HD Radio (HDR) is a trademark for in-band on-channel (IBOC) digital radio broadcast technology. HD radio generally simulcasts an existing analog radio station in digital format with less noise and with additional text information. HD Radio is used primarily by FM radio stations in the United States, U.S. Virgin Islands, Canada, Mexico and the Philippines, with a few implementations outside North America. HD Radio transmits the digital signals in unused portions of the same band as the analog AM and FM signals. As a result, radios are more easily designed to pick up both signals, which is why the HD in HD Radio is sometimes referred to stand for Hybrid Digital, not "High Definition". Officially, HD is not intended to stand for any term in HD Radio, it is simply part of iBiquity's trademark, and does not have any meaning on its own. HD Radios tune into the station's analog signal first and then look for a digital signal. The European DRM system shares channels similar to HD Radio, but the European DAB system uses different frequencies for its digital transmission. The term "on channel" is a misnomer because the system actually sends the digital components on the ordinarily unused channels adjacent to an existing radio station's allocation. This leaves the original analog signal intact, allowing enabled receivers to switch between digital and analog as required. In most FM implementations, from 96 to 128 kbit/s of capacity is available. High-fidelity audio requires only 48 kbit/s so there is ample capacity for additional channels, which HD Radio refers to as "multicasting". HD Radio is licensed so that the simulcast of the main channel is royalty-free. The company makes its money on fees on additional multicast channels. Stations can choose the quality of these additional channels; music stations generally add one or two high-fidelity channels, while others use lower bit rates for voice-only news and sports. Previously these services required their own transmitters, often on low-fidelity AM. With HD, a single FM allocation can carry all of these channels, and even its lower-quality settings usually sound better than AM. While it is typically used in conjunction with an existing channel it has been licensed for all-digital transmission as well. Four AM stations use the all-digital format, one under an experimental authorization, the other three under new rules adopted by the FCC in October 2020. The system sees little use elsewhere as the DAB standard predominates outside North America and has been in widespread use since the 1990s.
History This standard was meant to supersede other existing stereophonic standards on AM. iBiquity developed HD Radio, and the system was selected by the U.S. Federal Communications Commission (FCC) in 2002 as a digital audio broadcasting method for the United States. It is officially known as NRSC‑5, with the latest version being NRSC‑5‑E. iBiquity was acquired by DTS in September 2015 bringing the HD Radio technology under the same banner as DTS's eponymous theater surround sound systems. The HD Radio technology and trademarks were subsequently acquired by Xperi Holding Corporation in 2016 as part of its merger with DTS. HD Radio is one of several digital radio standards which are generally incompatible with each other:
FMeXtra was a competing U.S. standard, but has been stagnant since the 2010s. Compatible AM Digital (CAM‑D) for AM stations. Digital Audio Broadcasting (DAB), also known as Eureka 147, is the most common standard in Europe. Digital Radio Mondiale (DRM‑30 and DRM+ configurations) was intended mostly for shortwave radio. By May 2018, iBiquity Digital Co. claimed its HD Radio technology was used by more than 3,500 individual services, mostly in the United States. This compares with more than 2,200 services operating with the DAB system. A 400 kHz wide channel is required for HD FM analog-digital hybrid transmission, making its adoption problematic outside of North America. In the United States, FM channels are spaced 200 kHz apart as opposed to 100 kHz elsewhere. Furthermore, long-standing FCC licensing practice, dating from when receivers had poor adjacent-channel selectivity, assigns stations in geographically overlapping or adjacent coverage areas to channels separated by (at least) 400 kHz. Thus most stations can transmit carefully designed digital signals on their adjacent channels without interfering with other local stations, and usually without co-channel interference with distant stations on those channels. Outside the U.S., the heavier spectral loading of the FM broadcast band makes IBoC systems like HD Radio less practical. The FCC has not indicated any intent to end analog radio broadcasting as it did with analog television, since it would not result in the recovery of any radio spectrum rights which could be sold. Thus, there is no deadline by which consumers must buy an HD receiver.
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