In his lifetime (c. 1450-1517), the composer Heinrich Isaac was revered as second only to Josquin des Prez himself. Isaac probably had the better job: court composer to Emperor Maximilian I. Very unusually for the time, Isaac was employed only to write music, without any performing, teaching, or clerical duties (which helps explain his astonishingly large output, most of which is unfamiliar today). In the German-speaking lands at this time, music for the Mass was never through-composed, even for special occasions: half the verses were chanted, alternating with verses either set for choir by a composer or improvised by an organist. Only in Maximilian"s chapel would the music for choir (composed by Isaac) alternate with improvisations by the imperial chapel organist (Paul Hofhaimer and later Hans Buchner). Rebecca Stewart, Cappella Pratensis, and organist Wim Diepenhorst have set out to reproduce just such a Mass (for Easter Day) on this disc. There are more vigorous performances of Isaac"s luxurious choral writing (by the Tallis Scholars, for instance), but Stewart and her choir display the same clarity, distinctive vocal sound, and entrancing devotional quality you"ll find on their superb Josquin and Ockeghem discs. The impressive Diepenhorst (playing a 16th-century organ in Innsbruck) improvises his part following the principles and examples laid out in Buchner"s organ treatise Fundamentbuch. There"s only a little bit of plainchant on this disc (a pity, since Cappella Pratensis does it so well), and the alternation between the organ and Isaac"s five-voice writing makes for a rich (if not heavy) musical meal. You can always go on an Anonymous 4 diet later. --Matthew Westphal