As far as I can tell, yes the HDMI only accepts 1080i and 480p/i signals.
Here are some quotes from the manual that describe inputs:
HDMI: "The DIGITAL IN terminal can only be used with 1080i, 480i or 480p picture signals. Set the Digital Set -Top -Box DIGITAL OUT terminal Output setting to 1080i, 480i or 480p. For detailed information, refer to the Digital Set -Top -Box instruction manual. If you cannot display the picture because your Digital Set -Top -Box does not have a DIGITAL OUT terminal Output setting, use the component Video Input (or the S Video Input or Video Input). In thiscase the picture will be displayed as an analog signal."
Component: "This TV is capable of displaying 1080i and 480p DTV signals when connected to a DTV Tuner Set-Top-Box (STB). To view DTV programs, connect the STB to the component video input terminals (Y, PB, PR
) of the TV. Component color inputs provide luminance and separate color signal. Select the output of the STB to either 1080i or 480p."
Does this not mean that your STB or HD source needs to be outputting 1080i or 480p/i, even with component inputs? Most STB's can up-convert a 720p signal to 1080i or down-covert to 480p like the HD TiVo, can they not? If this is the case, George should be fine with the explorer recevier providing it outputs 1080i or 480p.
Does the onboard tuner convert incoming 720p signals to 480p?