ac3 delay, multiple vobs - clarification needed

[erdie]

ok, here's the story:

after ripping with smartripper/moviemode or DVDDecrypter you end up with usually 4-6 1GB vobs.

if you select vob #1 in DVD2AVI it automatically adds all remaining too. when you demux the ac3 by saving the project the ac3-filename will contain a certain delay value - "-66 ms" e.g.

now the question(s):
why is it that when I select only the last .vob of the rip and demux the ac3 the delay is different?

Wouldn't different (more than 50ms) delays for each vob result in desync audio when re-muxing with the encoded .avi (as according to different guides you only correct the delay DVD2AVI gives you for selecting the first respectively all vobs) ?

[hakko504]

The reason you don't get asynch audio is that the vobs overlap. The first audio of vob5 should be played back together with the last frame(s) of video in vob4 (if you have negative delay, vice versa if it is positive.)

Consider this situation: Number is the vob number and time starts at0 when the video starts.

Video: 1111...112222...22233...34444
Audio: 11111...111222...22333...34444
Then the demuxed audio from vob1 and 3 would have negative delay, from vob 2 positive delay, and the audio from the last vob would have zero delay.

[erdie]

k, if that is taken in account by DVD2AVI when demuxing ac3 from all vobs why is it that in some cases the delay reported by DVD2AVI just isn't right
(done many rips, happened with last one -for the first time - that's why I asked...)
(was -60ms for first/all vobs, and about -220ms for all others, and ~-220ms was what I had to set to get audio in sync...)

[roach76]

DVD2AVI just takes the first VOB file's delay and uses that one. The problem is though, as DVD2AVI adds the audio files together it doesn't take into account the differing delays, so you'll end up with video out of sync that are from different sources.

[erdie]

why is it then if you start demuxing in the middle of vob 1 the delay is still different ?

e.g.:

demuxing AC3 from the beginning, whole vob1: 0 ms delay
demuxing AC3 from somewhere in the middle of vob1: -114 ms delay
???

[hakko504]

Audio as well as video is compressed in chunks called frames. but an audio frame consists of multple samples in time, where a video fram,e consists of multiple samples in space. If these audio/video frames are not aligend, which they can only be if the time covered by a video frame is an even multiple of the number of audio frames (or vice versa). Since this is usually not the case, the delay will be different from one video frame to the next. DVD2AVI will get the audio frame with the start as close as possible to the video frame you have selected as the start point, calculate the difference in time and report that as the DELAY.

[erdie]

ok.
still, what's it with DVD2AVI reporting the wrong delay then?

[roach76]

The delays between each VOB are different and DVD2AVI is reporting them correctly. You only require the delay from the first VOB.
The only time you require the delay from any other VOB that follows them is when you mix the VOBs out of order or you join VOBs from different sources together.

When doing a movie the audio overlaps between VOBs creating differing delays, like the video at the start of 1 VOB might be played together with the audio from the previous VOB. But just like the video, the audio is in one complete stream so that is why you only need the first VOBs delay. Mixing different VOBs together can create gaps in the audio between the VOBs, for example, the audio might stop early in one VOB and is meant to continue in the next VOB. So if you end it there and add another VOB from a different source, you end up with a small bit of video without audio at the end of the previous VOB. But DVD2AVI assumes it is like a movie and is one long stream and will join the audio streams together to make it one and will create a synching issue in the next clip along.

So the moral of the story is you only need the first VOBs delay and if you are mixing different VOBs together work on them separately and join them together when they are in VOB form only so the gap that is meant to be there remains, unless you change the delays.