I'm trying to do some basic color correcting on some HD material on my HDTV monitor...but I'm baffled by the mini DVI port on my 20" aluminum iMac...what do I need? I found a mini DVI -- DVI cable, but how do I get a composite signal out of this thing? It says on the tech specs page for the iMac it supports it?? Any links to what I need. Thank you so much!! Mark.