I've managed to joing MPEG-1 files of different frame rated via TMPGEnc, but sometimes it doesnt work (error says the file is not compatable with the other file).

"Everything" else is the same between these files (I made, and compressed in VDub, then converted to MPEG).

Going back and re-changing the frame rates so the two files match solves the problem. Fine..............but why does it work with some, but not others???....is there a requirement that must be met when joining two files of differing frame rates?

Very puzzling. If it just plain did not work at all it'd be OK...but the sometimes it does sometimes it doesn't aspect is maddening. What is the trick I'm missing???