Someone tried a couple of years ago to get mpeg2 transcoding to work, and
could never get it to work properly. That's why Gavin (Beirdo) wrote the
mpeg2cut module for me to include in nuvexport. So far, avidemux is the
*only* program capable of true lossless mpeg2 cutting (without resorting to
cutting at a GOP, which is not good enough). Then, in his tests, he
discovered that lvemux was the only program capable of stitching the audio
and video back together without sync problems.
The problem (as I've painfully discovered and discussed before) is
that the ivtv cards change the audio and video relative timestamps (PTS,
DTS, etc). That information is at the PS level, and avidemux blindly
strips PS into A and V. If the sync offset isn't constant, it gets off as
the clip progresses
Unfortunately, afaik, lve does NOT work with fedora core 3 or later (along
with some other modern distros -- I forget the exact library conflict),
preventing mpeg2cut from doing its job.
Now, he's been working on extracting the relevant portions from each of those
programs into a single app (esp. so that it wouldn't require X or anything
like that), but it will likely take him awhile to finish.
It's a difficult problem that few programs have been able to
solve. ProjectX seems to be the closes, but I've heard the streams it
produces has other issues. In particular, think of cutting out a chunk of
a clip where the PTS changes... do you now have to parse *all* MPEG
packets thereafter to update to the new PTS less changes and the 4-5
minutes of commercials removed? Also, forward/backward GOP references get
ugly.
Ideally, that's what the lossless cutter would do, though. Parse
the whole stream, and either cut on GOPs fixing all timestamps along the
way, or even better would be to reencode around GOP boundaries at
cutpoints for frame-accurate edits.
I would also suggest that a "lossy" reencode to MPEG2 would not be
without merit at the point in time. People have struggled with getting
MPEG2 captures burned to DVD (sans commercials) for the same reasons as
above. As I understand it, transcoding from within Myth basically
decodes (using the myth playback engine with *can* parse A/V correctly),
then reencodes. Although it would be time consuming and introduce a
generation loss, adding an {MPEG2, MPEG4, RTJPEG}->MPEG2 as a transcoder
would be useful as a stopgap at this point.
For now, if all you want to do is save space, just transcode to mpeg4 with
mythtv (keep in mind, it's optimized for encoding speed, not quality), or if
you have the time, run nuvexport's xvid exporter (I prefer transcode, but it
defaults to ffmpeg). Nuvexport's default xvid settings should give you about
350M/hour (minus commercials) at 624x464 (weird res, but that's how the
calculations best come out after you crop 2% of the edges to get rid of
broadcast noise).
Speaking of that, anyone ever tried for a high-quality version?
Something like a 2-pass encode with denoising, etc?
-Cory
--
*************************************************************************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
*************************************************************************
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