Analogue step sequencer
Analogue step sequencers are simple devices. They consist of a few knobs (usually 8, 16, or 24, divided up into rows or columns of 8). Each knob specifies a voltage. The sequencer then reads these voltages in order, in sequence. It usually has the option to either cycle through all of them in series, or to cycle through whole rows or columns at once in parallel. It outputs whichever voltage or set of voltages it's looking at.
Going to the next step causes a sudden change in value, a "stepping" sound, but this can be smoothed out by placing a slew limiter in the signal path after the sequencer. In the context of pitch, this adds a portamento or slide.
The sequencer's tempo is usually controlled with a built-in clock, which sends out regular pulses, called triggers. Each trigger tells the sequencer to move on to the next value or set of values, known as advancing it. In most contexts, that's one pulse per sixteenth note, also known as four pulses per quarter note — 4 PPQN.
As the clock is just a simple pulse wave LFO, it can also double up as a gate, directly telling the synthesiser when each note should start and stop. This is achieved by simply connecting it to an envelope generator, and in turn connecting that to a VCA. Adjusting the clock's pulsewidth can change the note length. If the clock's pulsewidth can't be adjusted, then you can use any other LFO to both trigger the sequencer and act as a gate for an envelope generator.
The sequencer's step doesn't need to be advanced by a steady clock. It can be triggered by anything. For example, each manually played note could advance the sequencer using its gate. With an audio to trigger converter, even an external sound can advance it, such as acoustic hi-hats being played by the band's drummer. It needn't be mechanically timed.
As control voltages can be used to control anything, not just an oscillator's pitch, sequencers can also control anything. You could, for instance, use one to step through a sequence of different cutoff points for a filter. Looking at it another way, a looping step sequencer is an LFO that can draw arbitrary shapes, by manually setting the value of each part of the cycle. This is known, appropriately enough, as an arbitrary waveform generator.
You can even crank up the sequencer's tempo fast enough that it becomes an audible oscillator. Alternatively, by keeping it slow and setting it not to loop, a sequencer turns from an LFO into an arbitrary envelope generator.
As with all other modules in a modular synthesiser, a simple device that seemingly performs only one function can actually be used to do lots of different things, if you learn to think laterally enough.
- "Rhythmic Control of Analog Sequencers" John Duesenberry, Polyphony, Sep 1978, pp. 26—29