[mythtv-users] MPEG2 -> MPEG2 Transcoding

[Cory Papenfuss]

>>  	The lossless mpeg2 cutting should clean them up.  Then utils like
>> replex, dvb-replex, etc should be able to insert DVD NAV streams (by
>> demuxing or not)... then they're directly authorable.
>
> As far as I understood Geoffrey on the -dev list, remuxing will still
> potentially lead to bad sync for mythtranscoded mpeg2.
>
 	Due to bugs or cases where there are gaps in the audio stream as I 
understand it.  When ivtv cards hiccup, they fiddle with the video PTS to 
fix it.... not the audio (I think).

> I have made the following observation: If I demux (mpeg2desc) and remux
> (mplex) an original ivtv recording, then it's perfectly in sync.
>
 	Like I said before, it works for 95% of the people 95% of the 
time.  Try capturing from a home video camera tape... I'll be you'll have 
sync issues demuxing it like that.  It will be fine at first, but after 2 
hours or so, you'll likely have audio sync that's off by 10s of seconds. 
Unless of course your video camera playback device has a broadcast-quality 
TBC (time-base corrector.. not likely).

 	VHS captures also fairly likely to cause sync errors.  The 
problem is that the ivtv card expects to see *EXACTLY* 525 lines/frame at 
29.97Hz, where each line lasts *EXACTLY* 1/15734s = 63.556us. 
Consumer-grade tape devices have lots of jitter and produce craptastic 
quality "NTSC" (or "PAL") signals.  TV's don't mind if things get skipped 
a bit, but realtime capture devices mind a lot and have to deal with it 
somehow.

> If I do the same to a mythtranscoded mpeg2, I have to give an offset of
> -150ms to mplex.
>
 	The problem is not absolute sync.  It's sync that *CHANGES* 
throughout a capture.  The only type of sync error that is even 
acknowleged with the usual linux tools (mpeg2desc, mplex, transcode, 
replex, etc, etc) are what you mention... a static XXXms offset.   The
presentation timestamp that's part of the PS mux'd Audio and Video stream 
fixes missing video/audio bits by adjusting them periodically in the 
stream.  If you demux a PS, you lose that information.... so when you 
remux it you don't know that you're missing a video frame here or there. 
The result is sync that gets worse throughout playback.

 	That's why the lossless mpeg2->mpeg2 has been so difficult to do. 
Geoffrey parses the PS, compares the VTS/ATS, does some heuristics to 
decide if something's changed, and then adjusts the remainder of the 
stream to have a constant offset.... err something like that.  Bottom line 
is it cleans up the stream.

 	Oh... it also allows cutting, too... :)

-Cory

-- 

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* Cory Papenfuss                                                        *
* Electrical Engineering candidate Ph.D. graduate student               *
* Virginia Polytechnic Institute and State University                   *
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