[MEncoder-users] framerate denominator and numerator got much bigger after encoding

[JRaSH]

I transcoded an avi file frame-served by avisynth with xvid codec by
MEncoder. In the avs script, I manually changed the source framerate by
writing something lik "changefps(24000,1001)", which means the framerate
is about 23.976 with denominator to be 1001 and numerator to be 24000 (a
standrad NTSC film setting). But when I checked the ouput video file
after encoding, the denominator and numerator turned out to be bigger
than 100000000, though the calculated framerate is still approximately
23.976.

Could anyone tell me how I can control the behaviour of MEncoder so that
the denominator and numerator will stay the same as the source?

Thanks