The appeal of the vocal quartet New York Polyphony goes beyond simply the robust elegance and clarity of its singing — though that alone is formidable enough. For its free concert at San Francisco State University this weekend, part of the indispensable Morrison Artists Series, the quartet will perform music by some of the great names of the Renaissance, including Francisco Guerrero, Antoine Brumel and Jacob Clemens non Papa (who got that moniker so that no one would confuse him with the pope). [...] the program also includes a skein of contemporary works, many of them written expressly for the ensemble, that respond in some way to the Renaissance vocal tradition.