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

Tanu Kaskinen tanuk at iki.fi
Tue May 8 08:17:06 PDT 2012

On Tue, 2012-05-08 at 06:02 -0700, David Henningsson wrote:
> 2012-05-07 20:06, Arun Raghavan skrev:
> > The separate mics may eventually be useful for beamforming and
> > associated processing. Would your approach require changes to alsa-lib
> > again if we wanted to do that? If yes, it might be better to let
> > PulseAudio take care of this.
> The attached patch (untested!) would just add the possibility to do a 
> four-to-two channel mixdown when you open front:%f (for the cards 
> explicitly selected in the patch), so for four-channel beamforming you 
> could still open hw:%f in four channel mode.

That would still require special profiles in Pulseaudio...

So, we have three options on the table:

1) Have a generic 4-channel input mapping in default.conf.
   - Provides access to all four channels on all known and most unknown
   - Causes some unnecessary delay in startup on most systems.
   - May expose bugs in the remapping code.
   - Patch exists.

2) Have udev-triggered device-specific mappings.
   - Provides access to all four channels on the devices for which a
udev-rule is written.
   - Requires patching for each individual device.
   - Depending on the configuration file content, may expose bugs in the
remapping code.
   - No patches exist. I know how to make them.

3) Downmix to stereo in alsa.
   - Provides access only to downmixed audio.
   - Requires patching for each individual device.
   - No worries about remapping bugs.
   - An untested patch exists for one device. I don't know how to make
these patches.

The downmixing option doesn't sound very nice with iO4. Owners of that
device probably want to access all channels individually. I don't really
see much benefit in option 3 over option 2. The worry about the
remapping bugs is not very relevant, in my opinion.

I still vote for option 1, but I accept any of the options. I probably
won't write the patches, though, if we go with option 2 or 3.


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