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In a move that mirrors national radio trends, Tucson's Clear Channel stations planned to add four new formats today.

But hardly anyone will be able to hear them.

That's because the four new stations are being broadcast in a new "high-definition" format, which requires special radios that aren't yet widely available.

HD radio can be a complicated and technical concept, but it's being hyped as the biggest change to radio since the advent of FM.

Terrestrial radio still reaches vast numbers of Americans, but it needs to compete with newer technologies like the Internet, iPods, CDs and satellite radio, industry experts say.

For the past decade, younger listeners have been dropping off the dial, favoring other forms of entertainment. The amount of time 12- to 24-year-olds spend listening to the radio, for example, has plummeted by almost 25 percent since the early '90s, according to Edison Media Research.

Radio stations are slowly adding HD digital signals. Clear Channel's four new formats consist of "side stations" that piggyback on their existing analog signals. Among other local stations, 93.1 KXCI-FM and KZPT-FM 104.1 are broadcasting digitally but have not rolled out new side stations.

Experts believe HD could usher in a new wave of radio and eventually overtake standard FM as the way we listen.

Here's what you need to know.

Instead of the muffled voices and background hisses that go hand in hand with standard radio listening, HD radio is full and clear. FM stations come in like a CD, and AM stations sound like FM.

Why the improvement in sound quality? HD radio uses a digital signal, instead of the analog signal used by standard radio.

Because of the way digital radio signals are transmitted, digital radio doesn't suffer interference from buildings or get disrupted by bad weather, so the signal is clearer, says John Kelley, station manager for KUAT.

You may have experienced the digital difference with your cell phone — when you're in the boondocks and the phone switches to analog "roaming" mode, you get the same crackles and pops you hear when reception is weak on your standard radio.

● Digital radio means more stations.

HD radio could conceivably double the number of stations in any given market because when stations broadcast on a digital signal, they can split the allotted bandwidth and air two stations on the same amount of air space.

For example, with Clear Channel's new offering, instead of 93.7, listeners with HD radios now have the choice of 93.7-1 (with the same programming as the analog 93.7) and 93.7-2, a country side channel.

On the other side channels, 92.9-2 will play jazz, 98.3-2 old-school, hip-hop and R&B; and 97.1-2 will play Tejano.

To get to the side channel, tune to the original station on an HD radio. A touch of a button bumps the reception up to the side station.

Think of it like an office's Internet access, says Mike Irby, chief engineer for Clear Channel in Tucson. There's a set amount of bandwidth — but it can be chopped up to accommodate more than one person.

Right now, each station can split the digital signal into one side channel without losing quality. If the equipment continues to get better, it might be possible to divide each signal up three or four times.

● You have to have the right kind of radio.

Tucson stations — like KIIM-FM, which started sending out a digital signal in January, according to program director Buzz Jackson — have been broadcasting in HD for several months. Listeners just didn't know it.

That's because picking up the new stations — or existing ones in the new, crystal-clear format — requires a digital receiver. And hardly anyone has one.

Mass-market retailers aren't carrying them yet; they're easiest to find online. And they're still relatively expensive: Car radios cost at least $199; desktop models cost at least $250.

But prices are falling rapidly, Irby and other experts say. RadioShack could offer a desktop HD for $199 by as early as Christmas.

Retailers like Wal-Mart, Target and Best Buy will likely offer the new technology for $99 by next Christmas or even earlier, says Peter Ferrara, CEO of HD Digital Radio Alliance, a consortium of major radio companies that's been spearheading the rollout of HD technology.

But after the start-up investment, HD radio is free, unlike say, satellite radio, which charges a monthly fee, or iTunes, which charges by the song.

Tucson's new side channels are music only — like what you get when you turn on a cable music station — and commercial free until January of 2008. They ride the fence between locally and nationally programmed.

The Digital Radio Alliance drew up plans for 75 different formats. Clear Channel chose from that list, but then did extensive local research to decide exactly which songs to play, said Nikki Van Doran, director of marketing for Clear Channel.

The four side channels were designed for mass appeal and to coordinate with the existing stations, Van Doran said.

This is the "testing window," Van Doran said. "In 10 years, it'll be easier for someone to pick a bizarro format and make it stick. But right now, we're just trying to educate people."