Framedrop at start when saving MPEG-2 VOB files

[test32421]

Using the current 2.8.2 nightly in copy mode, I have noticed that with MPEG-2 .vob 720x480 29.970030 FPS files, simply saving them to a container results in a almost 1 second video stutter at the beginning. I first thought this happens when merging together several .vob files, but it happens even on just one. I don't know how accurate the FPS display in VLC is but with .mkv, I get 29.970628 FPS, with .mp4 29.967584, with .avi 29.970000 (doesn't support AR forcing by the way, I know the format is considerered obsolete but still).

However, if I instead cut the file, there is no visible stutter at the beginning, and if I select .mp4 output, VLC even shows the same FPS as with the input file. It seems to me this is some kind of bug in avidemux or a defect in the decoder (shown as Lavcodec). Maybe it only accepts 29.97 FPS and drops frames to get there? At any rate, playing the .vob file in VLC does not show this stutter at the beginning.

[eumagga0x2a]

I have noticed that with MPEG-2 .vob 720x480 29.970030 FPS files, simply saving them to a container results in a almost 1 second video stutter at the beginning.

Please provide a sample (just a 50, max. 100 MB large head portion of the file should be enough) via WeTransfer, Mega, Dropbox or Google Drive. Not sure what you mean by "simply saving them to a container" vs cutting. What matters, it is copy mode vs re-encoding.

FPS deviations originate mostly from dropped undecodable early B-frames in open-GOP streams.

[test32421]

I don't have an account with any of those services, but I'll look into it. And by saving to a container, I mean simply importing a .vob file to avidemux and doing nothing else other than select a container format and save it. But when I instead cut the beginning out, for instance, the "new" beginning of the output file does not seem to exhibit the issue.

And I'm aware of the issues with B-frames, but that is said mainly to occur with cutting operations from what I understand. If I just import a whole .vob file and save it, using copy mode, I don't understand why that would result in a changed framerate, but apparently it does.

[eumagga0x2a]

I don't have an account with any of those services, but I'll look into it.

No account (and no email address despite the website trying to suggest otherwise!) needed for WeTransfer for files less than 2 GB in size.

And by saving to a container, I mean simply importing a .vob file to avidemux and doing nothing else other than select a container format and save it.

Again, "save it" will either copy the video stream or re-encode it. The only bit of information this statement provides is that the segment layout was at its default state (the entire video selected).

Regarding VOB files from commercial video DVDs, I am not sure that an assumption that the layout of actual MPEG-PS streams matches file boundaries is valid. This may be fully intentional. In this case, loading VOB files in Avidemux may not produce expected results.