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Modular Synthesis for Beginners: Randomness



This month, we investigate how synth modules can make your music sound more human by harnessing random acts of chance.

The use of random elements in music goes back at least to the 18th century. Composers could assemble uncomplicated pieces such as minuets by rolling dice to choose what would play in each measure from among a menu of optional measures, any of which could more or less work. By the middle of the 20th century, aleatoric music had become somewhat mainstream in the world of classical composition. An early and well-known example was John Cage’s Music of Changes (1951). (The word aleatoric comes from the Latin word alea, which means dice.)

Take a Chance

Electronic music, whether it’s pop, new age, or minimalist, is often highly repetitive. By using randomness, we can add a little variety to a repeating sound pattern. We don’t need to specify exactly how much the sound will change from moment to moment or where the change will happen. We do, however, have to choose what will change (and what won’t).

Also, we can’t avoid deciding what the maximum range of any given change will be. For instance, if we’re using some sort of random signal to move the cutoff frequency of a filter, we will usually want to set the amount of randomness, along with its speed and shape.

At one extreme, a piece of music can be almost entirely controlled by random operations. At the other extreme, we may want to compose almost every part of the music very precisely, but leave a slight amount of randomness here or there to add flavor or give an automated process a more “human” feel.

Modular Synthesis: Randomness
Here are five of the randomizing modules in VCV Rack. Stoermelder Maze (upper left) is a two-dimensional, four-voice step sequencer that always chooses the pitch for each location at random. 23volts Cells (upper right) has 24 gate outputs, which are either high or low depending on the well-known computer “life” program. Vult Caudal (lower left) produces 12 different CV outputs based on equations for swinging multi-segment pendulums. Frozen Wasteland Probably Note (lower center) quantizes an incoming CV, but it can randomize its output slightly or radically depending on how various controls are set. The Stellare Modular Turing Machine is based on a Eurorack module from Music Thing; it has two stepped outputs and a bunch of gate outputs.

Many synthesizers offer one form or another of randomness. As usual, I’m going to use VCV Rack to illustrate some of the possibilities. You can download this month’s patch HERE. [Editor’s note: this patch requires a few modules that aren’t free.] I may forget to mention a few possibilities, so you can have fun discovering them for yourself. This month’s patch uses modules from Nysthi, Blamsoft, Audible Instruments, Frozen Wasteland, AS, Alright Devices, Impromptu Modular, Vult, Hora, Valley, Stellare Modular, Bogaudio, and mscHack.

Random Patches

Starting around 1980, a few hardware synths had a random patch command. Push a single button, and all of your instrument’s parameters are randomized; suddenly it would produce a sound nobody had ever heard before. Most of these sounds were garbage, but you could tweak or massage some of them to produce usable musical effects. One of the best implementations I’ve seen of this concept in modern software synths is in the Nostromo Rack Extension for Reason, from Lectric Panda.

It’s hard to imagine randomizing an entire patch in a modular synth. Too many cables would be hooked up in ways that made no sense. However, most VCV modules have a Randomize command in their right-click menu—even the mixers, though why you would want to randomize a mixer is hard to fathom. With step sequencers, though, randomizing knobs can be a great way of coming up with melodies or rhythms that you wouldn’t necessarily think of.


Most synths have noise generators. In a technical sense, noise is a very rapidly changing random signal. Computers can’t actually produce true random values, but any decent computer program can use mathematical processes to produce signals that seem perfectly random to us humans. The essence of noise is that the signal does not repeat. If you use an oscillator’s noise output to make a wind sound or a cheap hi-hat, in some sense you’re generating music using randomness, but this is a trivial case.


A sample-and-hold (S&H) module reads (takes samples of) some signal. It takes a new sample whenever it receives a clock signal. Then it sends that sampled value to its output until the next time it receives a clock pulse. The result is a stepped output. If the signal that is being sampled is noise, the steps will have random values. They may be close together or far apart, and you have no way to predict what the next step will be; there’s no pattern.

Random sample-and-hold has been a staple of synth sounds in pop music for many years. Most often, the S&H signal is applied to the filter cutoff frequency, but you can apply it to oscillator pitch or to other parameters just as easily.


Some step sequencers have a random setting. When the sequencer receives a clock pulse it will advance to a new step, but you won’t know ahead of time which step it will choose. This effect is typically less random than a sample-and-hold, because the sequencer output will usually have only a small number of fixed values. If you tune the steps to a C major chord, for example, the output will always be in C major, but you won’t know which of the notes in the chord will play at any given moment.

Smooth Random

A few modules can output signals that change slowly and smoothly, but in an unknown direction. Technically, this signal is a type of very low-frequency noise. However, it doesn’t have the jittery quality we associate with noise. I’ve used a module of this type, Vult Caudal, for a few things in this month’s VCV patch. Instead of using Caudal, you could produce something rather similar using three or four LFOs, patching them all together so that each of them is modulating the frequency and/or amplitude of a couple of the others.

Yes/No Switching

Another VCV module that’s very useful for randomizing the music is the Bernoulli Gate from Audible Instruments. When a Bernoulli gate receives a gate or trigger signal at its input, it passes the signal on to one of its two outputs…but you don’t know which one. It’s like a coin-toss. You can use a knob (or an external CV) to control the probability of a “heads” vs. “tails.” Using this module, you can produce drum patterns that have some variation.

You can find another form of probability-based switching in the Gate-Seq-64 module from Impromptu Modular. You can set up a repeating pattern, usually to play a drum sound, in which some of the beats always play, some never play, and others may or may not play.

Turing Machines

Confession time: I’m not even sure what a Turing machine is. It’s a thing in computer science, and after reading the Wikipedia article, I’m no wiser than before. However, VCV Rack does have a couple modules that were inspired by the Turing machine. These modules produce a stepped output suitable for changing the pitch of an oscillator. Sometimes the output repeats a short phrase over and over, and sometimes it switches to a new phrase. The result has enough repetition to draw listeners in, but enough variety to keep them interested.

And that’s the point, after all. If you can’t manage that, why bother making music at all?

To get started on Jim Aikin’s “Modular Synthesis for Beginners,” especially if you’re unfamiliar with the free modular soft synth VCV Rack, begin with his first column in the series, “READY TO RACK!

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