It's not a

Star Trek

convention, but the annual Electro-Music Festival arouses similar passion and commitment. Now in its third year, the event - for lovers of theremins, vintage synths and vocoders,

musique concrète

and circuit bending - is billed as the largest gathering of electronic-music enthusiasts in the country.

Around every corner you'll find unconventional instruments handled by the offbeat musicians who have mastered them. Thirty of them, 10 from Philadelphia or the vicinity, will perform tomorrow through Sunday at the Cheltenham Art Center. The $60 three-day pass also covers lectures, demonstrations, workshops and jam sessions. One could call it a conference as much as a festival - a bonanza of education, artist development, boosterism, networking and community building.

Electronic music may seem like a fringe pursuit, but it is all around us. Programmed beats, samples, synthesizers and the like are an integral part of popular music. The sounds at Electro-Music tend to be more obscure than that, although no one style predominates.

Greg Waltzer, a cofounder of the festival who is based in Stroudsburg, Pa., cites examples "from noise to space music, jazz to classical, conventional to experimental, composed to improvised." He will appear Sunday night with festival cofounder Howard Moscovitz in a trio called Xeroid Entity. (A full schedule is available at event.electro-music.com. Live streaming audio will be available the whole weekend at radio.electro-music.com.)

The theremin, invented in the early 20th century and generally considered the first electronic instrument, will be played by four disparate artists, including Kip Rosser, the festival's volunteer publicist. Hailing from Lower Bucks County, he'll present music by Chick Corea, Frank Zappa and others, in a duo with jazz pianist Tara Buzash. Another theremin artist, Kevin Kissinger of Kansas City, Mo., will offer an ambitious real-time version of his hypnotic loop-based work "Three-Legged Race."

Electro Music 07, a sampler CD, gives a good idea of the festival's scope, from the relatively mainstream dance vibe of Mark Jenkins and Jonathan Block to the splintered, hyperkinetic beat-making of Fluorescent Grey, also known as Robbie Martin, out of the Bay Area. Other standouts include the ominous ambient jazz of Gemini and the oddly organic abstractions of guitarist Warren Sirota.

Vytear, an outstanding local drum-n-bass artist who does not appear on the sampler, will perform Saturday night, unleashing music he describes as "super computer head . . . rave blast anthems and bouncing."

Electronic music emerged as a proper discipline in the postwar era, and contemporary classical composers seemed naturally drawn to it. Rosser says the music addressed "a fascination with being able to make sounds that had no precedent." As years went by, the adaptability of those sounds proved endless.

Avant-garde jazz musicians such as George Lewis began to work with electronics in the '70s, and they continue to do so. But it was the late Robert Moog, inventor of the Moog synthesizer, who did more than anyone to bring electronics into the popular arena.

Among some musicians there remains a strong Luddite impulse, a tendency to view electronic instruments as somehow less than real, even a crutch. But to a growing number of critics, fans and artists themselves, the creative legitimacy of electronics is no longer controversial.

With more and more people accessing a veritable candy store of computer software and other high-tech tools, electronic music is undergoing a wave of democratization and globalization.

"People all over the world are experimenting, not only with the instruments, but with new kinds of music," says Moscovitz, a composer based in Allentown, who created the electro-music.com Web site in 2003 as a community resource for this burgeoning new field. "There are young kids who are just as comfortable with John Cage as they are with John Lennon," he adds. "And they're mixing all this music together."

Watching the Web site take off, Moscovitz sensed a need for a "face-to-face event" that would unite the site's users, and the 19th-century stone edifice in Cheltenham, a self-contained venue with multiple gallery spaces, seemed ideal.

Listeners can float from room to room on a whim. "You're not expected to like all of it," Moscovitz says. "Within 20 to 30 minutes, there'll be something else going on."

The organizers are big on artist accessibility - performers are asked to offer brief introductions before sets. "It's too mysterious if someone just walks behind a bank of equipment and starts playing," Moscovitz explains. The artists will also field questions and mingle and absorb the work of their peers, for they, too, are paying participants.

A curious absence from the program: the knob-tweaking experimentalists associated with Bowerbird, a new and innovative Philly-based concert series that will host its 100th event Saturday night at the Pageant:Soloveev gallery (at 607 Bainbridge - see bowerbird.org for more details). At Bowerbird, you'll hear avant-garde music, some of which is electronic; at Electro-Music, you'll hear electronic music, some of which is avant-garde.

There's little if any overlap between the two constituencies. But both represent important developments in alternative culture, in Philadelphia and well beyond.

As Moscovitz puts it, the Electro-Music Festival is an opportunity to "support the music and educate yourself on new phenomena in the very early phases. It's like hearing a folk music concert in New York in 1959, before Bob Dylan was famous."

To hear samples of electronic music from artists at the festival, go to http://go.philly.com/albumsEndText