[FFmpeg-user] Convert PAL to NTSC

[S. Fisher]

Usually films that are shot at a rate of 24fps are converted to
PAL's 25fps by simply speeding them up; no frames are added.
This results in a shorter playtime and an increase in the pitch
of the sound.

So to convert to NTSC's 24fps (24000/1001), all we have to do
is slow everything down; we don't have to change the number of
frames.

Here's a method that works well for me.  On ffmpeg's command line
the following appears twice---once before the input file and once
before the output file:

-r ntsc-film

To slow down the audio, add this to the audio filters:

atempo=0.959040959040959