Formants
Been doing a little experiment this morning. I took wave 75 from the Prophet VS and resynthesized for my K5. This waveform is dang near perfect for choir sounds, at least around middle C. The problem is as you play higher on the keyboard, you run into the "chipmunk" effect. The reason for this is that an important element of the vocal sound is formants. These don't change with pitch, but when you transpose a vocal waveform, you are not only transposing the pitch but the formants as well. Voila! Alvin and the Chipmunks.
Formants are are peaks in the overall frequency response of an instrument, at least that is my understanding of them. They are caused by the resonance of the chamber in which the sound is originating. So for a violin, it is the violin body. As you play a violin, you are not, of course, drastically changing the shape of the violin body, so the resonant response of the body remands the same regardless of what note you play.
Maybe some of you have had experience with formant synthesis. Unfortunately, I haven't had a lot. But I thought one way of approaching the chipmunk problem with the Prophet VS waveform was to synthesize more than one waveform for various ranges of the keyboard, much like creating a multisample. So to create a waveform an octave above middle C, I took every other harmonic from the original. This has the effect of preserving the peaks and valleys of the harmonic profile. Or maybe a better way of saying it is that it compresses them.
Believe it or not, this worked. I had to tweak a couple of harmonics afterwards, but that's much easier that tweaking 40 or 50. So I have a nice choir sound an octave above the original. Problem is that I need one inbetween this range. Say a fifth above the original. No problem. Just take every 1.5 harmonics (this will mean taking the average of two harmonics when the count has a fractional part).
I've uploaded an example of using formants to synthesize a choir sound with my Kawai K5; it's in my folder. I created the patch the same way you would create a multisample of an instrument. I synthesized two sounds per octave and spread them out across the keyboard. This minimized the chipmunk effect.
The formants I used are:
F1: 590Hz
F2: 920Hz
F3: 2710Hz
I found a chart of formants here:
http://www.sfu.ca/sonic-studio/handbook/Formant.html
I used the "haw'd" womens formant to get a nice "ahhhh" sound.
The Kawai K5 doesn't implement formant synthesis, at least not in any sophisticated way. So to program the sound, I used the above formants and the overall harmonic profile shown on the above webpage (as well as the one on the webpage Dale posted a link to) as a guideline for setting the amplitude of the harmonics and let my ears do the rest.