HD Radio


HD Radio is a trademark for in-band on-channel 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 due to its reliance on the sparse allocation of FM broadcast channels in North America; in Europe, stations are more tightly spaced.

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 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.
HD Radio is one of several digital radio standards which are generally incompatible with each other:
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 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.

Technique

Digital information is transmitted using OFDM with an audio compression format called HDC. HDC is a proprietary codec based upon, but incompatible with, the MPEG-4 standard HE-AAC. It uses a modified discrete cosine transform audio data compression algorithm.
HD equipped stations pay a one-time licensing fee for converting their primary audio channel to iBiquity's HD Radio technology, and 3% of incremental net revenues for any additional digital subchannels. The cost of converting a radio station can run between $100,000 and $200,000. Receiver manufacturers who include HD Radio pay a royalty,
which is the main reason it failed to be fully-adopted as a standard feature.
If the HD receiver loses the primary digital signal, it reverts to the analog signal, thereby providing seamless operation between the newer and older transmission methods. The extra HD‑2 and HD‑3 streams do not have an analog simulcast; consequently, their sound will drop-out or "skip" when digital reception degrades. Alternatively the HD signal can revert to a more robust 20 kbit/s stream, although the sound quality is then reduced to conventional AM-level. Datacasting is also possible, with metadata providing song titles or artist information.
iBiquity Digital claims that the system approaches CD quality audio and offers reduction of both interference and static. However, the data rates in HD Radio are substantially lower than from a CD, and the digital signals sometimes interfere with adjacent analog AM band stations..

AM

The AM hybrid mode uses 30 kHz of bandwidth, and overlaps adjacent channels on both sides of the station's assigned channel. Some nighttime listeners have expressed concern this design harms reception of adjacent channels with one formal complaint filed regarding the matter: WYSL owner Bob Savage against WBZ in Boston.
The capacity of a 30 kHz channel on the AM band is limited. By using spectral band replication the HDC+SBR codec is able to simulate the recreation of sounds up to 15,000 Hz, thus achieving moderate quality on the bandwidth-tight AM band. The HD Radio AM hybrid mode offers two options which can carry approximately 40~60 kbit/s of data, with most AM digital stations defaulting to the more-robust 40 kbit/s mode, which features redundancy.
The digital radio signal received on a conventional AM receiver tuned to an adjacent channel sounds like white noise – the sound of a, or a large waterfall, or a strong, steady wind through a dense forest canopy, or similar.

All-digital AM

All-digital AM allows for two modes: "Enhanced" and "core-only".
  • In enhanced mode, the primary, secondary and tertiary carriers are transmitted, allowing for a maximum throughput of 40.2 kbit/s while using 20 kHz of bandwidth out to the station's 0.5 mV/m contour. Inside this contour, stereo audio along with graphics and text information can be decoded by the receiver.
  • In core-only mode, the station only transmits the primary carriers.
When the receiver can only decode the primary carriers in either mode, the audio will be mono and only text information can be displayed. The narrower bandwidth needed in either all-digital mode compared to hybrid mode reduces possible interference to and from stations broadcasting on adjacent channels. However, all-digital AM lacks the analog signal for "fallback" when the signal is too weak for the receiver to decode the primary digital carrier.
Five AM stations have operated as all-digital / digital-only broadcasters, either on an experimental basis or under new rules adopted by the FCC on 27 October 2020 that allow any AM station to voluntarily choose to convert to all-digital operation.
  • WWFD was granted special temporary authority from the FCC in July 2018 to broadcast all-digital. It continues broadcast in digital-only mode under regular authorization.
  • WMGG broadcast in all-digital mode from January 2021 until a new owner returned it to analog-only mode following a station sale in November 2021.
  • WFAS broadcast in digital-only mode from May 2021 until its license was deleted in October 2024.
  • WSRO broadcast in digital only-mode from December 2021 until the station went silent in March 2023. After a brief return to the air in 2024, its license was cancelled in March 2025.
  • WYDE broadcast in all-digital mode from September 2023 until June 2025 when it returned to broadcasting in hybrid mode.
The FCC requires stations that wish to multiplex their digital AM signals to request and receive permission to do so. WWFD experimented with using a digital subchannel, operating a second channel at a low data rate while reducing the data rate of the primary channel. In October 2020, the FCC concluded from WWFD's experiments:

FM

The FM hybrid digital / analog mode offers four options which can carry approximately 100, 112, 125, or 150 kbit/s of data carrying compressed digital audio depending upon the station manager's power budget and desired range of signal. HD FM also provides several pure digital modes with up to 300 kbit/s rate, and enabling extra features like surround sound. Like AM, purely-digital FM provides a "fallback" condition where it reverts to a more robust 25 kbit/s signal.
FM stations can divide their datastream into sub-channels of varying audio quality. The multiple services are similar to the digital subchannels found in ATSC-compliant digital television using multiplexed broadcasting. For example, some top 40 stations have added hot AC and classic rock to their digital subchannels, to provide more variety to listeners. Stations may eventually go all-digital, thus allowing as many as three full-power channels and four low-power channels. Alternatively, they could broadcast one single channel at 300 kbit/s.
FCC rules require that one channel be a simulcast of the analog signal so that when the primary digital stream cannot be decoded, a receiver can fall back to the analog signal. This requires synchronization of the two, with a significant delay added to the analog service. In some cases, particularly during tropospheric ducting events, an HD receiver will lock on to the digital stream of a distant station even though there is a much stronger local analog-only station on the same frequency. With no automatic identification of the station on the analog signal, there is no way for the receiver to recognize that there is no correlation between the two. The listener can possibly turn HD reception off, or listen to the distant stations and try to get a station ID.
Although the signals may be synchronized at the transmitter and reach the receiving equipment simultaneously, what the listener hears through an HD unit and an analog radio played together can be distinctly unsynchronized. This is because all analog receivers process analog signals faster than digital radios can process digital signals. The digital processing of analog signals in an HD Radio also delays them. The resulting unmistakable "reverb" or echo effect from playing digital and analog radios in the same room or house, tuned to the same station, can be annoying. It is more noticeable with simple voice transmission than with complex musical program content.
Stations can transmit HD through their existing antennas using a diplexer, as on AM, or are permitted by the FCC to use a separate antenna at the same general location, or at a site licensed as an analog auxiliary, provided it is within a certain distance and height referenced to the main analog signal. The limitation assures that the two transmissions have nearly the same broadcast range, and that they maintain the proper ratio of signal strength to each other so as not to cause destructive interference at any given location where they may be received.