How do you shoot on screen footage from a tv?

I've seen plenty of vids where someone is point their cam at a tv and recording it but when I try the tv screen gets big black line scrolling down it ?

Why does this happen? I know its possible to record a tv screen as its on the likes of gaming sites at latest events and theres plenty of vids like that on You Tube etc where epope,are clearly sat recording there tv (not plugged into it)

But when I try its practically impossible to see the screen?

Confused

Try changing the shutter speed on your camcorder that might get rid of the large black band that travels down the screen.

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that did it, thanx