[linux-dvb] AC3 over DVB

[Marcus Metzler <mocm@mocm.de>]

>>>>> "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.

Marcus

-- 
/--------------------------------------------------------------------\
| Dr. Marcus O.C. Metzler        |                                   |
| mocm@metzlerbros.de            | http://www.metzlerbros.de/        |
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 |>>>             Quis custodiet ipsos custodies                 <<<|