If you want to use analog synths/oscillators for xenharmonic music, there are MIDI-based options like the H-Pi Tuningbox, or the more advanced Silent Way soft/hardware. And there are modules like the Analogue Systems RS-130. Or you could also try a hack: maybe it's possible to hard sync your hardware with software-generated audio.
First a little warning: even if your synth or hardware oscillator has a good hard sync option on board (not hardwired/internal, but a dedicated audio input) you might still get poor results. The biggest challenge is to get the slave oscillator to respond properly to the incoming master signal over ~ say ~ three octaves or more.
And of course hard sync also produces it's own characteristic artifacts which could be a bit too much if you just want to tune an oscillator, and have no musical use for the 'classic tearing sync sound'. So you may want to reach for a filter to tame those wild timbres.
Having said that, the procedure is very simple. First, you get a good free microtonal softsynth, like the FMTS. Next, you program a melody, and have the softsynth play it using a square wave or a ramp. This audio signal is fed into the hard sync audio input of your synth/oscillator. If you don't like what you're hearing try these things:
- change the base-frequency of the slave osc;
- try transposing the softsynth-generated melody;
- try turning up the level of the master signal (a lot);
- change the waveform of the master signal (for example, the FMTS has a square waveform and two ramps, which produce quite different results)
The 'hard sync microtuning trick' allows you to create analog xenharmonic sounds in any tuning you can think of, without having to buy extra gear. Add analog pulse-width modulation, linear fm etc. and also try feeding the tuned audio into the cv-inputs of waveshapers.
