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Old 07-07-2004, 01:29 PM   #1
cracauer
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Registered: Jul 2004
Location: Boston, MA, USA
Distribution: FreeBSD, Debian/AMD64, Ubuntu/i386
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MPEG2/DVD/transcode question


Hi,

I would like to get a better understanding on some of the steps involved when transforming videos to make a DVD, transcode in particular.

Let me give the sequence I use first:

1) I use mencoder on a TV card

2) mencoder outputs an mpeg2 file and uses 48KHz audio
parameters: vcodec=mpeg2video:vbitrate=1800:abitrate=48

3) I use transcode on that file to split it up for mplex
transcode -i tv.mpg -export_asr 2 -ympeg2enc,mp2enc -V -M4 -o tv2

4) mplex -f 8 -o tv3.mpg tv2.*

5) then I use dvdauthor but that's not related to my question.


My question is in the transcode step. While mplex is obviously just copying the file, transcode is obviously recoding everything, doing mpeg2 encoding from scratch. The runtime of 246 minutes on an Athlon XP 2500+ leaves me to figure that.

Why is that? I understand that the transcode step is needed to extract the different streams out of the mpeg2 file. But why is it reencoding? Why can't it just copy the frames that the original mpeg2 file had?

Is there any way that I can avoid the loss of quality from the reencoding and spending the runtime?

Can I somehow use mencoder to directly generate files that are accepted by mplex?

Why is trancode so slow anyway? The mpeg2 encoder in mencoder is much faster.

Last edited by cracauer; 07-07-2004 at 01:36 PM.
 
Old 07-07-2004, 02:20 PM   #2
wapcaplet
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Registered: Feb 2003
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I don't have any experience recording from video cards, but if mencoder is already outputting an mpeg2 video file (with mpeg2 audio as well), there should be no need to re-encode anything. As long as the video is in a DVD-compliant format (mpeg2 video, mpeg2 48khz audio, at a standard DVD resolution like 720x480), it should be ready for dvdauthor-ing immediately.

As for the mpeg2 encoder in mencoder, I toyed around for quite a while trying to get it to work for doing conversion to DVD, with no success; AFAIK, transcode is calling upon 'mpeg2enc' of the mjpegtools package, and yes, encoding using it is rather slow (on my Athlon 1600XP, about 4x to 5x the length of the video). I've had better luck cutting out the transcode-middleman entirely, and encoding directly using the mjpegtools applications (there's a nice HOWTO for that). If you do manage to get good-quality mpeg2 video output from mencoder, and it is faster than mpeg2enc, please let me know how you did it!

But I digress; to answer your question, it should be possible to separate the video and audio streams using only mencoder. I'm not certain of the exact syntax, but the options you will want to look into are '-oac copy' and '-ovc copy'. The mencoder manpage also mentions some options 'rawaudio' and '-rawvideo', which seem to be related to demuxing (that is, separating audio/video streams). Sorry I can't be of more assistance.

A little shameless self-promotion here: I've written a script (and GUI) for converting to DVD, VCD, etc. Not what you want for what you are doing, though you may find it useful in the future.
 
Old 08-02-2004, 10:17 AM   #3
hwouters
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Registered: Aug 2004
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Hi,

I hope this script will help you:

----
#For encoding, I used the following command (this command may not work, but you got the idea):
mencoder -audio-demuxer 20 -oac lavc -ovc lavc -lavcopts vcodec=mpeg1video:vbitrate=6000:acodec=ac3:abitrate=448:aspect=4/3 -ofps=25 -srate 48000 -of mpeg -o /var/tmp/out.mpg in.avi

#For splitting (what you've been asking), use:
mencoder -dumpaudio -dumpfile /var/tmp/out.mpa /var/tmp/out.mpg
mencoder -dumpvideo -dumpfile /var/tmp/out.m2v /var/tmp/out.mpg

#to merge it back to a DVD-compatible mpg-file:

mplex -f 8 -o out.mpg /var/tmp/out.m2v /var/tmp/out.mpa

#To make a dvd-structure, use:

dvdauthor -o DVD out.mpg
dvdauthor -o DVD -T

#you can write it with isogrowifs or k3b now

---

Note that I used the mpeg1video instead of mpeg2video, because the mpeg2video gave problems for my dvd-player (it's like he doens't read the keyframes).

Greetings Hendrik
 
  


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