Conductor Pierre Boulez is noted for the clarity of his approach to 20th-century music, and when it comes to his own compositions the French innovator, like Stravinsky before him, is often his own best interpreter. So it'll be a treat for the apprentice musicians of the Civic Orchestra when Boulez leads the orchestra in a dress rehearsal of Rituel. Completed in 1975 as a memorial to the Italian avant-gardist Bruno Maderna (1920-'73), Boulez's close friend and kindred spirit, Rituel is typical of the experiments with sonority and texture the two performed. The piece has 15 sections: the odd-numbered are to be played slowly and coordinated by the conductor, and the even-numbered have a moderate tempo and are open to improvisation. (Only the even-numbered sections will be presented at this rehearsal, though the Civic will perform the entire work next April as part of festivities marking the composer's 70th birthday.) The traditional orchestra is split into eight ensembles ranging from one oboe to a brass choir of 14, each featuring a different timbre. Not surprisingly, Rituel demands precise timing and sensitivity to modernist nuances--two qualities a bit beyond the grasp of the Civic, which sticks mostly to standard fare. However, with the patient and pedagogical Boulez on the podium, the orchestra is sure to get plenty of insight. Also to be rehearsed is Bartok's The Miraculous Mandarin. Bartok is one of Boulez's heroes, and his enthusiasm shines through in all his takes on the Hungarian. After the rehearsal Boulez will deliver a lecture called "Score, Imagination, and Reality." The lecture will be held at 2:30 in the Rubloff Auditorium of the Art Institute, Michigan at Adams. Both events are free, but tickets are required for admission. Sunday, 11:30 AM, Orchestra Hall, 220 S. Michigan; 435-6666.