[Al Skierkiewicz]

In the business this is called cliff efffect. The effect occurs at a signal to noise ratio of 15dB. You get picture with 15.2dB and no picture at 15. Propogation is the same as analog since it is an analog signal (digitally modulated) that is transmitted. However, reflections (ghosting) cause considerable problems in the receivers. Since most digital channels were assigned in the UHF spectrum, line of sight communications is the generally the only acceptable way to receive DTV. Those living behind a hill or mountain that were able to receive analog transmissions with snow and ghosts will not receive DTV.
Since the signal is essentially a packetized MPEG format, loss of signal results in blocky video and loud distortion in audio. Many, if not most, stations are broadcasting multi channel signals consisting of one high def (720P or higher) program and one or more standard def programs within the same channel. My station is broadcasting one 720P, one standard def and two less than standard def program streams.