All editions are based on the original Formschneider prints of 1550 and 1555 with reference to Pätzig’s dissertation which details variants between motets in the CC and manuscript sources.
The chant sources for this music are not always easily determined. At times, the corresponding chant found in the Liber Usualis or the modern Graduale Romanum is an almost perfect match. I have often relied on the Graduale Patavienese, the gradual used by the diocese of Passau, to which Vienna belonged during the time that Maximilian was Emperor (available in facsimile in Das Erbe deutscher Musik, Vol. 87). The often-lamented and missing Constance missal may or may not have been the source for some chant melodies but as it unavailable that is only a supposition. According to Burn (2002) Isaac may or may not have used that source. Where the chant can be identified (or suspected) it is signified by an “+” above that note.
No attempt is made to indicate the original mensural signature in every part. The intention was to create in modern notation as clean and readable a score as possible. In the individual editorial notes for each piece I mention when unusual original notation occurs.
Text underlay, as in most early music, is problematic. My approach to text underlay follows these general principles:
a) Follow chant text when identified, shown by an “+” in these editions.
b) Ligatures in white mensural notation indicated a series of notes where a singer should not change syllables. Ligatures appearing in the Formschneider are shown in these editions by brackets. Every attempt was made to observe ligatures unless a manuscript source shows a variant.
c) Match text underlay in imitative passages.
d) Use the Formschneider underlay if it does not conflict with items a, b, and c above.
e) Repeated or added text not in the original is enclosed in brackets.
f) Other editorial changes are indicated by an asterisk and editorial note in the score.
Musica ficta is used sparingly. Tritones are voided. Parallel fifths are avoided. Leading tones are generally altered if the cadence resolves to a unison or open fifth. Isaac’s music might very well have been less modal than that of his contemporaries, owing to his closeness to the secular music of Italy, but there is no real proof of that. My justification for not altering more of the printed notes is simply that this music is the beginnings of that trend towards modern tonality and is still 75-100 years before that of Palestrina, Lassus, and Gabrieli.