I recently found out how to use MEncoder to split and trim those mpeg files into single or multiple clips.
MEncoder has two command line options, -ss and -endpos, which let you define the start or end position of the clip you want to extract.
Unfortunately, the default command doesn't work with mpeg files.
The work-around is to convert the mpeg file to avi format first:
$ mencoder original.mpeg -ovc lavc -oac lavc -o original.avi
Then, create a copy starting or ending at a given point in time, defined as hour:minute:second using either the -ss or -endpos options.
For example, to extract a clip from the 17 minute 50 second mark to the 57 minute 47 second mark from a one-hour file, these two commands will do the trick:
$ mencoder -ss 00:17:50 -oac copy -ovc copy original.avi -o clip_start.avi $ mencoder -endpos 00:39:57 -oac copy -ovc copy clip_start.avi -o clip.avi
Note that the -endpos was recalculated for the second command as 39:57, not 57:47.
That's because the clip_start.avi file is 17 minutes and 50 seconds shorter than the original, and so we need to recalculate the clip end position in terms of the new length.
The file clip.avi contains the clip from 17:50 to 57:47 extracted from the original file, and we can discard the intermediate clip_start.avi file.
It takes two commands because MEncoder seems to ignore the second -ss or -endpos option it finds, and uses just the first one.
It would be nice if it would just let us do this instead:
$ mencoder -ss 00:17:50 -endpos 00:57:47 -oac copy -ovc copy original.avi -o clip.avi