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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