Why does VDub[Mod] output 29.97 FPS NTSC to 29.971 FPS?

[MaximRecoil]

Recently when I was encoding some purely interlaced 29.97 FPS NTSC video to AVI (DGIndex + AviSynth + DGDecode.dll + VDubMod) I noticed that the finished AVI file always showed as 29.971 FPS in Gspot, though VDub's "File Information" dialog claims it is still 29.97 FPS. The strange thing is, if I go (in VDubMod) to Video > Frame Rate > Source Rate Adjustment and change the default setting of "No change (29.970 fps)" to "Change to" and manually input "29.970"; when it finishes the AVI shows precisely 29.970 FPS in Gspot and in VDub's "File Information" dialog.

[LocalH]

Does the 29.971fps AVI play through to the end without introducing audio skew? If the video leads the audio, or the other way around, you ought to fix it. But if it plays with perfect audio sync, then I wouldn't worry about it. That is odd that it goes to 29.971 however, because even if you round it up, the exact frame rate (30000/1001) rounds to 29.970.

[MaximRecoil]

Does the 29.971fps AVI play through to the end without introducing audio skew? If the video leads the audio, or the other way around, you ought to fix it. But if it plays with perfect audio sync, then I wouldn't worry about it. That is odd that it goes to 29.971 however, because even if you round it up, the exact frame rate (30000/1001) rounds to 29.970.
It plays fine with no sync problems both ways, i.e. with the default output @ 29.971 and when I force 29.970. I was a bit concerned about it on a hardware player though, that hooks to a TV, since a TV wants NTSC and only NTSC. Most likely the player can "fix" it on the fly but I don't see why it should have to. It doesn't make much sense that "No change (29.970 fps)" gives you a different framerate (29.971) than using the option just below it "Change to" and manually input "29.970"; you know, both of them saying "29.970" and all. I also noticed, when searching Google for "29.971" fps that this is a common frame rate for people encoding video to encounter, and also that certain applications (someone mentioned Adobe Premier) won't accept video with that non-standard frame rate. I think I am just going to force 29.970 fps to be on the safe side, unless anyone knows of a specific reason why I shouldn't:

This default setting gives you a 29.971 FPS AVI:

http://maxim.skyphix.com/29971.jpg

This manual input setting gives you a 29.970 FPS AVI:

http://maxim.skyphix.com/29970.jpg

[North2Polaris]

The following is from the Capture Guide, Section 6.2 available on this site:

NTSC: Due to a minor bug in VirtualDub the native NTSC framerate of 29.97 cannot be set. The capture will then be at a framerate of 29.971 fps, a framerate that AVI2SVCD will reject rendering the file rather useless. Therefore the fps MUST be set to 29.9697 to work with AVI2SVCD. (The option "Lock video stream to audio" only works for "Compatibility capture mode", so it doesn't do anything here.)

[MaximRecoil]

NTSC: Due to a minor bug in VirtualDub the native NTSC framerate of 29.97 cannot be set. The capture will then be at a framerate of 29.971 fps, a framerate that AVI2SVCD will reject rendering the file rather useless. Therefore the fps MUST be set to 29.9697 to work with AVI2SVCD. (The option "Lock video stream to audio" only works for "Compatibility capture mode", so it doesn't do anything here.) Is that relevant? I am not capturing video, I am encoding MPEG-2 video to AVI and setting the source framerate manually to 29.970 does give a file that is 29.970 FPS; but leaving it at "No change (29.970) doesn't.

That Guide did show up when I searched Google for 29.971 FPS as the third result BTW, I just didn't think it had anything to do with what I was doing, since I am not capturing video and I didn't have to set anything to 29.9697 FPS in order to get a 29.970 FPS AVI (I simply set the source rate manually to 29.970 as I stated above).

[North2Polaris]

Is that relevant?

@MaximRecoil,

Not sure if it is relevant, but I have always remembered that particular section of the Capture Guide (a "minor bug" in VirtualDub that has been mentioned over several versions of the Guide) and it seemed similar enough to bring to your attention. Wilbert, the lead author of the Capture guide, may know more about the origin of this statement and may be able to point you in the right direction.

North

[neuron2]

I suggest posting your query at the following site, where Avery will see it and, I'm sure, explain things.

http://forums.virtualdub.org/index.php

[Wilbert]

Wilbert, the lead author of the Capture guide, may know more about the origin of this statement and may be able to point you in the right direction.
It was added before we started adding stuff to it. I suggest, like neuron2 says, asking Avery the status of this. Perhaps it's corrected a long time ago.