Wednesday, 30 March 2011
Patch Tips #15 - Non-Linear Distortion
I recently stumbled across the Nord Modular's exponential shaper module while looking for something to bend a signal. The ShpExp works with transfer functions to generate non-linear distortion. This can give a sound edge, without totally shredding it. Of course, my next thought was whether it would be possible to patch something similar with my system. As usual, Rob Hordijk had the answer.
The trick involves generating a signal at twice the frequency of the source, which will then be used to modulate the amplifier. The easiest way to do this is to patch multiples of the source to both inputs of a ring modulator, effectively multiplying the signal by itself. This performs full wave rectification and, in the case of sine waves, frequency doubling. Patched to a linear VCA, a sine wave is bent like this:
Here's how it sounds:
Mono- then Duo-phonic signal. Clean Cwejman signal path (MMF-1 sine, VCO-2RM RM, VCA-2P)
Mono- then Duo-phonic signal. Dirty Doepfer signal path for added crunch (MMF-1 sine, A-133 as RM, A132-1 VCA)
Feeding the rectified, frequency-doubled signal back to the source, rather than the VCA, will bend the sine into a triangle, albeit with a different pitch. Try varying the FM-CV amount, other VCO waveforms or even polyphonic signals for seriously warped shapes.
I used the VCO-2RM and A-133 which, being dual, allows one to cascade and further multiply the modulator. Frequency doubling or rectification can also be achieved by feeding +ve & -ve signals to a min/max analogue logic module like the A-172 & Maths' OR processor or Doepfer's A-119 ext. input module, as per this Patch Tip. Like the A-133, Fonitronik's forthcoming mh31 and Makenoise's modDemix provide one-stop solutions.
I'm not sure if this process is in fact single-sideband modulation as the modDemix description suggests, but it's certainly closely related to Amplitude Modulation which will be the subject of the next Patch Tip.