Wednesday, 3 September 2014

Patch Tips #27 - An FM Equivalent

An analogue take on digital FM featuring the Toppobrillo Triple Wavefolder.

Today's Patch Tip is inspired by MitchXI's description of a digital implementation of FM:

" ... in a yamaha style set-up, an oscillator is simply a ramp that goes from zero to one for every cycle - a phase accumulator - that gets fed into a lookup table that converts zero to one values into a sine wave. if you add a second oscillator's sine output to the ramp wave before the sine wave function, you get an fm equivalent ... to have an fm modulation relationship, you simply add the output of one oscillator into the other oscillator's sine wave function. the only digital computations that need to happen are addition, multiplication (for mod index/amount), and a table lookup."

If, like me, you feel digital is cheating, here's an analogue solution:

For the look-up table we can use the TWF's saw-to-sine function. Patch your saw (carrier) to the TWF via a mixer and trim the bias on a single channel to achieve a clean sine. Apply your modulator via a VCA or directly to the mixer and trim the amount of 'FM' to taste. In this example I used two VCOs/ two TWF channels and one modulator. I start with just the one voice which I pan left when I introduce the second voice:


At high modulator frequencies the result is pretty good. Because the saw-to-sine converter is analogue, it is sensitive to variations in amplitude and DC offsets. And, like Phase Modulation, this method has it's limits: If the modulator is too slow the 'FM' effect will be negligible.

This patch won't replace your thru-zero FM or phase modulation VCO - the maximum possible index seems similar to standard linear FM. But if your oscillators only have exponential inputs, the Triple Wave Folder offers another unexpected method of dynamically changing the colour of your sound.

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