AM and FM radio stations use iBiquity’s in-band on-channel (IBOC) digital radio technology to transmit audio and data through a digital signal in conjunction with their analog signals. HD Radio is the trademark for this technology.
HD Radio was selected as the digital audio broadcasting method for the US by the Federal Communications Commission (FCC) in 2002. Officially known as NRSC-5 (NRSC-5B), it is the only digital system to be approved by the FCC for digital AM/FM broadcasts in the US. FMeXtra, Digital Radio Mondiale, Digital Audio Broadcasting and Compatible AM-Digital are the other digital radio systems.
HD Radio documentation uses the standard practice where the kilohertz signal rate is written next to its corresponding data transfer rate kbits/s. HD Radio allows for an all-digital mode and the system is used to simulcast both digital and analog audio within the same channel, by some AM and FM radio stations. Digital information was initially transmitted with the use of COFDM with an audio compression algorithm called Perceptual audio coder, and was later combined with an enhancer called Spectral band replication to improve audio quality at very low bit rates. This codec was branded as High-Definition Coding (HDC).
The AM Radio technology uses 20 kHz channel and overlaps 5 kHz into the opposite sideband of the adjacent channel on both sides while the FM stations subdivide their datastream into sub-channels of varying audio quality. HD Radio receivers cannot be used to receive DAB and DRM stations when moved overseas and require separate products for different countries. HD Radio specification is partly open but mostly private and offers an elegant transition from analog to digital radio.