This is an idea I had while my computer was out of commission, so I haven’t honed it very much, but I thought it was interesting enough to share. (Though I can’t imagine I’m the first person to think of this.)

While day-dreaming up a CS-800 (four layers, cross-mod, etc…), I thought it might be cool to have the option of using the filter outputs for FM, rather than the unprocessed other oscillators, like with the Minimoog or Prophet 5 (We’ll get to why those synths are that way, at the bottom.). Thor being semi-modular, I can try that!

You may recall that modulating a sine wave with another wave-shape is the same as modulating it with each of that other wave-shape’s harmonics. Why post-filter FM would be interesting is that you can be more selective in what those harmonics are than when modulating a modulator in traditional FM. The most obvious example is to use a highpass filter on the modulator, with the envelope changing its lower harmonics over time, rather than higher harmonics. Supposing you had a perfect bandpass filter, you could sweep through modulator ratios! (Albeit not at the same indexes. But Thor can do that, too, with its wavetables, if you have Reason and want to know what that would sound like.) A more realistically narrow band is also interesting, though, as is changing its width, by modulating the resonance. (The same goes for a notch filter.)

Using a filter without keyboard tracking possibly changes the modulator more than the keyscaling on a DX-style synthesizer (certainly a resonant bandpass filter would).

I’ve tried this both with and without the carrier filtered and made a little demo:

At this point (a few hours of tinkering with relatively simple patches), it sounds a bit like an FM-subtractive hybrid… because it is that. But I think this technique is worth exploring further and capable of more than the sounds in the demo, which can admittedly be a bit chip-tuney.

To those who would also like to try this, you’ll need a synthesizer that doesn’t just have filter outs as a modulation source but also the ability to route the oscillators along different audio paths, which gets us back to the Minimoog and Prophet 5. Feedback is common in FM – and you can get some cool sounds using the filter output to modulate the incoming oscillator – but keeping it in tune is dependent on using Phase Modulation (invented for this purpose, I’m led to believe), rather than “true” Frequency Modulation. As I understand it, the problem is that and oscillator output will almost certainly not average out to 0 volts (either due to harmonic content – pulse waves being the easiest to visualize – or output circuitry) and so will drag itself out of tune, if used to modulate its own frequency. (Based on my experiments, this may be an incomplete explanation.)

Alternatively, many modular syntheizers can do post-filter FM, though modular synthesizers come with their own problems, such as cost, lack of patch memory, cost, voltage vagaries, and cost.

But. The Prophet 12 has a DAC after each voice’s VCA and FM… so this could possibly be added in a software update, which would probably sound pretty damn cool.