[timgr]

Each real channel has one or more virtual channels multiplexed into the signal. Usually that means one HD channel and 2 or 3 SD channels, although I receive two RCH that have 4 SD VCH on them. I suspect there is not enough digital bandwidth on a single RCH for more than one HD stream. The signal is a compressed digital stream, encoded to the analog carrier. Your tuner locks on to the carrier and picks out the channels from the stream.

The cable companies pick these channels from the air or from satellite - I'd presume - and then send it down the cable using different encoding and more compression. Supposedly the additional compression is why these channels when distibuted by cable are of lower quality than the OTA TV. I get WGBH OTA here, and I was shocked at how good the picture quality is.