It’s actually a frequency ratio; relative to Unison at 1.000:
As mentioned above, this creates a nice stereo spread, or wide animation in mono. It’s the equivalent of around a 14 cent detuning; both above & below unison pitch. That gives you a little more than a quarter-tone spread over the three voices.
I was going for a classic Eventide rack emulation. Actually, two 1st-edition H910’s in parallel (feedback is synchronized between them). These were massive analog + barely digital processors, with fixed delay times, and characteristic pitch glitching.
Delay times are selectable with combinations of four switches; up to 112.5 ms. with all 4 engaged. Again, 15 ms. and 30 ms. were chosen for authenticity. They are just under the limit of being perceived as distinct ‘echoes’. Those delay times provide subtle doubling, and allowed the original rack devices some ‘breathing room’ for processing pitch shifts.
There are whole books written on modern DSP processes involved, with the history, and some alternative approaches. But if you’re interested in the classic Eventide / Clockworks devices, search for the Clockworks Legacy manual (eventidelegacy.pdf?). I can no longer find it on this Eventide website, but it’s out there on the ‘net. That document is a software manual, but it’s helped me to understand many of the H9 algorithm derivations. Another useful source involves going back into the H9 / PitchFactor ancestry (Eclipse manual and preset guide, etc.).
