[mythtv-users] MPEG2 -> MPEG2 Transcoding

Cory Papenfuss papenfuss at juneau.me.vt.edu
Wed Mar 29 11:38:23 UTC 2006

> Geoffrey Hausheer
>> but
>> he'll ask for some more information and a sample to figure out the
>> problem. I know he fixed quite a few bugs for me and others while 0.19
>> was still in SVN.
 	Me too, while it was just a test app.  I haven't had time to play 
with it or run my "kill mpg" clip through it yet.  I was looking pretty 
good awhile back.

> Right, and that's the most important part.  One of the primary reasons
> Geoffrey created the new lossless transcoder in Myth 0.19 was to allow
> simple commercial clipping without introducing A/V sync errors, since
> other available MPEG-2 tools did not properly handle issues in the
> PVR-x50 recordings.
 	It's even more subtle than that.  There's a few more 
things going on than clipping commercials without A/V sync.  The problem 
with most of the other tools is that the ivtv cards change the *relative* 
A and V sync offset throughtout a stream every once in awhile (generally 
due to bad captures from tape, noisy signal, etc).  That breaks most other 
utils.  AIUI, the lossless adjusts the offsets *everywhere* in the stream 
to be a constant value... not just at the commercial cuts.

 	Another issue is of course the commerical cutting and what happens 
if the sync changes inside a chunk that's cut.  Also, it's 
frame-accurate... so if you cut on a non-keyframe, it will convert 
non-keyframes in the original GOP to keyframes in the output... thus 
frame-accurate editing.

> So, basically, what the OP heard about cutting MPEG-2 applies to other
> tools, but Myth's lossless transcoder was created because of the
> problems those other tools had, so you shouldn't experience any A/V sync
> issues when using it.
 	... and had been a significant deficiency in MythTV since I 
started playing with it 2 1/2 years ago.  Lossless cutting/cleaning was 
the only significant technical hurdle to decent archiving, and DVD 
burning.  With that in place, DVD authoring becomes simple (since the 99% 
of the other linux MPEG processing tools won't barf on the streams now).



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

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