need help on using ffmpeg for video grab

[Long Wind <longwind2009@gmail.com>]

I have bt848 card and watch TV with xawtv
As another application shows:

Device 00: BT848A video (Lifeview FlyVideo /dev/video0)
    Port 00: Television
    Port 01: Composite1
    Port 02: S-Video
    Port 03: Composite3

I select "Composite1" in xawtv
I try ffmpeg with this command:

ffmpeg -s vga -f video4linux2 -i /dev/video0 t2.mpg

then I "ffplay t2.mpg", it shows green screen
maybe I didn't specify "Composite1" in ffmpeg command, but how to do that?
or maybe I didn't specify PAL, but how to do that?