Spoleto USA has become a fixture in Charleston. Chamber concerts a major attraction at the 12th festival

The Spoleto Festival USA, which just ended its 12th season, seems to have settled comfortably into the fabric of this historic Southern town. I attended the first and third festivals (in 1977 and '79) and had not been back since. The ambitious three-week assault on all the arts seems to have found its own special, consistently provocative blend of the unusual with the comfortingly traditional. I also found downtown Charleston to be startlingly changed, a fact the locals attribute directly to the festival.

Spoleto Festival USA was the idea of Gian-Carlo Menotti, the composer, who had founded the original Festival of Two Worlds in Spoleto, Italy, in 1958. Charleston's first festival was an uneasy event, mostly because the local citizens didn't quite know what to make of Menotti, his festival, or the crowds of music lovers it attracted, right from the beginning. And then there were often problems that arose because Menotti's forte lay in conception, not execution.

But it appears that the dark days are behind. The festival is strong in administration and organization, and Charlestonians have come to look upon Spoleto as the reason for the transformation of their city from a down-at-the-heels backwater into a handsome, shining light of Southern cordiality. The area that spreads away from the waterfront and up King and Meeting Streets has been completely turned around, and it now includes a huge but architecturally compatible hotel complex.

As for the performances, what was good about the festival 12 years ago is still as good, and the riskier ventures are of higher quality than in the past.

The centerpiece remains the chamber concerts at the delectable Dock Street Theater - a modern reproduction of an erstwhile 18th-century theater that stood a block from the current structure. The two programs I heard included works by Mozart (a quintet for oboe and strings), Brahms (the Op. 18 sextet), Schubert (a four-hand piece entitled ``Lebenst"urme,'' or ``Life's Storms''), Beethoven (``Kreutzer'' Sonata), and a world premi`ere of a handsome new solo cello piece by Leon Kirshner. The remarkable Carter Brey performed the Kirshner work brilliantly. Other musicians who shone in these satisfying performances included the exceptional oboist Douglas Boyd and the superb pianist Stephen Hough.

But Spoleto includes theater, dance, orchestra, even a circus, and these events happen all over town. I experienced a lovely staged performance of the 12th-century music drama ``Herod and the Innocents'' in St. Patrick's Roman Catholic Church. Under a sparkling new big-top tent in Marion Square, I watched the charming old-fashioned Circus Flora, named after the five-year-old elephant that is its sentimental star. Also included were a generous serving of assorted heart-stopping aerial acts, a splendid quartet of horses on which acrobats performed hair-raising feats, and even a trained buffalo!

The center of controversy at this festival was Martha Clarke's so-called theater piece, ``Miracolo d'Amore.'' Miss Clarke, who is the latest flavor-of-the-month theater conceptualist, must have meant to be chicly controversial, what with all the intentionally murky symbolism, nudity, mimed pornography, and an especially crass moment of blasphemy. Her piece is tedious, stale, and dated, as is so often the case with these art-for-their-peers creations.

Spoleto usually imports an orchestra for several concerts, and this year it was the Symphony Orchestra of Belgian Radio and Television (RTBF Orchestra). I heard its music director lead the players in Roussel's Third Symphony and the US premi`ere of Fr'ed'eric Van Rossum's Violin Concerto. The following evening, conductor Alkis Baltas was joined by pianist Fran,cois Thiry in a performance of Messiaen's rarely encountered ``Des Canyons aux 'Etoiles'' (``From the Canyons to the Stars'').

The orchestra is no better than any of our middle-rung regional ensembles, and they play with less collective spirit, at least under Andr'e Vandernoot's baton. But the Van Rossum concerto is an animated, agitated, engagingly scored work that allows the violinist Andr'e Siwy multiple opportunities to shine.

Messiaen's is a full-evening's work in three parts and 12 sections for reduced orchestra and expanded percussion. Though not his most consistently inventive score, it is nevertheless crammed with stunning moments, particularly the composer's musical translations of his beloved birdsongs. Mr. Baltas conducted with fervor; the orchestra seemed utterly committed to the score; and Mr. Thiry revealed an extraordinary affinity for the difficult music.

There are many things the festival still needs to address. Program notes are nonexistent, which is an egregious shortcoming. And Charleston still lacks a decent opera house: The Gaillard Municipal Auditorium is an unfriendly, though acoustically viable 3,000-seat cement barn of a hall, passably suited to orchestra concerts, but not opera.

Nevertheless, Spoleto Festival USA offers a unique opportunity for visitors to encounter a special city and provocative, enriching artistic fare.