They all do ATSC, that is the basic HDTV format. But it can be encoded (modulated?) in a couple different kinds of signal to get sent to you, one of them being QAM. I think the other major one besides QAM is 8VSB, or something like that. And that is the signal that you would receive from antenna, I think. I don't know all that much about this, just enough to be dangerous.