[alsa-devel] Can ASoC support SPDIF AC3 passthrough playback (non linear PCM) ?

Gabriel M. Beddingfield gabrbedd at gmail.com
Mon Jul 23 03:03:46 CEST 2012

On 07/21/2012 10:39 AM, Sangsu Park wrote:
> Hi,
> We're developing a platform where we have SPDIF output.
> And we want to SPDIF AC3 playback (non linear PCM).
> Our CPU is a kind of ARM architecture one and it has a SPDIF that can
> support AC3 passthrough playback.
> I've already know that ALSA can support SPDIF AC3 passthrough playback
> like follows,
> http://www.mythtv.org/wiki/Configuring_Digital_Sound_with_AC3_and_SPDIF
> But, I can't find that any ARM architecture CPU can be supportable for
> SPDIF AC3 passthrough playback.
> Are there anyone who have a SPDIF AC3 passthrough playback?
> or anyone who know the clue about SPDIF AC3 passthrought playback
> supportable on ASoC?

I'm currently working on a similar project -- and I'm learning about 
this myself, too.

As I understand it, you have to package the AC-3 into IEC 61937 format 
first.  The S/PDIF driver will then encode it into the IEC 60958 (S/PDIF 

You probably need/want a copy of IEC 61937-3.[1]  It looks pretty simple 
(add 4 headers to the front and some zero-padding at the end to meet the 
alignment requirements).  I don't know of any software examples for 
doing it[2] -- so if you find some, please share! :-)

As for feeding it to ALSA, I think that's what the 
SNDRV_PCM_FORMAT_IEC958_SUBFRAME is intended for.  However, in practice 
I've heard that you can usually configure the PCM device to something 
like an S16 format and things "just work."  (IEC 61937-3's payload is 
16-bit aligned.)

...and I think I've heard of some people shooting raw AC-3 and things 
"just work."  YMMV. :-)


[1] Digital Audio - Interface for non-linear PCM encoded audio 
bitstreams applying IEC 60958 - Part-3: Non-linear PCM bitstreams 
according to the AC-3 format.

[2] Nor do I know of anything in ALSA that will do it for you.

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