"I have a little secret," says Isaac Stern, reaching inside the shoulder of his sports shirt in a scene from "From Mao to Mozart," a documentary film about the violinist's 1979 tour of China. He triumphantly extracts a square of foam rubber. This and other tips for violinists, including demonstrations in which he mimicks what a student does wrong -- devastating for the offender, whom he afterwards hugs in compensation, but very funny for the audience -- are strewn throughout the film. It adds up to quite a nice little inexpensive, if second-hand, master class. Stern gave only one concert during the trip, but spent a great deal of time listening to Chinese musicians, especially very young ones, playing Western music. He found amazing technical skill, but a lack of "passion and variety of color" in the playing. More awed at rapid-fire demonstrations of Chinese gymnastics and other sports, the pudgy Stern shrugs, smiles and comments philosophically, "On the other hand, they can't play Mozart." There is a humorous vignette, in which he listens to a rather pompous analysis and then gently disagrees with the theory that Mozart's genius consisted of an understanding of the socio-economic conditions of his time. And there is a shocking exhange in which an innocent query about inadequacies among students in their late teens and early 20s leads to a teacher's personal testimony about the extent to which Western music was considered criminal during the Cultural Revolution. But it's Stern's charm that holds together this Academy Award-winning 84-minute film, which is not quite a piece on music and not quite a travelogue. Sulking because the only piano in Shanghai is unplayable, or smiling beatifically at the music he is himself producing, he's an irresistible traveling companion.