Hi Frank. Welcome to HTF!
I'm afraid you are among the group of people "Most likely to return their HDTV".
Standard def video is a 1940's television standard designed for a 9" black-and-white tube. The same RF signal coming from your CATV coax will work on a 1940's television.
Most people return their HDTV's because they look great in the store and nobody tells them they need a HD source. The physical connection cable - makes no difference at all with SD video.
You have some options:
- Call your cable company and find out about "basic" HD service. (My company tried to sell me a $69/month package, but I now get 'basic' cable with 7 HD stations and a PVR for $24/month - no contract).
- Look into Dish or DirectTV for their new-customer signups that include HD channels.
Give it a shot, and let us know what happens. (You are not alone in this problem and your story will help others.)