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Question: Getting 7290p on TC-P42X3

A confusion. I just bought this model and it seems wonderful. However, when I press the Information button, it tells me it is 480i. My cable box is set to 720p. What do I have to do to get this TV to show 720p? I cannot find a way to see the exact resolution, but the info button should say 720p, right? I have changed the aspect ratio, but still 480i. Playing a DVD still says 480i.

Thanks, Paul