HD Radio (HDR) is a trademarked term for the in-band on-channel (IBOC) digital radio technology. HD Radio is used by AM and FM radio stations – mostly in the United States, Canada, and Mexico, with a few implementations outside North America. The system transmits additional digital data associated with an existing radio station's standard analog signals, re-broadcasting the same signal in a digital format, with less noise. The digital data may also include up to three additional digital radio signals, which can be used to broadcast other stations within the same frequency allocation.
HD Radio allows for an all-digital mode. Three AM stations use it, one under experimental authorization, the other two under new rules adopted by the FCC in October 2020. All other AM radio and FM radio stations that use HD, simulcast both digital and analog audio on the same frequency (a hybrid digital-analog signal) as well as to add new subchannels and text information. HD Radio broadcasting's content is currently free-to-air, although prospective listeners still must purchase new receivers, in order to hear the digital content of the transmission.