We're driving along I-395 in Fairfax and the signal on ordinary FM radio starts "picket-fencing" -- radio-geek lingo for the fuzzy flashes of static that pop up as you pass a big truck or when you stop at a red light.

Jan Andrews punches the button on the car radio that switches the service from analog to digital and suddenly, all the static is gone. On WHUR, Prince's singing takes on a striking new clarity. Over on AM, an even more dramatic transformation occurs, and suddenly, on SportsTalk WTEM, we hear more of Tony Kornheiser's voice than anyone ever had reason to want to hear -- deeper, more resonant, without the light fuzz that's normally a constant presence on AM.

Andrews, an engineer at National Public Radio, and Mike Starling, NPR's vice president of engineering, have taken me out for a spin to listen to the next aural revolution, digital radio. Marketed as HD Radio by iBiquity Digital, the Columbia-based company that developed and owns digital radio technology, the new sound is already being pumped out by nine Washington stations, even though there probably aren't 100 digital radios in use in the region.

By this fall, the hype for digital radio will be omnipresent. Radio stations will run promotions giving away receivers; buying the units will still set you back somewhere from $250 to $1,700. By next year, when the industry expects to sell 2 million digital radios, prices may fall below $200.

The sales pitch will focus only partly on digital's superior audio quality, which makes AM radio sound like FM does now, and turns FM signals into CD-quality sound.

The big draw will be multicasting, the additional programming that digital technology creates: Every station now on the broadcast dial, whether commercial or public, will be able to add a second channel, and possibly a third. Tune, for example, to WAMU (88.5 FM), and you'd hear the current news and talk programming, but if you scanned forward on a digital tuner, you'd still be at 88.5, but would hear the station's second channel, which might offer the bluegrass music that once filled much of the station's airtime.

By summer's end, NPR plans to offer public stations five program streams to choose from for their second channels -- classical, jazz, folk, progressive rock and electronica. Public stations that have dropped music programming in recent years to focus on more lucrative news and talk shows might choose to offer listeners some of the music formats that have been vanishing from the free airwaves over the past decade.

Earth-based digital radio is coming a little late to save the day; the satellite radio providers, XM and Sirius, have won nearly 5 million subscribers in large part by offering dozens of digital audio channels of music you can't hear on terrestrial radio. But traditional broadcasters believe digital will put them back in contention for listeners' ears, especially since terrestrial digital radio, unlike satellite, has no monthly fee.

"Terrestrial radio with its local programming will be strengthened by multicasting, but we'll all be sharing time amongst a lot of new devices" as technologies keep evolving, said NPR's Starling.

Only a few stations have announced what they'll do with their second channels. A Chicago country station is using the outlet to broadcast tunes from new country artists. A San Francisco public station that now broadcasts primarily news and talk shows, will add Cantonese- and Mandarin-language programs. A public station in Pittsburgh that splits its time between jazz and news will counterprogram against itself, offering its music on one channel whenever the other is devoted to news.

Second channels will likely be jukeboxes at first, playing music without deejays or commercials, but Vicki Stearn, spokesman for iBiquity, expects more ambitious, local programming to evolve as the number of digital radios grows. Others aren't as optimistic. Satellite radio executives, for example, say free digital radio will appeal to listeners who are reluctant to shell out $13 a month for radio, but likely won't threaten satellite's success because radio companies won't want to spend money on the staff needed to create local new programs.

As with FM radio in the 1970s, digital radio will grow mainly in relation to its ability to attract listeners to new content; the cleaner sound alone didn't do the trick for FM, and it won't for HD Radio. But eventually, stations will broadcast only digitally -- Stearn says that point probably won't be reached for 15 years -- and every radio you now own will become a useless relic that you'll try to unload on eBay. It's called progress.