[alsa-devel] Required/preferred multichannel order for ALSA

[Takashi Iwai]

At Fri, 27 Apr 2012 10:21:37 +0100,
Liam Girdwood wrote:
> 
> On Thu, 2012-04-26 at 16:33 -0500, Ricardo Neri wrote:
> > Hi Takashi, list,
> > 
> > I am implementing multichannel support for HDMI on Texas Instruments' 
> > OMAP4. I would like to know if ALSA mandates a specific channel order or 
> > has a preferred one.
> > 
> > I tried to find some guidance in alsa-lib or in the alsa driver. All I 
> > could find is the order described in speaker-test: 
> > FL/FR/RL/RR/C/LFE/SL/SR. This order seems to be in use due to historical 
> > reasons [1]. It was also mentioned that an API to set/get the channel 
> > mapping was going to be implemented [2]; I tried to find it without 
> > success. As [1] and [2] are very old posts, I was wondering if the 
> > situation has changed.
> > 
> > My question arises from the fact that HDMI audio uses the channel 
> > ordering defined in CEA-861 section 6.6.2, which is different from what 
> > speaker-test expects. This also different from the order that SMPTE 320M 
> > specifies. OMAP4 is able to alter the channel mapping, so I could match 
> > what ALSA expects if such required/preferred order exists.
> > 
> > Thanks in advance for your comments.
> > 
> > Ricardo
> > 
> > 
> > [1].http://www.spinics.net/lists/alsa-devel/msg24444.html
> > [2].http://www.spinics.net/lists/alsa-devel/msg24495.html
> 
> A lot of hardware now can remap channels to suit the use case so it
> would probably be good to have an ALSA API to get/set the mappings.
> 
> This seems like it would be a good subject for the BoF.

Yes.  As I added to LPC 2012 wiki page, it's a topic I'd like to
discuss in the audio microconf, too.  It's a long-standing issue,
not only for embedded devices but also PCs, especially with HDMI/DP.

Actually I've had a few different (partial) implementations, but
nothing was so convincing yet.

BTW, for the time being, the remapping itself is done in alsa-lib
route plugin.  This works when the driver is unique and supports
only a single configuration.  Then you can define the routing table
beforehand.  Take a look at alsa-lib/src/conf/cards/*.conf based on
old AC97 codecs.

OTOH, if the driver is generic (e.g. HD-audio) and supports different
configurations, it doesn't work so easily.  A more generic API is
mandatory in such a case.


Takashi