I think the signals available break down into these groups:
A) 1 to (roughly) 125 - analog TV signals.
B) 126 and up - digital TV signals
My PVR-250 will tune up through channel 125 so I can see anything that is supplied on the cable.
For the digital channels it is up to the cable TV company what they do but I believe that mine possibly uses encryption on ALL digital channels. This is to ensure that if you buy your own settop box you still have to pay them a few dollars every month to view even the basic digital channels, much less the more advanced digital channels like HBO or HDTV.
To tune digital channels I am guessing that you'd need:
1) A tuner that can tune those specific frequencies 2) A way around the encryption, most of which are most likely illegal at least here in the U.S.