http://en.wikipedia.org/wiki/Subvocalization

Anyway, I've had the opportunity to revisit most of my Gesualdo recordings in the last couple of days. While the Curtis CD is indeed much better than the youtube clip, it's still somewhat wobbly compared to the (top of my list) La Venexiana, whose Fifth Book is also better than their Fourth. However I think it would be still true to say that there's a lot of work to be done on getting an ensemble of singers not just to take these matters seriously but to be able to project them coherently as an ensemble. I think it would help if whoever's involved in such things would try to get the sound of it into the singers' ears by means of a suitable instrument (which is after all how it was done at the time) - either by building one (hard but not impossible) or be simulating it electronically (dead easy).

So far I haven't found the microtonal composition software I remember, but I did find this!

Indeed it would! But it would need more insight than simply singing all intervals exactly in tune. As shown by one of the examples given there in a famous passage from Josquin's Ave Maria, singing all intervals "pure" causes all the rising tones inexorably to be minor tones with a value of 10/9 instead of 9/8. The result is unavoidable and severe comma depression - so while every simultaneity that exists is purely in tune, the result is that within a single phrase the base pitch drops by a full semitone! (The extract adds, at the end, what the pitch of the final chord should have been had pitch stability instead of comma depression taken place.)

Only by a) making certain of the tones of the scale be variously Major (9/8) and Minor (10/9), and b) knowing in advance which ones are to be which, can a piece be rendered both "in-tune" and "pitch-stabilised".

That is the real challenge that confronts a group of would-be syntonicists!

Baziron

It's not a silly question TF - it's a very fundamental one. But we can surely be certain that the "ideal" concept (whether it was compromised in actual practice - compositional or performance) was to maintain base-pitch stability. This is demonstrated by the numerous monochord diagrams that provide for this kind of tuning (e.g. Zarlino, Agricola etc.) wherein the structural basis hinges upon the constancy of pitch of the string (= base pitch). It was always the case that Pythagorean tuning strictly maintained its base pitch (hence the numeric charts giving all the unchanging interval values of the Pythagorean diatonic scale).

These very issues are explored in some considerable depth HERE, and it is worth listening to some - indeed all - of the audio examples. Especially in the case of Willaert, a case is made there for viewing the control composers themselves exercised for providing music that, while being sung "in tune", also maintained base pitch. This is because they actually understood the effects of comma depression, and made contingent arrangements to deal with it in the way they actually structured their music!

Baziron

Baziron, this is absolutely fascinating! Thank you for posting this.
I'll have to print it off (can't cope with reading from a screen) and read it through in some depth before I can hope to say anything remotely intelligent about it.
I get the feeling (backed up by my undergraduate studies) that Willaert is a composer about whom there is much more to be said/written, and I, for one, would be really interested to hear/read it.