[mythtv] Lossless MPEG2 editing!
manu
eallaud at yahoo.fr
Fri Jan 21 11:14:15 EST 2005
Le 21.01.2005 05:08:35, Cory Papenfuss a écrit :
>> Sorry to chime in... but I posted a bug report to point out that
>> mythtranscode does not work for me with cutlists (it does a great
>> job of transcoding RTJPEG->MPEG4 though). Is there another way to do
>> that (almost as easy as transcoding directly from the frontend)?
>> Thanks,
>> Manu
>
> The quick and most accurate answer is probably "No." I'm not
> completely sure what you're asking for. A program for hand-editing
> out commercials is avidemux... works pretty well. It doesn't need to
> reencode, but it can.
>
> Regarding lossless MPEG2 cutting, there are a number of issues:
>
> - Funky MPEG2 streams... between ivtv/DVB/HDTV/Dig Cable, there are
> lots of different bits within a "standard mpeg2 stream." There can
> be *lots* of other streams and formats of wrappers, packets,
> substreams, etc all under the umbrella of an "MPEG2 capture"
>
> - GOP vs. non-GOP accurate edits. MPEG2 streams have I, P, and B
> frames. Only the I frames can "stand alone." The other two rely on
> previous/future combinations of other frames to generate a frame.
> That means that cutting arbitrary frames is difficult to make a
> lossless operation. Some re-encoding around the cutpoints is usually
> necessary, even if cutting on GOP boudaries. Read the GOPchop docs
> for more on this.
>
> - GOP length: Real-time encoders (e.g. ivtv-based cards) don't have
> the benefit of noncausal filtering (read: they can't predict the
> future), so they make compromises quality of the video for a given
> bitrate. AFAIK, the hauppauge cards and ivtv driver combination only
> produce one GOP sequence, which is about 15 frames long. That's
> roughly 1/2 second.
>
> - Sync: Audio and video packets don't have to be very close to each
> other. It's probably likely that a good MPEG2 stream cutter will also
> have to be a rudimentary player to figure out which streams and
> packets are really necessary to fully decode something. Since MPEG2
> streams are both pseudo-realtime and packetized, they have
> Presentation Time Stamps embedded to allow for prebuffering of data.
> This can then be "presented" at the correct time. If you take a
> chunk out of the middle, all PTSs past this point now need to be
> tweaked.
>
> I'm sure I'm forgetting some things, but those are some that
> I've run across. If it were easy, it'd be done by now.... :)
>
Thanks a lot for the explanation. I knew it was not straightforward,
but I didn't know it was soooo bad ;-)
Well my question was about cutting with/without reencoding in myth,
because a while ago I used this feature (mythtranscode) and recently it
was not working anymore. But someone told me it is back on its feet
now, so I am happy. Obviously it seems that a good editing tool is not
an easy thing to code.
Thanks,
Bye
Manu
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 189 bytes
Desc: not available
Url : http://mythtv.org/pipermail/mythtv-dev/attachments/20050121/b60f3d16/attachment.pgp
More information about the mythtv-dev
mailing list