> > AFAIK, no, it was specific to the codec model.
> 
> Ok, then we can only hope for Kailang to supply this information if 
> possible. And if not possible we could attempt the workaround (when/if 
> we agree on it...) for these devices as well?
> 
> >>> Note that in alsa-lib, the HD-audio "default" is already set up to
> >>> copy left-channel for mono streams.  You can see a line setting
> >>> "route_policy" to "copy" in HDA-Intel.conf.
> >>>
> >>> Thus, when ALSA apps run without PA, it'd work in both stereo and
> >>> mono.
> >>
> >> Assuming the right channel is muted, yes. But not in the current
> >> implementation.
> >
> > It should work no matter whether the right channel is muted or not.
> > The plug layer will use only the left channel when a mono stream is
> > recorded  since route_policy=copy is set.  Remember that it's about
> > "default" PCM, not about "hw" PCM that PA uses.  We don't touch "hw"
> > intentionally because it's really intended to be a raw access.
> 
> 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.  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.

> >> By not making a change in the ALSA layer, it will still be broken for
> >> any ALSA apps who record the Internal Mic as a stereo signal. They will
> >> get a broken result as the right channel will be phase inverted. That's
> >> why I think this is better dealt with in the ALSA layer.
> >> Would a zeroed right channel be less broken than a phase inverted right
> >> channel? I think so.


Takashi