I just noticed a couple of mistakes in this thread. One...HD is 1/100th the power of analog, not 1/10th. But this isn't that important, because NOISE isn't the concern that it is with analog. All the radio needs is to be able to distinguish a one from a zero...which requires much less power.
Also omeone said that power was the determining factor for coverage. It's only one of them, and not necessarily the most important one. The presence of first adjacent channels is probably more of a limiting factor.
And finally someone equated bitrate with power. 48k is a measure of BITRATE (the number of bits per second for the stream..in the case of 48k it would be 48 thousand bits per second, or 48 kilobits per second), not the power of the HD signal. If you receive HD1, you should receive HD2 (if present) as well. The bits are interleaved...a part of ONE signal, not a "separate subcarrier". That's analog thinking