Patch: Tesla Coil

Is started working on the patch aiming to create a sound similar to a Tesla coil or to electrical discharges in general (the colloquial German term would be britzel). As far as a know, musical Tesla coils are wired to release a discharge every time the acoustic signal crosses zero. For a sinusoidal signal, this would basically mean, that discharges are triggered with twice the frequency of the sine (or maybe at the same frequency, depending on the realisation of the zero-crossing detector). This means for an A at 440 Hz, 880 (or 440) discharges per second are triggered. But how does a single discharge sound, and how do we program the SY-300 to sound the same way?

So a single discharge basically sounds like an impulse, a bang if you want, caused by the air rapidly expanding as the current flowing through it heats the air up. The signal we are looking for is called an impulse train, although the term is usually used in a more abstract sense. With a synth we can approximate an impulse train by using a PWM (pulse-width modulation) signal, with a very low (or high) pulse width, basically giving us a highly asymmetrical rectangle function. We can then pitch in down an octave or two and we should start hearing individual impulses.

But we are not finished jet: You see, not all electrical discharges sound the same. In fact they vary greatly in level as the typically hit random places in the vicinity of a Tesla coil. There are several ways to model this behaviour. For the example below, I chose multiplication (aka. ring modulation; see my post on another patch) with a broadband noise signal on the second oscillator. I added some distortion and compression as well, to make it less clean sounding:

For some additional fun I configured the expression pedal to control the cut-off frequency of a resonant lowpass filter. Also CTL1 starts an LFO which affects the same parameter using an S&H (sample and hold) signal. CTL3 holds all oscillators so you can play around with the filter parameters.