strange things happening with the audio tracks on dvd ripping.

[nido]

Hello

I've walked into this curiousity when ripping one particular DVD. Please note
that I have managed to convert the DVD into the format that I wish it to be.
This is not a request for illegal activities but rather a curiousity about how
this came to be.

I've copied the data from the DVD onto the harddrive with `vobcopy -l`; which
copied the whole first track to my harddrive. I used the vobfile extracted that
way as a source.

I ripped the audio out like this:
ffmpeg -y -i $vobfile -vn -map 0.1 -ac 2 -acodec libvorbis -ab 128k -ar 44100 sound1.ogg
ffmpeg -y -i $vobfile -vn -map 0.2 -ac 2 -acodec libvorbis -ab 128k -ar 44100 sound2.ogg

The rate conversion is because at 48000hz a vorbis stream chokes. I use 2 channels
because the encoder will crash otherwise. I've been using these settings on
single-audio video streams for a while with success.

The video hard been ripped like this:
ffmpeg -y -i $vobfile -an -vcodec libtheora -b 1024k -me_method full -croptop 80 -cropbottom 80 video.avi

according to mplayer...
video.avi length is 02:12:46
sound1.ogg length is 2:12:46.2
sound2.ogg length is 2:12:33.2

according to lsdvd, the dvd length is 2:12:33.250
according to mpalyer, the dvd legnth is 2:12:33 as well

So here's the first question, where did these ~13 seconds of material come from?
It is on the beginning of the stream; because when I delay the audio of the
second stream, it all works out. However; I don't see the video.avi file
displaying 13 seconds of nothing or soemthing. Any ideas on this would be
greatly appreciated.




Going further:
It seems after muxing them together with mkvmerge; that sound2 misses the first
~13 seconds of audio. the mux command used is as follows:
mkvmerge -o out.mkv --title "title" --chapters "file made with dvdxchap" --aspect-ratio 0:2.4 video.avi --language 0:eng sound1.ogg --language 0:dut sound2.ogg --language 0:eng subs-1.idx --language 0:dut subs-2.idx

using the above command; the first audio stream is perfectly synced, the second
one is ~13 seconds early (or late, it's hard to make out without much reference)

to compensate for the sync i did this:
mkvmerge -o out.mkv --title "title" --chapters "file made with dvdxchap"
--aspect-ratio 0:2.4 video.avi --language 0:eng sound1.ogg --language 0:dut
--sync 0:13000 sound2.ogg --language 0:eng subs-1.idx --language 0:dut subs-2.idx

The second stream was now more or less synchronised nicely; it needed about 100
or 200 more to get in sync perfectly, however; the first audio stream is now out
of sync (1 second); so the final mux was something like this:
mkvmerge -o out.mkv --title "title" --chapters "file made with dvdxchap" --aspect-ratio 0:2.4 video.avi --language 0:eng --sync 0:1000 sound1.ogg --language 0:dut --sync 0:13000 sound2.ogg --language 0:eng subs-1.idx --language 0:dut subs-2.idx

Have any of you noticed similar behaviour from mkvmerge? any idea where the
desynchronization of the first stream came from when I added the sync command
for the second? It seems like something that should be reported in a bug; but
given the curious material that spawned this problem, I am hesitant to so do
now. I've had this problem with two DVD's (harry potter 4 and 5; the dutch
editions); I haven't tried this method of encoding on different dvd's yet so I
can't say how that will turn out, but any suggestions are welcome


EDIT: should it be relevant; this is done on Ubuntu Jaunty beta; all tools save ffmpeg are from the repository. ffmpeg can be either svn version of yesterday or the repository verison; it doesn't matter.
Also; extracting the ac3 streams with tcextract delivers audio files of the same length as the encoded oggs. The same counts for mplayer -dumpaudio.