These editions are based on the original Formschneider prints of 1550/1555 with reference to Pätzig’s dissertation, which details variants between the Formschneider and manuscript sources. The method used to transcribe these editions was defined in my dissertation, which drew heavily upon Gossett (1974).
Chant sources for this music are not easily determined. At times, the chant found in the Liber Usualis or the modern Graduale Romanum is an almost perfect match. But I have also had to rely upon the Graduale Patavienese, that gradual used by the diocese of Passau, to which Vienna belonged during the time when Maximilian was Emperor (available in facsimile in Das Erbe deutscher Musik, Vol. 87). The missing (and oft-lamented) Constance missal was probably the source for some chant melodies but as it is unavailable that remains 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 a clean and readable a score as possible in modern notation. In the 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) Every attempt was made to honor ligatures unless a manuscript source shows a variant. Ligatures in white mensural notation indicate a series of notes where a singer should not change syllables. Ligatures appearing in the Formschneider are shown in these editions by brackets.
c) Text underlay in imitative passages is matched regardless of the print source.
d) The Formschneider underlay is used if it does not conflict with items a, b, and c above.
e) Changes from the Formschneider text or any 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.