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Modular Synthesis for Beginners: Step Sequencers

Jim Aikin

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Automate your modular system with step sequencers.

Playing music with our hands, feet, and breath is a wonderful thing. Sometimes, though, you want to set up a pattern on a machine, sit back, and let the machine play the music for you. We have two ways to do this: with recorded audio, and by sequencing.

Today we have sophisticated MIDI sequencers that will do just about anything you might imagine. The first sequencers, however, were simple devices. The sequencer would step through a series of notes, usually eight or sixteen of them, one at a time. When it finished the series, it would repeat. And repeat again, and again, and again.

Stepping Out

Early synth groups like Tangerine Dream used step sequencers to create endless psychedelic soundscapes. To be honest, I never found this type of music (a form of minimalism) very interesting. I like to spice up my step sequences with a few variations.

Even the word notes in the description above is a bit too cavalier. The first sequencers had rows of knobs, and each knob could be set to some arbitrary value. The output was in the form of an analog control voltage. A knob for each step in the sequence dictated the voltage level of that step. The row of knobs could control pitch, thereby producing notes, or some other sound parameter.

Basic sequencers of this type are still found in many of today’s modular synth systems. But there are many variations on the theme. Some step sequencers have several rows of knobs, with an output for each row. You can use these outputs for playing chords, for example. Or use one row to program a series of notes and a second row to add accents to certain notes.

Most step sequencers let you set the length of the sequence to a value other than eight or sixteen steps. Some sequencers let you program rests on certain steps, so as to create a less monotonous rhythm. And some have a separate gate or trigger output for each step that you can switch on or off. These outputs are another way to create rhythmic variations.

First Steps

In its simplest form, a step sequencer is a row of knobs (or perhaps sliders, but let’s call them knobs). Each knob has its own setting, which is a signal that the synth can use. We may as well call them voltages. The sequencer will have either an internal clock or an input for an external clock signal. It will probably have a few other controls, such as a start/stop button and a knob for controlling the internal clock’s speed.

Each time the clock ticks, the sequencer advances to the next knob or column of knobs—that is, to the next step. The sequencer reads the value(s) of that step and sends the value(s) to the output(s).

There’s nothing special about the clock. A modular system can use any rising-edge voltage, such as the signal from an LFO square wave, as an external clock.

Some modern step sequencers provide an output that’s already quantized to equal-tempered half steps. Assuming you’re playing normal music, this is quite handy. If the output is not quantized, then you’ll need to run the signal through a quantizer module and send the quantizer’s output to the oscillator’s pitch input.

Digging In

With a hardware step sequencer, you can easily turn the knobs while the sequence is playing, thereby changing the pattern. When the sequencer exists only in the form of software, you’ll probably find that turning the knobs with a mouse is less intuitive, less tactile, and therefore less satisfying, but you can certainly do it. With a software sequencer, however, you may be able to change the pattern automatically.

There are many ways to do this—far too many to cover in an introductory tutorial. In VCV Rack, you’ll find quite a variety of sequencing modules.

These are four of the more unusual sequencing devices in VCV Rack. Mog Network (upper left) has 16 knobs and one output, but each cell has two gate ins and four gate outs. Cells can trigger one another in complex ways, resulting in patterns that are far longer than 16 steps. The Frozen Wasteland Quad Algorithmic Rhythm Generator (upper right) produces gate signals on four channels using Euclidean processes to activate gate positions in each row. Computerscare I Love Cookies (lower left) has 26 knobs and 26 CV inputs, which can be sequenced in any order in up to six pattern rows. TheXOR Renato (lower right) has independent clock inputs for its X and Y directions, and each of the 16 cells can be switched on or off using a CV input.

Some sequencers can randomly choose one of two knobs on each step (Geodesics Entropia). Others can walk through a two-dimensional grid with separate clock inputs for stepping in the X or Y direction (JW Modules GridSeq, TheXOR Renato). Some sequencers have patterns that can branch in various ways (Mog Network, Sckitam MarkovSeq). A few can output curves and mutate the pattern (ZZC Phaseque, DHE Modules Curve Sequencer), while others play multi-voice chords on some or all steps (JW Modules NoteSeq). You’ll find sequencers that override one or more of the knobs with an external input (computerscare I Love Cookies), sequencers that change direction or select steps at random (Nysthi Squonk), and sequencers that are specialized to send only gates rather than note pitches (Impromptu Modular Gate-Seq-64, Frozen Wasteland Quad Algorithmic Rhythm Generator).

Variations on a Theme

In the software world, it’s also common to find step sequencers that can store and switch among numerous patterns. By chaining patterns or choosing among them with an external control signal, you can create a song arrangement with an intro, verse, chorus, break, and so on. Building a song using one of these devices is generally less fun and less intuitive than using a multitrack MIDI sequencer, but it can be done.

One of the many interesting things you can do with step sequencers is to use two or three at once, each set to a different length, using each of them for a different part of a composite sound. For instance, you might set up a repeating pattern of eight pitch steps but add an accent once every five steps (by sending a gate from a separate sequencer to an envelope generator and sending the envelope to the filter cutoff). You might also give it an octave shift in a pattern of eleven steps (by mixing the pitch output the first sequencer with a pitch output from your third sequencer). That would produce a pattern that only repeats every 8 x 5 x 11 = 440 steps.

I generally run all of my step sequencers from a single clock source. In VCV Rack, Impromptu Modular Clocked is a good choice; it has three extra outputs, each of which you can set to a multiple of the basic clock rate. If you’re feeling bold, you might try running several sequencers at unsynchronized clock rates. That will yield a mixed pattern that never quite repeats.

The Starting Gate

Once the sequencer is running, all will be well. But getting it to start in a reliable way is not always easy. Most step sequencers have Reset inputs; if you send a rising-edge signal to the Reset, the sequencer will always start at the same step when it starts receiving clock signals. One of the more frustrating things about VCV sequencers, however, is that they don’t all respond to Reset commands in the same way.

Let’s say the sequencer is stopped. It receives a Reset command. And then the external clock signal starts arriving. What will happen then? Ideally, we would want the sequencer to respond to that first clock signal by playing step 1. Some sequencers do that. Others, however, reset to step 1 when they receive the Reset command. And then when the first clock signal arrives, the sequencer advances immediately to step 2. On the first cycle through the sequence pattern, you won’t hear step 1.

This is not a huge problem once you understand your gear. But it illustrates the fact that you do have to learn what your modules are capable of. The things you don’t know can definitely mess with your mood.

Hands On

In this month’s video (back at the top), I’ve set up a simple sequencer patch using AS Seq16. This module is pretty much a clone of the 8-step sequencer that’s one of the stock VCV modules. It has more steps and a couple extra features, but compared to more visionary sequencers, it’s vanilla. To illustrate a couple ways to make step sequences sound more interesting—hypnotic, but not boring—I’ve added a few wrinkles, including audio delay and some modulation signals controlling the oscillator. The notes stay the same, but the sound moves around.

This month’s patch uses uses modules from AS, Count Modula, Impromptu Modular, Audible Instruments, Bogaudio, Surge for Rack, Alikins, mscHack, Valley, Stoermelder, Noobhour, and Nysthi. All of them are free. The noobhour quantizer is interesting: you can activate different notes in different octaves. It always spreads the incoming CV signal across its active range, so you can change the pattern in the middle octave by clicking on a button or two in the top or bottom octave, thereby causing the incoming CV to be distributed in a different way.

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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