As far as HD radio, you have to be able to get a fairly strong signal on the main carrier, in my experience with a portable Insignia FM HD radio, must be 3 if not 4 bars out of 4, with no multipath. The HD1 signal which is also the same program the analog signal carries will revert back to the analog signal if the HD1 signal isn't strong enough, however the HD1 signal is a lot better in dynamic range, pretty much CD quality. For the stations that have HD2, HD3, and sometimes an HD4 subcarrier, it's all or nothing, just like digital TV. If the signal "fades out" on you, there's nothing until you get the signal back. HD2, HD3, and HD4 are generally less quality that the HD1 signal, similar to TV subchannels that are generally SD while the main .1 channel is generally HD. The only one I know of locally that goes all the way to HD4 is WKSU 89.7. WNWV used to have to HD3, but since going back to the Wave, dropped all their subs. I tried my portable in the car, using it's wire as an antenna, and spotty reception unless you're within maybe 5 miles of the stick. HD radio broadcasts at only 10% of the effective power output of the main analog carrier, in reality 1/100th of the wattage. However DTV puts out 1/10 of the power it did on analog for roughly the same signal area. The FCC may someday allow HD to go to 10% of the main analog carrier's power which will then approximate the analog signal's coverage, or at least allow the stations to increase it somewhat. I never tried a true HD car radio, but would guesstimate it would work fairly well in strong signal areas with occasional drop outs or reverting to analog if listening to an HD1 signal.