Thanks to everyone for replying! On 04/27/2012 07:12 AM, Takashi Iwai wrote:
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.
Thanks for the advice! I could remap the channels succesfully with the alsa-lib route plugin. It increased the CPU usage when using speaker-test, though; from 2% to ~5%, according to top. I guess this is acceptable for now.
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.
Maybe I can statically remap the channels in the driver to align with speaker-test and post in omappedia how to remap if someone needs to. This in case their HDMI sinks supports a different HDMI layout.
Thanks!
Ricardo
Takashi