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.


Brian Biggs said...

In addition to the Modemix and mh31, Intellijel has the µMod.
Nice work again Navs, adding value to my modular.

Unknown said...

navs, great patch but if you don't mind me asking what are you referring to when you mention +ve and -ve?

Navs said...

Cheers, Brian. Another one to add to the list!

Thanks, Aaron. Do you mean the bit about using OR logic? Sorry, I should have been more specific. I meant if you feed an oscillator and an inverted version of itself (positive & negative) into the A-172 etc. The Cwejman VCO-2RM has both directly available, so I must have been thinking of that.

Anonymous said...

I wonder if I'm doing something wrong- I put my multed sine to both inputs of my dalek modulator and it definitely didn't double the frequency. I did though get inspired to use my VCAs as waveshapers and got some pretty exciting results using a Maleko/Wiard Oscillators with different phase offsets- Triangle into VCA Audio Square into VCA CV as well as basically every permutation plus an LFO on the phase mod. It's very "bye bye AFG"

Navs said...

Great that the post inspired you - I've always wondered which process was behind the AFG's alternative waveforms.

Odd about the RM. I wonder if it's specifically due to the Dalek. I used DC-coupled RMs. Did you try DC mode? Hard to know with the LW modulators as there is an amount of x-mod/ feedback etc. built-in that, IIRC, can not be fully overridden.

Fagunlessavyfav said...

Hi- Weirdly after months I went back and tried this patch again. I'd commented that it didn't work right on the Dalek Modulator but yesterday I tried it in "destroy" rather then "obey" and it worked like a charm. I'd have though "obey" would have been the way to go.

Sharonsr said...

Hello Navs
Can please fix the audio links , they are missing.
Thank you