The centrepoint of subtractive synthesis, the filter is the subject of adoration, debate… and this month’s modular synthesis class

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When we speak of ‘analogue synthesis’ we usually mean analogue subtractive synthesis. There are, though, many other non-analogue forms of subtractive synthesis - the typical sampler fits that description. You see, the ‘subtractive’ bit has nothing to do with how a synth makes its sound, but how that sound is manipulated once generated.

In the case of subtractive synthesisers, an oscillator generates a waveform by analogue or digital means. That waveform likely consists of the fundamental frequency, plus harmonic and inharmonic frequencies. Said waveform is then spat through a filter, which is used to hack away at those frequencies. The filter’s actions might be defined by modulation sources such as low frequency oscillators, envelope generators, or performance controls like velocity sensitivity or keyboard scaling. More on that below.

It’s this hacking away at (or subtraction of) a signal’s frequencies that gives subtractive synthesis its name. But a filter is also often the defining aspect of the instrument being used. Most readers know that Moog’s synths are prized for the rich, characteristic sound of their transistor ladder-based filter circuit. Roland’s TB-303 is also liked for the raw, squelchy sound of its filter. Below, we’ll use Cherry Audio’s Voltage Modular Nucleus to show a well-appointed filter module in action.