[mythtv] A prelude to transcoding MPEG2->MPEG2

Geoffrey Hausheer ou401cru02 at sneakemail.com
Sat Dec 6 00:17:14 EST 2003

I have started looking at enhancing the transcoder to do
commercial-clipping on MPEG2 streams without reencoding them to MPEG4.

I have finally invested a little time in understanding the MPEG2 format,
and have a better idea of what is needed.
I've looked at GOPChop, which is the closest thing I've seen to this
capability, but it leaves the problem of having to cut to the nearest GOP
(I-Frame).  When we faced this problem with MPEG4 streams, I decided to
create a new I-Frame at the cutpoint, and reencdoe to the next I-Frame,
then do some magic to get the key-frames right.

With MPEG2 we have the same options: 1) cut to the nearest GOP, and use
micro-jumps to get to the exact frame, or 2) reencode a new GOP
consisting of a new I-frame at the exact start-point up-to the next real

As far as I can tell, the MPEG2 format doesn't require GOPs to all be the
same size, so we could keep valid streams and do (1).  However, it
requires an MPEG2-encoder (there is one in libavcodec, though quite a bit
of code will be needed to use it), and it may turn out to be difficult to
get compatibility with all MPEG2 streams.  Doing (2) would be quite a bit
easier, but it means that watching the videos outside of myth, (or
recording to DVD) you could end up seeing ~0.5 seconds unwanted video
between the commercial cut.

MPEG2 is further complicated by B-Frames, which means that the end-point
needs to be cut to the nearest P-Frame or I-Frame, so you could end up
with a few extra frames after the start of the cut-point in either of the
above methods.

There is also the question of how audio and video are muxed together.  My
understanding is that the 'PES' consists of a series of GOPs and audio
sections.  So I am not sure how to get frame accuracy without building
new PES and possibly needing to reencode the audio.

The simple truth seems to be that MPEG2 is a pain in the ass to deal with

This is about as far as I've gotten so far, which isn't anywhere near
enough to actually implement anything.

Considering the complexiity of keeping an MPEG2 stream compliant, I think
the easier approach will be the GOPchop approach.  I was considering just
importing it wholesale (it is GPL) but it looks like it'll take a fair
amount of work to extract the GUI parts and make a library from it, so it
may be easier to just duplicate the GOP modifying functions in myth.  On
the flip-side, chopping to exact frames would give a much beter overall
solution, but is likely to be challenging to make work for the PVR250,
DVB, and HDTV crowds.

Anyhow, this is all to say that we're no farther along in the goal of
chopping mpeg2 streams then we were 3 months ago.

However, if anyone knows where I can find a reasonable spec for the MPEG2
stream format, I'd be much obliged.  My google searching has turned up
lots of useful stuff, but not enough for me to really understand how to
begin (for instance I've been able to find zilch on the actual PES and
GOP formats.

Also, everything I've looked at has been MPEG2-PS related.  the 'TS'
variety seems to be quite a bit more complex, and if DVB uses it (as I
suppose it does...'TS' being 'Transport Stream'), it will cause much woe,
so if anyone knows anything about that, it'd be useful too. For instance,
has anyone tried using GOPchop on a DVB stream?


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