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This Month’s Module: Ian Fritz’s Chaos Generators

Chris Meyer

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If you want the predictability of an LFO with the pattern variations of random modulation, just add chaos. 

One challenge in music composition is finding the right balance between crafting a repeating motif to give the listener something to hang on to, and generating enough variation to keep them interested. If you’re too close to either extreme, you risk losing your listener’s attention. In a modular synthesizer patch, finding this balance is akin to choosing between a typical repetitive low-frequency oscillator (LFO) or a random control voltage generator as a modulation source.

You can take a middle path: chaos-based generators. It’s easy to assume chaotic means “really random.” In fact, chaos generators combine features of both periodic and random control voltages. This video compares periodic, random, and chaotic control voltage sources in a couple of applications.

Common Chaotic Behavior

A typical chaos generator module contains a feedback loop, akin to a self-oscillating filter. And when viewed over just a couple wave periods, the output of a chaos module can look quite similar from cycle to cycle, like an LFO.

The kink—literally—is that a chaotic system has a nonlinear element in that feedback path. This is what throws the system off, causing it to create variations over time. For example, look at the similarities and differences between cycles of the oscilloscope trace below.

Another characteristic of chaotic systems—chaos-based synthesizer modules included—is that relatively small changes in their parameters can generate very different results. For example, the driving oscillator’s coarse frequency changed only a slight amount between the image above one the image below; note the radical differences.

A third feature of most chaos modules is that they have multiple outputs. These outputs are typically taken at different points in the feedback loop, between different elements of the circuit. The overall system may be oscillating at a given rate. However, the voltage waveform at these points will usually look quite different…such as the green and blue traces below.

Ian Fritz’s Chaos Module Designs

Ian Fritz has been one of the leading lights in the modular DIY community, sharing numerous schematics and ideas. One of his areas of expertise is applying chaos theory to synthesizers. His website dedicates a sizable chunk to the subject.

If you’re not into building your own modules from scratch, several of his designs are available in both kit and assembled form. Two outlets sources are Elby Designs, which uses through-hole components that are easier to solder but end up requiring more depth in your modular case, and nonlinearcircuits, which uses small surface-mount components to create skiff (shallow case) compatible modules.

I’ve owned three of Ian Fritz’s chaos-based Eurorack modules. Here is a quick summary of their main characteristics, plus a few tips on using them. You can also hear demos of some of his designs on Ian’s YouTube channel.

ChaQuO

This is the module used to create the images above. It combines a quadrature sine wave oscillator with Ian’s EZ Chaos Circuit. On the most basic level, you can think of it as two resonant one-pole lowpass filters (integrators, actually) with a pair of diodes in the feedback path. It features three outputs taken from after each of the two integrators plus after the nonlinear element.

The driving sine-wave oscillator has a frequency range of 0.03 Hz to 3 kHz. Think of the chaos circuit’s Rate control as the cutoff frequency for the resonating filters. Setting it between just below the driving LFO’s frequency and an octave or two above is a sweet spot for creating some very interesting beating patterns and other interactions. The Damp, Couple, and Gain controls affect how the loop is closed for the feedback path; they interact to provide different results. The ChaQuO module no voltage control, but the quadrature LFO has its own outputs, and you can patch your own signal to drive the chaos side of the circuit.

Chaotica

At first glance, Ian Fritz’s Chaotica looks like a version of ChaQuO with control voltage inputs for Rate, Damping, Gain, and a new Offset control, but under the hood it uses a more complex circuit. It has three integrators plus four non-linear circuit elements, and does not need a separate LFO to drive it. Each of the three integrator sections has its own output.

The Chaotica has a few additional features. When a gate or trigger is sent to its Reset/Inhibit input, it immediately clamps all of the integrators to 0 volts. When the gate or trigger goes low, the chaos engine is allowed to take off again from scratch, acting like a delayed LFO.

With the Eyes switch set to 1, Chaotica’s X output is prone to stick to positive voltages; setting Eyes to 2 allows X to have pronounced negative excursions as well (Y and Z are bipolar). If the Tame/Wild switch is set to Tame, the X output (the green trace in the image above) looks like a cross between an ADSR and a sine wave output; Y resembles a cross between a sine and a mathematical sinc impulse (function the blue trace); and Z (the magenta trace) has a sharp rise and wobbly fall like a filtered sawtooth wave. Set that switch to Wild, and the system exhibits pronounced resonant ringing with higher swings in voltage overall.

The Hypster

The Hypster features four voltage-controlled integrators and non-linear elements patched in a ring, with outputs taken in between each integrator/non-linear element pair. It also features voltage control over the familiar Rate, Gain, and Damp parameters, as well as a pair of switches to alter its behavior. And like Chaotica, it may be varied from audio rates to minutes-long evolutions.

Of the three, The Hypster may be the most “LFO-like” for those new to chaos, with fewer controls and a gentler evolution between cycles. For example, the Wild/Mild switch has a much more subtle effect than in Chaotica. The more interesting control is Squirrelly/Curly, which adds more upper harmonics of complexity in the Squirrelly position.

In ChaQuO and Chaotica, the X outputs often have the most “character” and are most responsive to changes in parameters. In The Hypster, this role is taken by the U output (the yellow trace in the image above). It has the largest voltage excursions, often clipping against Eurorack limits of ±10 volts. Reducing Gain and increasing Damping tends to restrict the +U output to negative voltages (each output also has an inverted output available).

Controlled Chaos

As I hinted in the introduction, I’m always trying to find that balance between creating changes to keep the listener engaged, but not making them think my music (and sounds) consist of just pointless meandering. Chaos generators fill that bill for me, providing both repeating motifs and variations without user intervention. They require a bit of study to wrap your head around, but I’ve found the effort to be well worthwhile.

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Synth and Software would like to thank longtime modular user and former synth designer, CHRIS MEYER, for his contribution. Chris is the force behind Learning Modular, where he teaches others how to master modular synthesis.

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