Wes Leatherock asks,
IBOC stands for "in band on channel", and is the digital radio broadcating system used in the US. It is marked as "HD Radio" (for hybrid digital, not that other HD) and is licensed from Ibiquity, a CBS-affiliated company. It uses compressed digital streams on subcarriers via the regular AM or FM transmitter.
Digital audio broadcasting has been rather a flop in the UK, where it uses its own frequencies. IBOC (HDR) is not selling all that well in the US either, but the same radios do receive analog broadcasts, AM and FM, too. I don't know why so few HD radios are on the market; perhaps the license fee is too high.
***** Moderator's Note *****In relation to telecom, I wonder if digital radio sales are flat because music players are being integrated into cell phone/PDA devices?