Changing the frame rate

Sometimes it's useful to change the rate at which an AVI clip is played back and VirtualDub offers several ways of doing this.

Simple adjustment

One the Video menu select Frame rate.... Tick the Change frame rate to (fps) box and enter a new frame rate. Lower than the original for slow-motion, higher for speeded up video. In this example I'm changing a 25fps PAL clip to 12fps, approximately half speed.

The sound will go out of sync with the picture if you do this, but I don't care in this case because I'm going to add a new soundtrack later.

This feature doesn't create or destroy any frames, it just plays the originals at a different speed, so you can save with Video -> Direct stream copy.

Frame rate decimation

Throws away every second frame (or two out of three frames etc.). This option is useful for creating clips with a small file size without harming the still picture quality too much. You will lose audio sync so must recode (Video -> Full processing mode but not Audio) to get it back.

Inverse telecine

This applies to 30fps (60 fields interlaced) NTSC format clips made from 24fps feature films (ie. most US/Japan sourced DVDs). The frame rate conversion used for this is called 2:3 pulldown. What this means is the first frame from the cinema film is spread over two video fields (half-frames), the second over three fields, the third frame two fields again, the fourth frame three fields, and so on.

This process introduces blurriness, juddering on slow pans, and "combing" interlacing artefacts on freeze-frames. VirtualDub provides options to undo the damage. And the very best of luck in getting it to work! For more information see this Wikipedia article on 2:3 pulldown.