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Step-Sequencing Basics

July 1, 2014
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Fig. 1. The granddaddy of the modern step sequencer is the Serge TKB. Introduced in 1980 and still being built by Sound Transform Systems, the TKB has pressure-sensing touchpads and other advanced features.
Mechanical devices that can play hands-free music have a long history—the player piano, for instance. When analog synthesizers took the music world by storm in the 1970s, the tool of choice for automating patterns of notes was the step sequencer.

Compared to MIDI sequencing, step sequencers may seem primitive. But today’s models can do some surprisingly sophisticated tricks. In this article we’ll introduce the concepts behind step sequencing and look at a few of the modern variations.

Some digital music systems, such as Propellerhead Reason, include step sequencers. Without getting into the pros and cons of software emulations, we’ll concentrate on the real thing—hardware step sequencing as found in a modular analog synthesizer.

Baby Steps The frequency of an analog oscillator is increased or decreased by changing the level of the control voltage being sent to it. In most systems, increasing the voltage by one volt boosts the oscillator’s pitch by an octave.

Fig. 2. The two Pressure Points modules in the lower row (built by Make Noise), together with the narrow Brains module to their left, form an eight-step sequencer with touchpads. Above the Pressure Points is a Trigger Riot from Tiptop Audio, a digital module that produces rhythmic trigger/gate signals for analog sequencing.
On a step sequencer, you’ll find one or more rows of knobs (see Figure 1), usually with eight or 16 knobs per row. For each row, there’s a voltage output. In use, the sequencer steps along the row or along the columns of knobs, one knob or column at a time. At each step, the voltage level set by a corresponding knob is sent to that row’s output. The output can be patched to the CV input of an oscillator. Each time the sequencer selects a new knob, the frequency of the oscillator changes, producing a new musical pitch.

When the sequencer reaches the last knob in the row, it hops back to the other end, and the pattern of pitches repeats. If there are three rows, the three knobs in each column can be adjusted so that the oscillators receiving their voltages will play the notes of a chord. The voltages can be used for other purposes too, such as changing the filter cutoff frequency or the attack of a voltage-controlled envelope generator.

An incoming signal tells the sequencer when to step forward to the next knob. This signal is called a clock, trigger, or gate. Typically it’s a sharply rising voltage, such as the leading edge of a square wave coming from an LFO (low-frequency oscillator). The sequencer senses the edge of the square wave and responds by moving to a new step.

Generally, the clock signal is also used to gate one or two envelope generators. These will control the filter cutoff frequency and/or the level of a VCA (voltage-controlled amplifier), thus giving a musical shape to the sequencer’s series of pitches.

That description would fit the sequencers of the 1970s, as used by groups like Tangerine Dream to produce hypnotic soundscapes. But an endlessly looping set of eight or 16 notes can get boring pretty quickly. So let’s spice things up.

Making it Musical Rather than let the LFO that is providing the clock signal cycle at one tempo, use one of the sequencer’s row outputs to control the frequency of the LFO. The pattern of notes will still repeat endlessly, but now some of them can be longer or shorter than others, creating a rhythm. For irregular, non-repeating rhythms, the LFO frequency can be controlled by a different voltage source, such as another LFO.

Some sequencers have a CV input for up/ down direction. When this input is receiving a high voltage, the sequencer will step backward rather than forward. By sending this input a signal from a different LFO, we can get a “two steps forward, one step back” pattern. This is more interesting than a static pattern, and the pattern will continue for more than eight or 16 steps before repeating.

Another CV input, when it receives a high voltage, will cause the sequencer to reset to step 1 on its next step. This can shorten the pattern sometimes, but not always, again producing a more interesting and varied phrase. If the sequencer has a touchpad for each step, it may be able to reset to the most-recently touched step rather than always jumping back to step 1. Touching and holding a pad will cause the sequencer to stop advancing and hold that step until you lift your finger.

When a sequencer with touchpads isn’t receiving a clock signal, it becomes a rudimentary keyboard. Each time you touch a pad, the voltages from the knobs directly above that pad will be the ones sent from the row outputs. Because voltages are general-purpose control signals, the knobs can be used for whatever you like—changing the rate of an LFO or the speed of an envelope generator’s attack or decay, for example. Such keyboards aren’t velocity-sensitive, but even so, they can be used for expressive performances.

On some sequencers, each column of knobs is associated with its own gate output. When the sequencer is on a given step, it sends a high voltage to that step’s gate output. This signal can trigger an envelope generator, so some of the steps can sound different from others. The envelope might open up a subsidiary VCA, for example, sending a modulation signal to the oscillator to change its tone.

Getting an analog sequencer to play in tune is a challenge, because the knobs can make fine adjustments: They’re not stepped. A module called a quantizer makes it easier to produce standard musical scales. This receives the output of a sequencer’s knob row and adjusts it up or down to the nearest 1/12 volt. With oscillators built to the one-volt-per-octave standard, the quantizer will ensure that the sequencer’s output is in tune.

Giant Steps The future of analog modular synthesis looks good. Many small companies are building visionary new modules, including a few that interface with a computer in novel ways. Though step sequencers were once considered an obsolete technology, today it’s clear they’re here to stay.

Jim Aikin has written hundreds of product reviews and tutorials for Electronic Musician and other magazines over the course of more than 30 years. His books on music technology include Power Tools for Synthesizer Programming (Hal Leonard Publishing) and Csound Power! (Cengage Learning).

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