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

David Henningsson david.henningsson at canonical.com
Tue Feb 28 15:19:16 CET 2012

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 

>> 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.

David Henningsson, Canonical Ltd.

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