The shaman of postmodern jazz hasn't visited Chicago in years, and that alone should impel you to his doorstep next week: Ronald Shannon Jackson's continually evolving music may give you either palpitations or merely a headache, but you sure won't be bored. As a drummer, Shannon can create rhythms that are actually unique in their layered metric complexities--but he's just as likely to settle into a simple Texas blues. As a composer, he has done more than anyone else with the Harmolodic Concept of his former employer Ornette Coleman, writing some of the most exhilarating speedlines--but also some of the most purely gorgeous ballads of the decade. (And I haven't even mentioned his recitations of Shakespeare and Michael Harper, accompanied by his own drumming.) The Decoding Society once carried horns and the guitar army known as Vernon Reid; the current version, a leaner machine, plays a meaner blues, strips the melodies down to brass tacks, further punishes the boundary between jazz and rock. The Society now sports two guitars, two basses, and (naturally enough) enough rhythmic heart to swing the Tabernacle Choir. Thursday, Biddy Mulligan's, 7644 N. Sheridan; 761-6532.