The quest for the ultimate theremin sound - LOL! ;-) .. I do not think there is any "fixed" sound which will make everyone happy.. so I have spent the last 3+ years developing a theremin with tone adjustment possibilities I hope will cater for all 'tastes'.
If I was to write about all the technical aspects which influence theremin sound, I would probably crash this site! ;-) .. But I think there are some fundamentals - These are my opinions only.. I may be wrong!
1.) Wave-shape (harmonic content) must vary as the pitch varies.. But should not become "gross" at any point - Usual mechanisms by which this is achieved include oscillator syncing at low frequency difference, syncing producing an increase in harmonics, which reduce as frequency increases.
2.) Ideally, harmonic content should also be slightly affected by volume level.
The harmonics from oscillator syncing only affect the audio output on purely analogue theremins - Mixed signal or digital theremins must vary their harmonics by some other means..
Other forms of deliberate (and often pleasant, but also often quite often horrid) distortions of the post heterodyned audio waveform can change the 'base' charactaristics of the tone - with a theremin like the EW, both syncing and adjustable distortion are at play.
My approach is different - my focus has been on pre-heterodyne wave shaping.. One has independent adjustment on the levels of the first 5 harmonics, and also each harmonic can increase or reduce as a function of frequency.. I was planning control over 16 harmonics, but this was nightmareishly complex, and I find 5 harmonics are sufficient for a good audio pallette.
But the biggest discovery I made was when I accidentally introduced frequency modulation on my H1 (pitch only)"theremins".. Feeding the audio signal back in an adjustable way, and causing this to modulate the variable oscillator frequency, gives astounding richness to the tone - this modulation can be taken from before or after the VCA, allowing it to be constant or volume dependent.. It is real easy to do.. Simplest way is to vibrate the theremin at the audio frequency (bolt a speaker or one of those audio transducers to the bottom of your theremin and drive it hard!) (This is not the way I am doing it - but there will be a microphone input to allow FM from acoustic sources such as a speaker playing the theremin sound - or voice or whatever).. This causes the antenna to vibrate, causing frequency modulation.
My theremin modules will be available by Aug.. I will be launching them at Hands-Off 2011. I had intended them to be on sale by now - but made a big mistake of having some extremely special inductors made in China (they were the only quote I could afford) The samples were great - but the parts supplied did not match the samples at all (one winding had inductance of 15uH rather than 85uH, for example) .. I cannot get anyone to make these parts before July - So I have been forced to redesign the boards to use other parts.
Drop me an email (<- see picture) and I will keep you informed.. I might have modules available before Aug..
Fred.