Dec 23, 1998 at 12:00 am

This is music strict, pure and demanding. Four unaccompanied voices of the Hilliard Ensemble — David James, countertenor; Rogers Covey-Crump and John Potter, tenors; Gordon Jones, baritone — triumphantly negotiate the homophonic challenges of the Renaissance composer Orlando di Lasso.

Born Lassus in Mons, Belgium in 1532, he moved to Mantua, Italy at the age of 12 and therefore was at the center of the effort to create a new music. In the Renaissance, the creative spirit was spurred by the rediscovery of ancient Greek models. This was most difficult for composers, since only theoretical Greek texts had survived — no actual compositions were known. Two different interpretations emerged: a conservative diatonic method and a radical chromaticism. It is possible to see much of the subsequent course of Western music as the pursuit of these different compositional — and therefore different-sounding — methods.

"Missa pro defunctis," in the conservative style, is a requiem intended to be sung by clerics. Though chordal rather than monadic, it has the ethereal and hypnotic qualities so favored by burnt-out marketers of Gregorian chant.

"Prophetiae Sibyllarum" is much more interesting, much more radical. Utilizing all 12 tones of the chromatic scale, the work was intended for cognoscenti. The words of the introduction announce the chromatic program and the music demonstrates it — the listener is taken through 11 of 12 possible different tonal centers in the first eight bars! Each of the remaining 12 sections is set to a Latin verse composed by early Christians to "prove" that an ancient Greek soothsayer, or Sibyl, had foretold the birth of Jesus. Hence, Renaissance rhetoric and new music coalesce.

No non-professional musician-clerics could negotiate — as the Hilliard Ensemble can — the special singing required by these works. The four singers keep just intonation — mathematically precise tones, thus mathematically precise intervals — so that the work sounds in tune. Though the work progresses chordally, each vocal line is so skillfully woven with the others that no performer must sing difficult or awkward tonal progressions.

The listener will not be helped by the comforting presence of familiar classical forms — these were far in the future. Rather, given a grasp and memory of the work up to that moment, one will discover the pleasure of not being able to anticipate the next moment of sound.