From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Gabriel M. Beddingfield" Subject: Re: Can ASoC support SPDIF AC3 passthrough playback (non linear PCM) ? Date: Sun, 22 Jul 2012 20:03:46 -0500 Message-ID: <500CA2F2.1090801@gmail.com> References: Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: Received: from mail-ob0-f179.google.com (mail-ob0-f179.google.com [209.85.214.179]) by alsa0.perex.cz (Postfix) with ESMTP id 8CBDB265018 for ; Mon, 23 Jul 2012 03:03:57 +0200 (CEST) Received: by obbeh20 with SMTP id eh20so8137019obb.38 for ; Sun, 22 Jul 2012 18:03:58 -0700 (PDT) In-Reply-To: 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, sangsu@gmail.com Cc: android-platform@googlegroups.com List-Id: alsa-devel@alsa-project.org 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. :-) -gabriel [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.