[i]You said digital Theremin and I jumped in discussing MIDI. I'll go ahead and keep the post as-is since it may answer some questions for you.[/i]
Hi!
Most Theremins have a very wide frequency range spanning over six octaves.
However, there is the "frequency range" and then there is the "playable range" of frequencies.
On a top-of-the-line Theremin, such as an Etherwave Pro, the linearity of the pitch antenna allows playability across nearly a five octave range. A range switch on the Epro allows octave transpositions +/- one octave rendering a seven octave range.
At the high end (near the antenna) the antenna sensitivity increases such that the notes are too close together for precision playing -- at the far end the sensitivity tends to decrease.
Most Theremins have around a 3 1/2 octave minimum playable range.
One of the issues with MIDI is the choice of using CC events for the pitch or to quantize the pitch response into discreet note-on events.
I have wondered if it is possible to implement linearity in software by mapping the values from a non-linear antenna to linear output values.
There is much interest in MIDI theremins although, other than the Ethervox (no longer manufactured) I haven't seen great reviews for other MIDI theremins. If you can implement a MIDI theremin that is flexible and usable for precision playing as well as sound-effects, you will likely generate interest in such a product.
As for me, I'm considering just ordering a Paia MIDI brain (CV to MIDI converter) and using the CV outs from my Theremins to drive the MIDI brain.
Good luck to you.