The "HD" in HD Radio previously meant "hybrid-digital," but according to iBiquity Digital Corporation, it doesn't stand for anything; some news reports have mistakenly said it stands for "high-definition," as in HDTV. The two technologies are only similar in that they both replace analog with digital signals. HD Radio is iBiquity Digital Corporation's trademarked name.
As opposed to analog signals, which are prone to interference by anything from buildings to the weather, HD Radio is a digital signal and doesn't suffer from interference. HD Radio approaches CD-quality sound on FM stations and makes AM stations sound like regular FM stations, according to Crutchfield.com. HD Radio can be used by both AM and FM stations and, as of April 2009, 1,889 radio stations broadcast HD channels.
You must have an HD receiver to listen to HD Radio. There are around 100 models of HD radios, and they can be purchased wherever standard radios and other electronics are sold. Car, tabletop and home-theater receivers are all available. Because the technology is relatively new and most listeners don't yet have HD receivers, stations that broadcast in HD simultaneously broadcast in analog.
Unlike satellite radio, which requires a monthly subscription fee, HD Radio is free; the only expense is the receiver.
In addition to higher-quality sound, HD Radio offers multiple radio formats on one frequency. This is known as "multicasting." Stations that broadcast in HD refer to their secondary stations as HD2, HD3 and so on, so a pop station on the FM frequency 103.7 could also broadcast a rock station on 103.7-2. People with standard radios could only hear the original station, but those with HD receivers could tune their dials to the second signal. HD stations broadcast in a variety of formats, including country, news, Spanish, classical, oldies and more. Some stations are experimenting with new formats, and many operate commercial-free.
HD Radio's Tagging feature operates in conjunction with iTunes: If you're listening to an HD station and you hear a song you'd like to hear again, hit the "Tag" button and the song will be downloaded to your docking iPod. You'll see the song in the tagged section and, if you'd like, you can purchase it from iTunes.
Stations broadcasting in HD usually publicize this information on the air. HD Radio also offers an online list of U.S. HD stations at its website (see Resources).