On 7/22/2012 8:03 PM, Gabriel M. Beddingfield wrote:
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 protocol).
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. :-)
There are two modes really - one where you send 16-bit data with the 61937 format. somehow the hardware add the preambles and CUVP bits - one where you create the complete 32-bit frame using the iec958 alsa-lib plugin Both work with my HDAudio output, the latter is more flexible if you want to set the C/U bits
Note that this is all supported in PulseAudio, and gstreamer can package AC3/DTS data for you. mplayer also does. -Pierre