[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
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