[MEncoder-users] -fps usage (sync question)

[Rick C.]

Hello,

I have a general question.  For the most part mencoder works for me successfully, but every now and then I do a conversion which is slightly out of sync (audio and video).  Since this is a general question I don't have a specific example to post but using something basic like this:

mencoder /test.mov -o /test.avi -oac mp3lame -ovc lavc -lavcopts vcodec=mpeg4:vbitrate=1500

My question is with these random videos that are slightly out of sync it seems using -fps to force the demuxing frame rate fixes the issue.  In these cases normally I get a frame rate on the source video using mediainfo and pass that to -fps.  Does this make sense at all and if so can I just do this every time?  Also even if I don't use -fps and if I don't pass an output frame rate using -ofps will that cause the output video to be out of sync?  Anything to help me understand better what could cause a video to be out of sync would be great!  Thank you,

rc