For Buddy Wachter, playing the banjo is much more than pickin' and chawin'.

Wachter, who solos Saturday with the Baltimore Symphony Orchestra at its final Oregon Ridge concert of the summer, has spent a lifetime and a career proving that the banjo is as versatile, as lyrical and as expressive as any Stradivarius.

By banjo, he means the four-string banjo. There is also a school of five-string banjo aficionados. The difference between the instruments is, of course, more than the superfluous (or, as the five-stringers would have it, the sine qua non) string.

"There's an old definition," says Wachter, who then switches to a broad Appalachian accent: "Five-strang yew got fangers; four-strang yew got notes."

The banjo was invented by African slaves in the antebellum South. Essentially the frame of a shallow drum with calfskin stretched across it, it had a fretless neck (it acquired frets around 1860) and five gut strings. It was played with bare fingers.

At the turn of the century, when its sound became distinctive to ragtime, someone thought of stringing the banjo with piano wire to make it more audible. But piano wire is far too tough for fingers, so banjoists acquired picks.

Here's where the family tree branches. The five-string banjo is played with finger picks, like false nails made of tortoise-shell. When the four-string banjo dropped the fifth string, which is a drone played with the right-hand thumb, the player could use the now-free thumb for leverage on a flat pickabout the width of a pocket comb.

A good player can make the flat pick as articulate as finger-picks, says Wachter, who can produce 15 notes a second, fast enough to play whirlwind pieces like Paganini's "Moto Perpetuo," a virtuoso violin work that he'll perform on Saturday's concert.

The real difference is not in what can be played with the two techniques, nor even in the slightly different tunings of the four- and five-string banjo, but in the kind of sound that emerges.

Though the banjo (however many strings) is now strung with stainless steel, the crisp strumming technique of finger picks makes the instrument ideal for the broken-chord harmonies of bluegrass. The high-water mark of the five-string banjo was probably "Dueling Banjos" from the film "Deliverance," which fixed its twang and brilliance in our ears.

The four-string banjo, with its softer sound, disappeared during the Swing Era: "not a happy time for the banjo," says Wachter. And it has never been as popular again as its more audible cousin.

But Wachter, 45, is a four-string banjo champion of many years' standing.

"There are some guys who play the five-string really well, and it has lovely, lilting, haunting notes," he says. "But I'm not as moved by it. Using one pick you get a more dynamic sound, more fire and power. To me, it's a lot more versatile and challenging."

A native Baltimorean, Wachter studied mandolin, guitar and banjo with Conrad Geblein, a Johns Hopkins professor who directed both the Baltimore Mandolin Orchestra and the Hopkins Banjo Club.

In 1968, when he was 15, Wachter auditioned for Fred Waring and His Pennsylvanians when the famous glee club played in Baltimore. Thanks to a special early graduation worked out with Overlea High School, he was able to join the Pennsylvanians for a two-year tour.

Since then, with a repertory that mixes classical and jazz works, he has played with pops orchestras under such conductors as Arthur Fiedler and Erich Kunzel; and with jazz greats such as Max Morath and Count Basie's All-Stars. He's even "dueled" with Bela Fleck, who is to the five-string banjo what Wachter is to the four-string.

Wachter believes in the intellectual respectability of the banjo. He concertizes internationally under the auspices of the U.S. Information Agency's "Arts America" program, which showcases folk as well as classical arts.

At his studio on a farm in Sparks, he's completing a video instructional course on the four-string banjo. And as part of the BSO's Arts Excel program, he goes into Baltimore public schools to teach science concepts such as the properties of sound -- which are easy to demonstrate on the strings of the banjo. "I try to show them that acoustics is not just something that lives in a book somewhere," he says.

His life's goal is to show that the banjo is the equal of any orchestra instrument.

"My job is not to be Mr. Solo and have [the orchestra] back me up," he says, "but to integrate it. I'm just beginning to do that. I'm a novice at it, but I just love it."

Saturday's program includes transcriptions of violin showpieces such as Sarasate's "Zigeunerweisen" ("Gypsy Airs"); classical works by Brahms, Chopin and Liszt; Broadway tunes; a Stephen Foster set; a ragtime set; and a bluegrass set.

Audience members are encouraged to bring their own banjos for a play-along jamboree.