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

[Arun Raghavan]

On Mon, 2012-05-07 at 13:02 -0700, David Henningsson wrote:
> 2012-05-07 03:47, Tanu Kaskinen skrev:
> > On Mon, 2012-05-07 at 09:46 +0530, Arun Raghavan wrote:
> >> On Tue, 2012-05-01 at 20:09 +0300, Tanu Kaskinen wrote:
> >>> Changes in v2:
> >>>   - Call the mapping a generic 4-channel input mapping
> >>>     instead of a 4-channel mic array mapping. The mapping
> >>>     will be used also by sound cards that have two stereo
> >>>     input jacks, so in those cases talking about mic arrays
> >>>     is wrong.
> >>>   - Added a comment about using the "hw" device name.
> >>>
> >>> BugLink: https://bugs.freedesktop.org/show_bug.cgi?id=45813
> >>> ---
> >>>   src/modules/alsa/alsa-mixer.c                    |    1 +
> >>>   src/modules/alsa/mixer/profile-sets/default.conf |    9 +++++++++
> >>>   2 files changed, 10 insertions(+)
> >>>
> >>> diff --git a/src/modules/alsa/alsa-mixer.c b/src/modules/alsa/alsa-mixer.c
> >>> index abb12ee..8b54f75 100644
> >>> --- a/src/modules/alsa/alsa-mixer.c
> >>> +++ b/src/modules/alsa/alsa-mixer.c
> >>> @@ -3888,6 +3888,7 @@ static int mapping_verify(pa_alsa_mapping *m, const pa_channel_map *bonus) {
> >>>           { "analog-surround-61",     N_("Analog Surround 6.1") },
> >>>           { "analog-surround-70",     N_("Analog Surround 7.0") },
> >>>           { "analog-surround-71",     N_("Analog Surround 7.1") },
> >>> +        { "analog-4-channel-input", N_("Analog 4-channel Input") },
> >> Since this is limited to some specific hardware, shouldn't we set up
> >> udev rules instead of always probing?
> > "All devices supporting 4-channel input" isn't very specific hardware
> > IMHO,
> 
> I agree with Arun on this one. 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...?
> 
> > and I doubt udev can be used to query the amount of supported
> > channels.
> Hmm...that raises a thought I didn't consider before. Why don't we do 
> this in alsa-lib? If this varies between devices, we could create e g, 
> for 4-channel mic hardware A, we set up a alsa-lib ttable to have 
> "front" record two channels, the two left ones mixing in to the left and 
> the two right ones mixing into the right. Or in what way do we actually 
> use all four channels today?

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.

-- Arun