Frame count per second, and TDA question

[spiritgumm]

I was trying to match a video clip to a dvd that is 704x480, 256 audio. I got it to work in SUPER, after trying Winff and gui4ffmpeg. When I previewed the clips in TDA 1.6, the Super was like other video I've seen - each thumbnail was "15" apart from each other. I had assumed that meant each thumb represented 15 frames per half second (you can frame-forward thru one thumb). The thumbnails for the Winff clip were 12 frames each. The gui4ffmpeg thumbnails were 18 each. Visually, the gap between the Winff thumbs was gone, whereas the gap between the gui4's thumbs were wider than the Super, so I guess that makes sense in a graphic way...but what do these numbers mean? Should I be worried about them? Is it okay to combine video that has different fame-counts per thumb? Is there a way to control them during conversion in those programs?
Example of Winff clip in TDA:
http://img444.imageshack.us/img444/1738/tdajm0.jpg

[guns1inger]

I suspect that TDA shows an image per I frame, or the first frame of each Group Of Pictures (GOP). Depending on the encoder and the settings it uses, this can be every 12, 15 or 18 frames for DVD compliant mpeg.

As far as authoring is concerned, so long the video is compliant there should be no issues.

[daamon]

the Super was like other video I've seen - each thumbnail was "15" apart from each other. The thumbnails for the Winff clip were 12 frames each. The gui4ffmpeg thumbnails were 18 each.

Visually, the gap between the Winff thumbs was gone, whereas the gap between the gui4's thumbs were wider than the Super, so I guess that makes sense in a graphic way...but what do these numbers mean?

Do some reading up on what a "GOP" is (Group Of Pictures) in the Glossary (the link is at the top-left of the page). Each GOP starts with an I-frame and then has a number of frames following it - these frames are partial frames based on preceeding and following frames. That's how the file is made smaller, or compressed.

What you're seeing is MPEG encoded with GOP lengths of 12, 15 and 18.

I had assumed that meant each thumb represented 15 frames per half second (you can frame-forward thru one thumb).

No. Each thumb is the I-frame from the start of the GOP.

I suspect that TDA shows an image per I frame

It's exactly that.

Should I be worried about them?

No.

Is it okay to combine video that has different fame-counts per thumb?

Yes, as long as they're all PAL or all NTSC and all DVD complaint.

Is there a way to control them during conversion in those programs?

Yes, you can specify GOP lengths in some encoders. I don't know Winff or gui4ffmpeg, but have SUPER and can't see that it's possible to set the GOP length.