transcoding mpeg2 -> mpeg2 (was: RE: [mythtv] DVB, (new) recording profiles, and transcoding)

[Geoffrey Hausheer]

On Fri, 3 Oct 2003 17:20:21 +0100, "Edward Wildgoose
Edward.Wildgoose-at-FRMHedge.com |mythtv/1.0-Allow|"
<pfnahiui2x0t at sneakemail.com> said:
> I also found:
> http://mpgtx.sourceforge.net/
> 
> Which references: gop-fixup
> http://mlug.missouri.edu/~markrages/tavi/index.php?page=GopFixup
That software looks like it has had no activity in the past year.  In any
case, I don't think changing the header timecodes is difficult (I already
do this for mpeg4 streams).  I'd prefer to not rely on different programs
for the transcoder if at all possible, and I have seen nothing so far
that requires one.

I haven't looked at the MPEG2 spec stuff, but as I recall, I frames (the
sync frames which contain a reference for the next several delta frames)
can exist anywhere, but the key frames (which sync audio and video) are
expected at a fixed rate (and a key frame is always an I frame).  These
are set at 15 frame intervals for ntsc, (and 12 for pal I believe). 
Since I'm no MPEG2 expert, I'm not sure if these are equivalent to GOP
frames or not.  Last I looked though, myth used the key frame spacing
data to do fast-forward/rewind, so at least it was expecting them at
regular intervals.  Until I dig into the mpeg2 stuff more, I can't really
say any more (as it is, it's quite likely that everything I've said so
far is wrong...) :)
.Geoff