Weird VDub behaviour while cutting AVI

I've got this movie in AVI format which I am converting to SVCD. I've already done it once and tried to cut it afterwards with TMPGEnc's MPEG Tools but I couldn't get it to work. I don't know why, but maybe the computer ran out of memory or something, who knows.

Anyway, I decided to get around the problem by cutting the AVI instead and encoding the separate portions. I followed a guide on how to do this with VirtualDub.

The movie has 159004 frames and I want a 20 second overlap at each cut point once I've converted it to PAL. So the cut points I had are:

0-53251

52751-106253

105753-159004

What I get out of VirtualDub is segments with the following lengths:

53251 (correct)

53612 (should be 53502)

53406 (should be 53251 again)

I've done this three times with the same result, so I'm pretty sure I'm not delusional. Has anyone got any idea why this is happening, and is there a better tool out there for cutting AVI's which is a good deal more accurate than VirtualDub?

Could vdub be 'moving' the cutpoints to Keyframes?

Why not use TmpGenc's source range feature to encode the seperate parts instead, saves cutting the movie at all. Just do two seperate encodes, one for each half with appropriate settings in the source range.

Originally Posted by bugster

Could vdub be 'moving' the cutpoints to Keyframes?

Why not use TmpGenc's source range feature to encode the seperate parts instead, saves cutting the movie at all. Just do two seperate encodes, one for each half with appropriate settings in the source range.

Yeah I've tried this before though. Using source range the first MPEG plays perfectly while all subsequent MPEGs have a jerky motion to them. I don't know why this happens, but it does. I just put it down to a bug in TMPGEnc.

Originally Posted by bugster

Could vdub be 'moving' the cutpoints to Keyframes?

That sounds possible. The same "shifting" of the exact cut points I specify occurs whether I specify the points by frame number or time location.

What is a "keyframe" exactly anyway? There's no definition for it in the glossary.[/quote]

Originally Posted by mats.hogberg

...or set the range in VirtualDub and frameserve to TMPGEnc?

/Mats

Will this work if I'm *not* using the AVI file for the audio input?

I'm modifying the audio in GoldWave first and using a WAV file as the audio input. I know it works when you do the complete file as one encode operation, but will it still work if I'm only doing a portion of the complete file in a specified range?

How will TMPGEnc know to take the right section of audio when the audio is not being frameserved through VDub like the video?

VirtualDub will cut at the closest key frame for files using encoding techniques that use keyframes (MPEG for example). Scrolling through a preview of the file with VD, hold down the shift key and you will go from key frame to key frame only. You can probably use this to validate what you are seeing with your cutting.

I would never try to explain to you what a key frame is in detail- many people understand much more about the technique than I and there is documentation everywhere. As a very top level summary, a key frame contains all the full 'picture' for that frame. The intermediate frames contain only changes from the last frame. Thus, given an intermediate frame only, you cannot produce a full picture without also having all the previous intermediate frames back to the last key frame. There are lots of more complicated means of implementing this but the technique is one of the main means to provide the compression in formats such as the MPEG family. YOu can generally adjust the number of key frames (such as one every 15). This impacts the level of compression and may impact the 'jerkyness' of any fast forward or rewind (as the application used will likely only jump from key frame to key frame). Again, this is a very simple explanation and there are lots of exceptions and complicated extra techniques that can be thrown in (bi directional for example).

VD takes the simply approach by only cutting at key frames rather than try to build up the full frame to the selected cut point. As others have mentioned, there are lots of alternatives if you really need to have the exact number of frames - otherwise make your cut points based on the actual key frames and leave it at that.

Thanks for your explanation of what a keyframe is and confirmation that this is what is happening in VDub. I understand and appreciate that. Your explanation is more than good enough for my purposes.