HD Radio is a relatively new system that allows conventional (or terrestrial) AM and FM stations to broadcast their content over digital signals. Appropriating an abbreviation from high-definition TV, HD Radio offers better sound quality than AM and FM radio bands. It also allows stations to add more programming via several additional channels that can be broadcast “alongside” a station’s main frequency. Stations often use these subchannels to provide traffic or weather information, or diverse music content.

And just as with AM and FM, once you have a compatible radio, there is no subscription fee for the service. As of early 2008, there were more than 1,500 HD Radio stations across the U.S., providing coverage to 83 percent of the U.S. population, according to iBiquity Digital, the company that developed the technology. Still, HD Radio is only in its infancy; it’s used by just under one in nine U.S. radio outlets.

Unlike digital TV, HD Radio broadcasts won’t replace analog broadcasts (at least not in the near future) but will run parallel to them.

Stations supporting HD Radio simultaneously broadcast analog and digital versions of the same programming over the same frequency. With a regular radio, you’ll hear the usual analog version. With a HD-compatible radio, the radio tunes in to the digital programming on HD stations and the analog signals for non-HD stations.

If a HD Radio signal becomes too weak, the radio will automatically switch over to the parallel analog signal.

Here’s more on how HD Radio differs from regular analog radio:

It offers better sound quality. In our tests, HD Radio can live up to its promise of improved sound. The HD Radio sound quality delivers deeper bass, higher treble, more stereo separation, and a greater dynamic range (the difference between the loudest and quietest sounds) than FM or AM signals.

At its best, HD Radio pushes FM sound quality closer to that of CDs and makes AM broadcasts resemble those of analog FM.

Moreover, the HD signal from AM stations is in stereo, and there is no background noise—the hiss or crackle you occasionally hear with standard radio.

Programming is more varied. Of the more than 1,500 stations that have converted to HD Radio in the U.S., more than 750 have added a second (and some even a third) subchannel with different programming than the station’s main frequency.

You tune in the subchannels with a tap of a toggle or a turn of the tuning knob; they appear in the display with a designation such as FM2.

The subchannels typically carry programming of a different music genre than the main service. For example, Chicago-based WVAZ (102.7 FM) offers adult album alternative music on its HD1 broadcasts and gospel on its HD2 subchannel. WKTU, a New York station that plays hip-hop and rap on its main channel, uses its HD2 service to play country music—a genre that’s all but absent from the New York radio landscape.

(For a full list of HD Radio stations and their formats, visit www.hdradio.com.)

In many cities, the subchannels are bringing welcome diversity to the airwaves, and according to iBiquity, stations will eventually be able to broadcast up to eight subchannels.

Still, don’t expect anywhere near the variety you get with satellite radio. Most HD Radio stations carry no commercials on subchannels, but the main channels carry the usual commercials.

You get more information. As with satellite radio, HD Radio stations can show you the song title, artist, and other data on a display. Some stations also use the display to provide local traffic, weather, stock prices, news alerts, and more.

In the future, using technology to identify specific receivers, a vehicle could receive designated signals, whether additional audio channels (concerts or extra sports channels, for example) or specialized services, like “closed-caption” text for the hearing impaired. Such services would be broadcast by a station along with its main channels.

Tag and buy. Some HD stations enable “tagging,” which lets you flag a song for later purchase through iTunes. This requires an iPod with a dock and a radio with tagging capability.

Expect some service interruptions. In our testing around the New York area, we had little trouble tuning in many New York-area HD Radio stations. With some, however, we received the analog signal but not the digital one. When the digital signal for the main (HD1) service wasn’t strong enough, the radio efficiently switched to the analog broadcast.

When the digital signals faltered for an HD2 subchannel, however, programming simply stopped, resulting in a pattern of interruptions.

HD-compatible aftermarket radios for the car are on sale from Alpine, JVC, Kenwood, Sony, and others, typically with prices of $120 and up.

Your choices include in-dash HD Radio compatible head units, tuners that connect to radios designated HD Radio Ready, tuners that connect to factory-installed audio systems behind the dash, and transportable units that can be used both in the car and at home.

Some automakers are offering HD Radio systems as factory options in certain models. BMW, for example, offers HD Radio in all of its models, and the Ford Motor Company offers it in nearly all of its 2008 Ford, Lincoln, and Mercury models.

Table models for home from Boston Acoustics, Cambridge Soundworks, Jensen, Sangean, and Sony have been pricey, at $200 and up. The first $100 home model, from the manufacturer Radiosophy, set a new price point for other makers to match.