JAZZ has been called the great American art form of this century and, while there have been many great musicians from other countries, its place of origin is not in dispute. Jazz was born in New Orleans. At the turn of the century, New Orleans was, as it is today, a musical and cultural melting pot. The African rhythms from 300 years previously had been kept alive by the plantation slaves, and these gradually became mingled with the rhythms of the Caribbean. The enduring tradition of the New Orleans brass band, best exemplified today by groups such as the Dirty Dozen Brass Band, dates back to the end of the Spanish-American war. Street parade brass music is still much a part of the city's everyday life and a New Orleans funeral, like an Irish wake, is more of a party than a good many weddings. Jazz evolved in New Orleans and was spread to the rest of the states by New Orleans' musicians. It has no single inventor, although Jelly Roll Morton, a Creole, whose real name was Ferdinand Lemott, staked a claim to it and was the first great composer of the genre. Louis Armstrong, the first great improvisational genius of the music, was also a son of the city and made the first great jazz records with King Oliver and later with the Hot Fives and Hot Sevens before going on to achieve international fame as a well-loved popular entertainer. New Orleans is still a city that lives and breathes music, according to Jon Cleary, one of the world's finest exponents of the city's tradition of piano music, who is appearing at the Hongkong Jazz Club from Wednesday to Saturday this week, people in NewOrleans often don't so much walk down the street as dance. Cleary's style of blues-based piano has its antecedents in great New Orleans' pianists such as Professor Longhair, James Booker, Allen Toussaint and Dr John.