I think there's something wrong with your TV. I've never heard of an HDTV that will not accept 720p signals. That would be a crazily bad product to try to sell. ESPN, ABC, and FOX are all on 720p.
All HDTV tuners must accept ATSC broadcast signals in all the various versions including 480i/480p/720p/1080i. However many sets do not display 720p or 1080i (usually doing one or the other), instead converting the ATSC signal to the one they do. And although almost all will accept 1080i component signals, many sets, esp CRT older then 4 years old, will not accept 720p component. Even a few HDMI sets will not handle them.
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