Updating MPEG-2 timestamps when chopping

[lc204]

I'm trying to understand what needs to be updated in the mpeg when extracting several pieces within an mpeg and then joining them together. Besides the SCR in the pack header and the PTS, DTS...is there anything else I'm missing? Does the GOP timestamp need to be updated as well? Thanks.

[Guest]

It depends on your purpose. E.g., in the broadcast world usually nothing is done -- there is just a big discontinuity when a new stream begins. The AV sync process is smart enough to ignore it and resync on the new stream of timestamps. After all, the PTS is a limited size so it eventually wraps. Thus decoders have to be able to handle sudden discontinuities without going haywire.

What is your purpose?

[lc204]

I want to chop up an mpeg, join it programatically, and then transcode the result to wmv using 3rd party software. I've adjusted the SCR, PTS, and DTS for the packs that I've parsed out and the mpeg plays fine in windows media player using intervideo's codec. However when I transcode it to wmv using a 3rd party software, I lose the audio and the duration is off. It could be the decoder used for transcoding is more strict. It'll transcode the non-edited mpeg fine with no problem so I'm guessing I must be missing something. I'm still learning the structure of mpeg2 so excuse me if my understanding is wrong.

[Guest]

As a workaround to get the stamps corrected, try this approach. Demux the audio and video streams from your edited MPEG and then remux them.

It's hard to know what your encoder is barfing at so maybe it's better to finesse the problem this way.

[lc204]

Thanks for the help. Is there a tool that logs the errors in the stream when demuxing or remuxing while it's correcting? I want to pinpoint the problem rather than relying on a workaround.

The logic I'm using to parse the packs I want is to look at the SCR for each pack and if it falls within a range, I copy it out and adjust the timestamps. I've also included the first pack from the original which includes the system header and stream map. I'm doing this without demuxing which I think is the reason why I'm having problems.

[ronnylov]

You could try projectX which logs and corrects errors while demuxing.

http://project-x.sourceforge.net/

[lc204]

Thanks! I'll definately give it a try.

[Schmendrick]

A good way is to split your original source stream and correct any minor faults at the same time using project-x as ronnylov has suggested you can also demux it at the same time. With MPEGSchnitt ( http://www.doom9.org/Soft21/DigiTV/Mpeg2Schnitt087.zip ) you can join the different clips and remux them together.

In the case of ts-files you can use tsremux to recalculate the PTS-values (and DTS values??) of a ts-file merged from different ts-clips.

Schmendrick

[lc204]

Thanks. I was hoping I could split and join without demuxing. Also, all the tools I've seen don't provide command line options for automatting either demux/remux or split/join given a list of mark points.