How HD Radio Works

Close up photo of a radio station microphone in a recording studio.
HD Radio promises higher quality and more stations. aire images / Getty Images

Radio hasn't really changed much in the past 80 years or so. The last "big thing" was the widespread adoption of Frequency Modulation (FM) in the 1960s and 70s. Now a new radio format is set to add features to your radio listening experience: HD Radio. This new technology promises higher quality sound, more stations with the same amount of bandwidth and extra information beamed straight to your radio as a digital signal. HD Radio is not without its detractors, however, and it has a long way to go before it can be considered a success.

When most people first hear of HD Radio, they often assume it's like HDTV, but for radio. HD Radio is a very different technology, however. Unlike the "HD" in HDTV, the "HD" in HD Radio is not an abbreviation for "High Definition." Although some have said that it actually means "Hybrid Digital," according to iBiquity, "HD" is a trademarked brand name and doesn't actually stand for anything [Source: Crutchfield Advisor].


HD Radio won't replace traditional radio broadcasting -- it will be offered alongside it. So if you have no interest in what HD Radio has to offer, you don't have to worry that someday your trusty stereo receiver will stop picking up radio stations. However, the purchase of a receiver that can accept and interpret an HD Radio signal is necessary if you want to enjoy the extra features allowed by the new technology.

­ Just what are those new features? An increase in sound quality is immediately apparent. An FM radio frequency can only carry a certain amount of information, which is why FM sound quality is considered lower than CD quality. Amplitude Modulation (AM) frequencies carry even less information, which is why virtually all AM stations broadcast in mono. Their signal can't carry stereo information.

HD Radio sends more information over the same radio frequency by digitizing the signal. A digital signal can then be compressed by a computer, much like an MP3 is a compressed version of the much larger WAV file that is on a CD. Digitizing allows FM stations to broadcast with near-CD quality sound, and HD Radio AM stations will sound just as good as FM stations do now. An HD Radio receiver can also zero in directly on the digital transmission, eliminating interference and signal "reflections" off of buildings. The result is clear sound without any static, pop or hiss.

Another application of HD Radio is the ability to multicast multiple channels on the same signal. Multicasting also refers to a form of Internet routing architecture, so the multiple channel transmissions allowed by HD Radio might be more accurately described as multiplexing (iBiquity chose the term "multicasting").

The extra bandwidth of an HD Radio signal leaves room for completely separate channels operating on the same frequency. Here's how it works: Suppose your favorite radio station is 102.5 FM. When you tune in, you pick up the regular broadcast -- classic rock. Your HD Radio receiver indicates that the station is multicasting. You turn a dial to select the alternate channel, where they play nothing but blues. Or your favorite talk radio station might have separate, simultaneous broadcasts featuring news and weather, sports talk and political commentary. ­

Current HD technology allows the addition of one extra music channel and three extra talk channels before the sound quality begins to suffer. The extra stations are referred to as HD2 stations. If an HD Radio receiver loses the digital signal, it seamlessly fades back to analog FM until the digital signal returns.

­ An HD Radio receiver can display text data that is sent alongside the audio signal. This could take the form of artist and song information about the music being played or sports, weather or stock updates.

No radio stations are planning to go all-HD. They'll continue to broadcast regular analog signals along with the HD signals, so old radios will be able to pick up AM and FM stations with no problems in the foreseeable future.


HD Radio Signal

The RF spectrum of an FM radio station transmitting an analog stereo signal
Image used under GNU Free Documentation License

The radio waves that carry analog AM and FM signals are limited in the amount of information they can convey, both because of the nature of the waves themselves and the spacing of stations on the frequency band. FM stations are separated by 200 kHz, while AM stations have just 10 kHz between them. Alternate channel spacing of FM stations gives them 400 kHz to work with. The signals broadcast by radio stations are actually spread out on either side of the frequency into Upper and Lower Sidebands (USB and LSB).

The larger spacing between FM stations gives them enough room to broadcast higher quality audio in stereo, but not enough for true CD-quality audio. An FM signal could carry enough information for a higher-quality signal, but it would take up so much bandwidth that there would be room for far fewer stations on the dial.


HD Radio solves this problem by compressing the digital signal so more information can be sent along the same amount of radio bandwidth. Initially, iBiquity used a compression algorhythm called PAC (Perceptual Audio Coding). However, audio compressed with this method led to complaints about poor sound quality, so in 2003, iBiquity changed to HDC (High-Definition Coding), a different compression method that allowed for higher sound quality.

­ Although HD Radio is touted as providing CD-quality sound on the FM airwaves, the compression of the digital signal is a "lossy" method. That means that it discards some of the information in the signal, resulting in a reduction in overall fidelity to the original sound. Much of the discarded information is not perceptible to human ears, and the final product may sound very close to CD-quality, but technically it is not the same sound as on the CD.

The digital signal is sent out piggybacked onto the analog signal on the same carrier wave (the wave sent out by the transmitter that gives a radio station its frequency on the dial). A third signal can also be added -- this carries text data. It is an in-band on-channel (IBOC) system because the signal is piggybacked onto the analog signal and doesn't use up extra bandwidth. If the system was an in-band adjacent-channel (IBAC) system, the bandwidth would be increased by using space on nearby channels (and causing interference if another station was trying to broadcast there).

The RF spectrum of a broadcast FM radio station transmitting an HD Radio signal
Image used under GNU Free Documentation License

Implementation of HD Radio on the station side requires significant modification and upgrading, with cost estimates ranging from $30,000 to $200,000 [Sources: Crutchfield Advisor and Wired]. Because HD Radio is a proprietary system owned by iBiquity, stations also have to pay an annual licensing fee of $5,000.


HD Radio Receivers

An HD Radio Receiver
Photo courtesy Amazon

So far, the number of HD Radio receivers currently in homes or cars is low -- about 100,000 units. iBiquity hopes to sell four times that many before the end of 2006. One reason for the low market penetration was that first-generation HD Radio receivers were extremely expensive, costing more than $1,000. Today, you can buy car stereos that support HD Radio for about $200, and home theater receivers that can pick up HD Radio don't cost much more than other high-end receivers. There are also reports of a Kenwood home receiver that costs $100.

As of October 2006, reported 609 stations in the U.S. broadcasting some form of HD Radio. About half of those stations were multicasting, and very few were using data casting.


HD Radio has not had a smooth ride. In addition to compression method changes and cost issues, the technology has had to deal with controversy over the FCC's decision to name it the primary standard for digital radio in the United States, competition from other digital radio formats, complaints that the sound and reception quality don't live up to expectations and problems with interference with other signals.

Many people within the radio industry have voiced concerns that iBiquity's system was granted primary status by the FCC, because perhaps another digital radio format could perform better. Some claim that the people in charge of deciding that HD Radio should be the primary U.S. digital radio system are also the same people who have invested heavily in the technology, creating a conflict of interest [Source: Wired].

Eureka-147, a digital radio format popular in Europe, uses a separate frequency band for digital broadcasts instead of piggybacking the digital signal onto analog broadcasts. There have been attempts to get the FCC to open that band to digital broadcasting in the United States. Kahn Communications reported the development of a system to offer higher-bandwidth AM radio without many of the problems faced by HD Radio on the AM band, and without requiring new receivers to hear it.

While HD Radio is supposed to use the same amount of bandwidth as the original analog station, there have been complaints about interference, particularly with AM. AM waves travel much farther at night due to the bending of the waves within the ionosphere (See Why do you hear some radio stations better at night than in the day? for more.). Because of this, the FCC does not allow HD Radio broadcasts at night on AM stations. During the daytime, adjacent stations are not allowed to broadcast in HD because they create too much interference. Even on the FM band, the digital signal sometimes creates an audible sound on adjacent channels, particularly for hobbyists who try to tune in distant FM signals (known as DXing).

Those who have purchased HD Radio receivers have reported some problems. At times, the audio compression causes a noticable reduction in sound quality. Some radio stations are not properly processing their signal for digital transmission, causing distorted sound. At times, the volume level or timing of the digital signal is different from the analog version, resulting in problems when the receiver has to switch between the two [Source: Crutchfield Advisor]. Other users have found that digital signals are difficult to tune in, that multicasting seriously impacts audio quality, or that stations broadcasting in HD Radio suffer a loss in quality of the analog signal.

For lots more information on HD Radio and related subjects, check out the links on the next page.


Frequently Answered Questions

Is HD Radio Free?
Yes, HD radio is free. broadcasters can choose to offer additional content and features for a fee, but the core HD Radio technology is free.

Lots More Information

Related HowStuffWorks Articles

More Great Links


  • Dotinga, Randy. "Sagging Radio Plays Digital Card." Wired, June 14, 2005.,1412,67821,00.html
  • Ferency-Viars, Robert. "What You Need to Know about HD Radio." Crutchfield Advisor.
  • Junko, Yoshida. "Last-Minute Changes Blur U.S. Digital Radio Spec." TechWeb, August 15, 2003.
  • Nave, Rod. "Broadcast Signals."
  • Shapiro, Leslie. "Review of the Panasonic CQ-CB9900U HD Radio Receiver." Crutchfield Advisor, November 30, 2004.