[linux-dvb] Re: AC3 over DVB

["Dr. Werner Fink" <werner@suse.de>]

On Tue, Jun 08, 2004 at 07:12:57PM +0200, Marcus Metzler wrote:
> >>>>> "Hamish" == Hamish Moffatt <hamish@cloud.net.au> writes:
> 
>     Hamish> Hi, This question is probably off-topic, for which I
>     Hamish> apologise. However, I wonder if anyone can help me with
>     Hamish> getting AC3 sound over DVB-T?  I want to pass it out to
>     Hamish> S/PDIF, not decode it in software.
> 
>     Hamish> It seems that Xine can do AC3 passthrough over S/PDIF,
>     Hamish> though I never got any sound out when I tried it. I get
>     Hamish> stereo sound (PCM) over S/PDIF ok. I suspect that its AC3
>     Hamish> stuff has been only tested with DVD.
> 
>     Hamish> MPlayer can supposedly do it too. I tried it, but it
>     Hamish> segfaults almost immediately on the station with AC3
>     Hamish> sound. (I just put the AC3 PID in place of the audio PID
>     Hamish> in channels.conf.) Once it got as far as the amp detecting
>     Hamish> "Dolby Digital 2/0", then crashed, but never again.
> 
>     Hamish> It seems like MythTV should also do it, but I've had even
>     Hamish> less luck with that, so I thought I'd get Xine or MPlayer
>     Hamish> working firt.
> 
>     >> From my Googling, it seems that AC3 is carried differently on
>     >> DVB versus
>     Hamish> ATSC. I wonder how it is carried on DVD? I wonder if Xine
>     Hamish> can only do ATSC (or DVD), and not DVB..
> 
>     Hamish> Hamish -- Hamish Moffatt VK3SB <hamish@debian.org>
>     Hamish> <hamish@cloud.net.au>
> 
> The AC3 PES format on DVB is a little bit different than in a DVD VOB.
> The private stream 2 PES that are used for AC3 have a 4 byte header in
> a DVD VOB which gives you a stream id and an offset to the first AC3
> frame in the PES. This header in the PES payload and is missing in DVB
> PES because you don't need them for stream identification (you don't
> really need the offset anyway). This could be a problem for some
> programs that expect the 4 byte header.
> I thought that most programs would take care of this, but I recently
> found out (when someone sent me an example file for debugging replex)
> that e.g VDR does not set the 4 byte header when transforming from TS
> to it's PES stream recording format. I don't know if this was from an
> older version or if the problem is still there.

The DVD uses a sub stream header within a private stream 1 PES, I've
never seen one in the PS2.  Which DVD does this? AFAIK PS2 frames
are for navigation data, arn't they?

Nevertheless bitstreamout and AFAIK the AC3 over DVB patch vor VDR 
can handle both DVD and DVB AC3 data.  For an overview how to get
the AC3 data out from DVD streams see

     http://mpucoder.kewlhair.com/DVD/

Beside this mplayer AC3 handling had a bug on the last look
I've done into the source. The constructed PES frame for
sending to the DVB card was simply wrong. Don't know if this
is in the current version fixed now.


         Werner 

--
AC3 loop through sound card http://bitstreamout.sourceforge.net/
Howto http://www.vdr-portal.de/board/thread.php?threadid=1958
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