HDTV won't work for my computer. What's wrong?

Alright so my video card supports HDTV. I've got the little HDTV dongle that came with the card. Today I decided to try this out, but for some reason all i get is staticy wavy colored lines on my tv. I checked and made sure all the colors were properly hooked up. And I've got the latest driver for my video card. Do I need a separate driver or some sort of software in order to use HDTV from my computer? I'm runnin an 8500GT. I'm using a normal RCA cable(red,white,yellow) because they're all the same, aren't they?