From mboxrd@z Thu Jan 1 00:00:00 1970 From: Pierre-Louis Bossart Subject: Re: Can ASoC support SPDIF AC3 passthrough playback (non linear PCM) ? Date: Mon, 23 Jul 2012 14:36:49 -0500 Message-ID: <500DA7D1.7060507@linux.intel.com> References: <500CA2F2.1090801@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: Received: from mga03.intel.com (mga03.intel.com [143.182.124.21]) by alsa0.perex.cz (Postfix) with ESMTP id 8D292260317 for ; Mon, 23 Jul 2012 21:36:50 +0200 (CEST) In-Reply-To: <500CA2F2.1090801@gmail.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: alsa-devel-bounces@alsa-project.org Sender: alsa-devel-bounces@alsa-project.org To: alsa-devel@alsa-project.org List-Id: alsa-devel@alsa-project.org 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