How HD Radio Works

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).

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.

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, HDRadio.com 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.