[pulseaudio-discuss] [PATCH] alsa: Remove four channel input profile

[Alexander E. Patrakov]

08.08.2014 15:11, Tanu Kaskinen wrote:
> Ok, if a surround map actually improves things, even though it doesn't
> make conceptual sense, feel free to apply this patch.

No comments here, as I cannot test it.

> Would you agree that it would be good to modify the remapping/remixing
> logic so that if one side of the remapping has only unspecified
> channels, then the mapping should be simply channel index based, i.e. if
> the other side has channels aux1,aux2,aux3,aux4 and the other side has
> front-left,front-right, then aux1 should be mapped to front-left, aux2
> to front-right and aux3 and aux4 should be discarded? Pretending that a
> 4-channel mic array has rear channels doesn't make sense - if an
> application records a stereo stream, I don't think remixing the
> pseudo-rear channels to the stream is likely to have any better result
> than just picking the two first channels of the array.

It would be nice to see what each channel of that device actually 
contains if the sound source moves from left to right (i.e. whether the 
layout is actually stereo compatible). Or maybe that's device-specific - 
e.g. what if one connects a TetraMic to a USB sound card?

According to http://www.core-sound.com/TetraMic/2.php , the 4-channel 
layout happens to be stereo-compatible.

> A separate issue that came to my mind is that I'd actually prefer using
> the mono channel position to mean "undefined". That is, instead of using
> maps like aux1,aux2,aux3,aux4, I'd like to use mono,mono,mono,mono. The
> remixing logic of course then needs to be adapted to treat mono channels
> as undefined instead of thinking that all four channels should be mapped
> to all the counter party channels like I think is currently done (if
> there's only one mono channel, that should be treated as a special case
> with the same semantics as are currently used). The name "mono" is
> unfortunate, because it implies that there is only one channel in the
> stream, so having multiple mono channels in one stream appears strange.
> It would be nice to have "unspecified" as the name instead - I think
> this can be made an alias. Something to note is that alsa doesn't have a
> "mono" at all in its position enumeration, but it does have
> "unspecified".

This applies to playback, too.

> pa_simple_new() without a channel map should ideally result in an
> unspecified channel map, not a surround map. Changing this requires
> fixing the remapping logic first, though.
>
> If you agree that these changes would be a good thing, I'll add that to
> my "list of things to do during the next decade" (it's a long todo list
> of random lower priority things that I work on occasionally in fifo
> order).

May I look at the other items in this list?

-- 
Alexander E. Patrakov