You may want to go into the settings for the receiver and disable/uncheck the boxes for 480i/p so that only HD-level signals are passed on to your TV. If you watch a non-HD program, your box will upconvert the signal to the lowest available/matching resolution you have allowed in the settings.
When I had my 2nd HR20 installed several months ago, the installer had intentionally go into the settings and disbaled everything except 480p, claiming that it was the ideal setting for watching HD. I just nodded and let him leave then immediately went and undid what he had done. So if you never touched those resolution settings, you may want to double check them.