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

David Henningsson david.henningsson at canonical.com
Fri Jun 22 17:27:51 CEST 2012

On 06/22/2012 02:46 PM, Takashi Iwai wrote:
>   /*
> + * Inverted digital-mic handling
> + *
> + * First off, it's a bit tricky.  The "Inverted Internal Mic Capture Switch"
> + * gives the additional mute only to the right channel of the digital mic
> + * capture stream.  This is a workaround for avoiding the almost silence
> + * by summing the stereo stereo stream from some (known to be ForteMedia)

Stereo stereo? It's "stereo" in stereo! :-)

> + * digital mic unit.
> + *
> + * The logic is to call alc_inv_dmic_sync() after each action (possibly)
> + * modifying ADC amp.  When the mute flag is set, it mutes the R-channel
> + * without caching so that the cache can still keep the original value.
> + * The cached value is then restored when the flag is set off or any other
> + * than d-mic is used as the current input source.

Ok, I get it now.

There is a slight drawback with this (IMO a bit hacky) patch, and that 
is that you can get a short signal spike on the right channel when you 
turn "Capture Switch" on while "Inverted Capture" is off. Hopefully it 
is not a practical problem.

Thanks for looking at this long-standing problem :-)

David Henningsson, Canonical Ltd.

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