Comcast said i can’t but i want high def.
NO [shorter answer!!}
If the Comcast box has component connections for your cables (Red, Green, Blue) and your HD TV has component connections, You can connect the two items and try. You will also need red and white audio cables to get the sound into your TV. Your HD signal will be limited to 720p, not full HD.
If the cable box is not specifically HD, attaching component cables will deliver a standard definition signal to your TV at 480p. It is the best signal you can get from a non-HD box.
If your TV does not have a place for the component cables, but only accepts red, white, and yellow RCA-type cables, those are ‘composite’ cables and do not carry HD signals.