transcoding mpeg2 -> mpeg2 (was: RE: [mythtv] DVB, (new) recording
profiles, and transcoding)
Mike Wilcox
mike at trouble.org.uk
Thu Oct 9 03:00:50 EDT 2003
Edward Wildgoose wrote:
> By the way, I did a test transcode of a show into mpeg4. The sound ends up out of sync as a result. I was cutting commercials front and back, so I can't rule out that it wasn't caused by this.
>
From what I've read, the MPEG stream put out by the DVB cards is known
as PVA, and has timestamp headers to allow the Audio & video to be
synchronised, and for the synchronisation to keep working even after
some drop-out on one or both streams.
My experience is that the MPEG2 decoder in Myth doesn't quite keep A and
V in sync (and turning the experimental A/V sync to ON makes it worse).
> There is something slightly wierd about DVB audio it seems. The sound always starts 1-2 secs behind in myth and then catches up over the next 2-3 secs. There is some code in avformatdecoder which is adjusting timestamps to try and do the catchup. However, I assume that mythtranscode is using the myth decoder? Just not sure why the audio isn't getting sync'd...?
>
From my experiments, it seems that the MPEG video player in Myth also
doesn't cope very well with dropouts in the video - any small problems
in the stream can lead to a lasting effect of "jumpiness" in the
recording (the whole thing playing, but jumping forward a second, then
back half a second, then forward etc...). If you FF over the dropout,
the stream just runs normally.
However, the Audio decoder carries on normally, and copes well with
dropouts.
I though Myth used the avformatdecoder for MPEG2, but I could be wrong.
> Is there anything else you might suggest to fiddle with?
>
Some alternatives for you to try. A couple of external programs I have
played with - one Windows (PVAstrumento), one Java on Linux (X08) -
which include as their main job the error-correction and A/V sync of DVB
streams.
I've been adapting the nuvexport script to author Video DVD, and using
the X08 stuff to help - it does a great job at both error correction and
synchronisation!
http://www.offeryn.de/dv.htm#pvas
and
http://forum.lucike.de/viewtopic.php?t=54&sid=64c8d5f7d3f42c8706a0ad04338cec12
(Sorry - it's mostly in German. The download link is near the bottom).
Cheers,
Mike
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