Missa 'O Quam Glorifica', Sanctus (Richard Taruskin and Cappella Nova)

In 1504, Robert Fayrfax (1464-1521) earned his Doctor of Music degree at Cambridge. A note in the Lambeth Choirbook says that he wrote the Missa 'O Quam Glorifica' "for his forme in proceeding to be Doctor" (spellings modernized). That period was very fond of rhythmic complexity, and the Missa 'O Quam Glorifica' amounts to the "Ne plus ultra" of that complexity.
The cantus firmus is the plainsong hymn O Quam Glorifica, a hymn in honor of the Virgin. Where it is present, it is placed in the second-lowest part, in very long notes, so we are never really made aware of it as a melody. (It's quite beautiful.) Above it, the other parts present a mirepoix of notes in small values, now duple, now triple, and usually in cross-rhythms with each other. The mastery which the piece demonstrates seems to lie in keeping the simultaneties in order in spite of all the activity.
Fayrfax was using the hymn in its English version, according to the Sarum (Salisbury) service books. Here is a recording of the hymn in a slightly different version. gregorian-chan...
The Missa 'O Quam Glorifica' was on the Harmoniae Britanniae concert which Cappella Nova presented in November of 1980. This particular "deep purple" (score) had faded very badly. It seemed to be most visible when scanned in grayscale, and, even at that, it required a bit of touchup with a pencil.