adding subtitles to a avi file

hi, i have been trying to add subtitles to an avi file using virtualdub but the video quality always turns out bad and the film stops so far through. the avi file is xvid, i'v tried doing it in divx and xvid both ways have same problem, divx ends up about 200mb less than the original and the xvid ends up about 350mb less. the subtitles themselves seem to be in sync ok, they are in a .ssa file. any ideas as to what i'm doing wrong please?

Hi! Did you create the both your original .AVI and your .SSA subtitle file from the same source (e.g. a commercial DVD)?

no there not from the same source

With all of the problems you've listed (poor video quality, playback stops, resultant combined file shortened), I'd suggest that you go back to the original source and re-rip it - and compress/combine video, audio, and subtitles as one step, perhaps in VirtualDubMOD...