Subtractive synthesis

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Subtractive synthesis is a method of creating a sound by removing harmonics, characterised by the application of an audio filter to an audio signal. For example, taking the output of a sawtooth generator and using a low-pass filter to dampen its higher partials generates a more natural approximation of a bowed string instrument than using a sawtooth generator alone. Typically, the complexity of the source signal and the cut-off frequency and resonance of the filter are controlled in order to simulate the natural timbre of a given instrument.

Subtractive synthesis is historically associated with analogue voltage controlled synthesizers such as the Moog synthesizer due to the simple circuitry required to generate the most common source signals: square waves, pulse waves, sawtooth waves and triangle waves. Modern digital and software synthesizers may include other, more complex waveforms or allow the user to upload arbitrary waveforms. Some synthesizers may use a form of pulse width modulation which dynamically alters the source for a richer, more interesting, more organic tone.


Examples of subtractive synthesis

A human example

An easy way to understand the basis of subtractive synthesis is to consider one’s own personal ‘synthesizer’ — when a human speaks, sings or makes other vocal noises the vocal folds act as an oscillator and the mouth and throat as the filter. Consider the difference between saying or singing “oooh” and “aaah” (at the same pitch). The vocal folds are generating pretty much the same raw sound in either case — a sound that is rich in harmonics. The difference between the two comes from the filtering applied with the mouth and throat. By changing the shape of the mouth, one varies the cutoff frequency of the filter, so removing (subtracting) some of the harmonics. The “aaah” sound has most of the original harmonics still present; the “oooh” sound has most of them removed (or, to be more precise, reduced in amplitude). By gradually changing from “oooh” to “aaah” and back again, one can simulate the ‘sweeping filter’ effect that is widely used in electronic music and which is the basis of the ‘wah-wah’ guitar effect (which is so named for its similarity to this vocal ‘filter’).

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