[pulseaudio-discuss] [PATCH v2] alsa: Add support for sound cards with 4-channel input.

Tanu Kaskinen tanuk at iki.fi
Tue May 8 07:42:41 PDT 2012

On Tue, 2012-05-08 at 06:07 -0700, David Henningsson wrote:
> 2012-05-07 19:09, Tanu Kaskinen skrev:
> > On Mon, 2012-05-07 at 13:02 -0700, David Henningsson wrote:
> >> Besides, it is not obvious to me what the
> >> channel mapping should be either, and couldn't that vary between
> >> different 4-channel hardware? Is the aux* just causing all inputs to be
> >> added for whatever the client wants to have...?
> > What do you mean by "added"? Mixed together? No, remixing is disabled
> > with aux channels (not sure about mono downmix, though).
> Hmm, so if the source output client requests a stereo stream (left, 
> right), and all we got is aux0, aux1, aux2 and aux3, what does the 
> client get in his stream? Silence?

I don't know. Possibly. But if that's the case, then the remapping code
should be fixed so that when connecting a stereo capture stream to an
all-aux source, the first two channels will be used.

> > Not having any
> > remixing may perhaps cause other problems, but if that's the case, it's
> > then a separate bug. I really think the input channel maps should be
> > all-aux by default (possibly even with stereo mics, binaural equipment
> > being a rare exception), because the channel semantics don't make much
> > sense with most input devices (except with PA's monitor sources).
> I disagree. Stereo mic recording in different types of setups is not 
> that uncommon and should be tagged as such. Line In recording (both 
> analog and digital) is also set up as left and right.

You're right. Especially with line-in the content may often have a
specific channel map (e.g. recording music from another device). I think
defaulting to "left, right" channel map makes sense for stereo input
devices after all. But for more channels I think an all-aux map is still
the best.


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