you can use it with just one cable, what happens is one of the tuners doesn't work, essentially you lose the capability of recording one show while watching another, you can still record on one tuner. The pain in the butt though is that when you change the channel on a 630, it automatically switches to the next tuner. So if tuner 1 is on TSN and change to CTV, CTV will not be on tuner 2, so if your missing one tuner, you will get a no signal message. So essentially you have to change the channel twice to get to where you want to go. Hope this makes sense.