Record dvb-t on Ubuntu linux

Hello!!
I'm using a Ubuntu linux and running Mplayer with a dvb-T USB CARD
The USB CArd works correctly and I can use the tools scan, dvbstream,
etc....

I want to record my favorite dvb-t channels and I found in docs the way to
do it:

mplayer -dumpfile r1.ts -dumpstream dvb://CHANNEL

Or in avi file:

mencoder -o r1.avi -ovc xvid -xvidencopts bitrate=800 -oac mp3lame
-lameopts cbr:br=128 -pp=ci dvb://CHANNEL

The second doesn't work, but it isn't the problem, the problem is that I
want to do this record at night, programing it with cron or at

My question is that I can't found a way to do a  record with mplayer of a
determined duration, as 50 minutes.

I've being searching and I found and way to do this for recording audio with
mims, which supports this option:

and if this option isn't available; I've thougth doing a script and launch
it with cron, but how can i stop it after a determined duration??

Thanks in advance.

NOTE: I'm not very experienced in Linux and, at least, sorry for not very
correct english....I hope you understand it!!


Bye!!