"Sanctus . . ." great, slow-rolling triplets building to a tremendous climax punctuated with piercing clarino trumpets and pulsing tympani, ebbing and then building again: "Sanctus . . ." It is one of the peaks in Bach's monumental Mass in B Minor, music that requires an angel chorus to receive its full due. But yesterday in the Andrew Rankin Memorial Chapel at Howard University, the University Chapel Choir showed how close a well-trained group of humans can come to the celestial ideal.

Tommorrow is the 293rd anniversary of Bach's birth, and alert musicians are already begining to get ready for the extravaganzas that we can expect in 1985 (which will also mark Handel's 300th birthday). On the evidence presented yesterday, conductor J-Weldon Norris is one of those alert musicians, and his preparations include not only a fine-tuning of his performers' purely musical skills but a baroque music should be performed.

His chorus (working with a professional orchestra in yesterday's performance) has 42 voices, a good size for idiomatic Bach and a resource he refined still further by setting apart an 11-voice chamber choir for lighter texture. His six soloists were all well-chosen, particularly the eminent soprano Mattiwilda Dobbs and countertenor William Jones who sang the Solo alto parts.

At the present level of musicological enlightenment, it is no longer a rarity to hear a countertenor, but a voice as impressive as that of Jones, linked with his mastery of Style, will always be a rarity.

The performance was not quite perfect: a little more orchestral rehearsal would have helped, for example, but the total effect was most impressive and the chorus in particular was superb.