I like how the HD doesn't actually stand for High Definition. Now, it doesn't really stand for anything (like DVD now stands for nothing), but originally it was Hybrid Digital. Because, as Alan said, the digital signal is piggybacked on the analog information.
A friend of mine had the same thing happen with his cable service. He had a cable modem installed, and the company ran a new line just for the computer. Later he put a TV near the computer and just split that one. He noticed that the cable modem line didn't have the local commercials inserted and instead got all national ad, when the other TVs in the house would be playing some cheaply produced ad.
