transcoding mpeg2 -> mpeg2 (was: RE: [mythtv] DVB, (new) recording
profiles, and transcoding)
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
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
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
(Sorry - it's mostly in German. The download link is near the bottom).
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