I was asked by a neighbor about the difference between what is digital about Digital TV (DTV) and what is digital about Digital Satellite. He didn't understand why only a couple channels on his High-Definition DirecTV receiver were in HD. I managed to explain that HDTV is only a subset of DTV, but then he still thought that the rest should still be another form of DTV. I understand that that's not the case, but I couldn't give a clear explanation as to why. Anyone have one?