The latest version of u-he Zebra2 (2.3) adds an on/off switch for “voice drift.” This is great news for anyone who cares about alternate tunings. In earlier versions of Zebra, there was a small hard-wired random pitch variation from note to note, which made predictable microtuned intervals impossible.
Zebra2 can also load .tun files (which you can create using the Scala software), so the on/off switch had me salivating for about ten minutes. But then I started thinking (again, as I have often in the past) about composing for microtunable instruments.
The bottleneck of the Zebra/.tun implementation, which is inherent in the underlying MIDI specification, is the concept of a “scale.” A scale is an ordered series of frequency ratios. Typically, a scale repeats every octave (that is, at a 2:1 ratio). Scala allows you to define “octave” in other ways, and there can be an arbitrary number of steps in the scale, but whatever scale you define, it will contain the same ordered sequence of ratios, which will be used by Zebra or any other microtunable MIDI synth until you load a different scale.
If your scale has more than 12 notes, good luck playing it on a MIDI keyboard. This can be a real brain-twister.
Right now I’m leaning back in the direction of using Csound for composing with microtunings. Compared to Zebra and a MIDI sequencer, Csound is very slow and balky to use (though Steven Yi’s wonderful front end, which is called blue, speeds it up a bit). But with Csound, there’s no need to define a “scale” ahead of time. The precise frequency of each note can be separately defined.
This gives the composer enormous freedom. You can start a piece with one group of related frequency ratios and then, after writing a phrase or two, add notes with new ratios that weren’t in your original group. In Csound, you can easily define a scale if you want to, but you don’t have to. The composition can move through the infinite space of frequency ratios rather than being tethered to a scale that was chosen ahead of time.
This kind of freedom is what the creative artist needs. “I’m out of red paint” is not a good thing for the painter. In the same way, “I need 45/32 for this chord, so I’ll have to load a different piece of software, create a new scale, copy it to the right directory, then delete Zebra from my project and add it again so it will load the new scale, after which I’ll have to edit the MIDI track because now my scale has more notes in it than before, so most of the MIDI note numbers will be wrong” is not a good thing for the composer.