Can anyone explain (VirtualDub query)

[STilez]

I had a DVD to convert from VOB's to AVI's. I set up a VirtualDub batch to convert each VOB to AVI. Then I tried to append the AVI files and weird stuff happened:

1) The DVD contained the usual 5 VOB files. When I took AVI #1 and appended AVI #2, VirtualDub also automatically appended all AVI's 3-5 in the folder even though not asked for. I tested this by moving AVI's 1+2 to a separate folder, loading AVI#1 and appending AVI#2 and it was fine. When I had AVI#3 in the same folder and appended #2 to #1, VirtualDub also appended #3 as well. Why is it trying to append files I haven't told it to?

2) When I merge (append) the 5 resulting AVIs, by the end (2.5 hrs) the audio track is about 2 seconds out of sync. It turns out VirtualDub created all 5 AVI files with the audio stream about 0.2 - 0.6 seconds shorter than the video stream. I'd have expected any conversion timing errors to be milliseconds or hundredths of a second not tenths or quarters. Why do such large differences arise when recoding the VOB's to AVI's and how do I make sure keep the streams are in sync and the same length?

Thanks for any help!

VirtualDub 1.9.11, source = VOB with AC3, video recode = DivX 6.9.x, 2 pass, 1400 kbit, audio recode = lame CBR, 160kbit, SSE2 processing enabled, Intel core 2 platform

[Baldrick]

You shouldn't just convert vobs like that. You should use a converter that can open the entire dvd folder/ifo like autogk(convert to avi divx/xvid) or xvid4psp.

If you want to use virtualdub you can try vob2mpg and then import the mpg in virtualdub.

[STilez]

You shouldn't just convert vobs like that. You should use a converter that can open the entire dvd folder/ifo like autogk(convert to avi divx/xvid) or xvid4psp.

If you want to use virtualdub you can try vob2mpg and then import the mpg in virtualdub.

Useful, thanks! Can you explain a bit more, what's going on so I can understand it better?

I thought VOB's were a form of MPEG file ("strict subset") and VirtualDub with MPEG plugin certainly reads them as if they are MPEG containers. So where's the sync drift coming in? How are these other programs avoiding the problem when VirtualDub is creating AVIs with (slightly) different stream lengths for audio and video? Is it a codec issue, a VDub issue, or...??

[jagabo]

The audio and video don't have to start at the same time and don't have to have the same length in a VOB file. By converting individual VOB files you may not retain the audio skew.

I would use DgIndex, Mpeg2Source() in AviSynth, then VirtualDub.

[STilez]

The audio and video don't have to start at the same time and don't have to have the same length in a VOB file. By converting individual VOB files you may not retain the audio skew.

Hmm. Skew would be understandable (and easily fixed) but this isn't skew. It's failure to preserve stream duration when recoding.

The audio is in sync at the start of the converted file. It gradually goes out of sync during the recoded movie which it doesn't in the source VOB's. The audio and video streams have been recoded to frame rates that are about 2 parts in 10,000 different from what they should be, or 1.5 seconds in 2 hours. It could be as simple as VirtualDub using insufficient precision, or inappropriate rounding.

Confirming this can happen with VirtualDub I recoded the audio only of an AVI from PCM to MP3 using the Lame codec, and the audio track length changed from 7645.91 to 7645.82 seconds. 0.1 seconds is noticeable, so either the start or end will be laggy in the recode.

I guess what I'm wondering is, it seems that audio track length is not sufficiently accurately preserved when VDub recodes the audio. It's sufficiently imprecise to allow audible lag to creep in during a 1-2 hour recode, and track lengths to noticeably change. But surely others would have noticed if that was the case, and VDub fixed? I understand a different program might convert VOB's better (I guess that's what you're saying), but I'm confused why VDub isn't handling conversions in the first place with the precision one might expect.

[jagabo]

MPG files don't have to use the same frame rate throughout. VirtualDub and AVI assume a constant frame rate.