Subtractive synthesis is a method of sound synthesis in which partials of an audio signal (often one rich in harmonics) are attenuated by a filter to alter the timbre of the sound. While subtractive synthesis can be applied to any source audio signal, the sound most commonly associated with the technique is that of analog synthesizers of the 1960s and 1970s, in which the harmonics of simple waveforms such as sawtooth, pulse or square waves are attenuated with a voltage-controlled resonant low-pass filter. Many digital, virtual analog and software synthesizers use subtractive synthesis, sometimes in conjunction with other methods of sound synthesis.^[1]

Examples of subtractive synthesis

The following is an example of subtractive synthesis as it might occur in an electronic instrument. It was created with a personal computer program designed to emulate an analogue subtractive synthesizer. This example will attempt to imitate the sound of a plucked string.

Whilst the following example illustrates how a desired sound might be achieved in practice, only the final three stages are really subtractive synthesis, and the early stages could be considered to be a form of additive synthesis.

First, two oscillators produce relatively complex and harmonic-rich waveforms:

Waveform #1 Problems playing this file? See media help.

Waveform #2

Pulse-width modulation is added for a dynamically changing tone:

PWM waveform #1

PWM waveform #2

The two sounds are mixed. In this case they are combined at equal volume, but any ratio could be used.

Combined waveforms

The combined wave is passed through a voltage-controlled amplifier connected to an attack, decay, sustain and release envelope. In other words, its volume is changed according to a pre-set pattern. This is an attempt to emulate the envelope of a plucked string:

Enveloped sound

Then pass the sound through a shallow low-pass filter:

Low-passed sound

In this case, to better emulate the sound of a plucked string, the filter cutoff frequency should start in the mid-range and to low. The effect is similar to an electric guitar's wah pedal.

Final sound

Some arpeggios

In real music production, there is often an additional step. An oscillator with a very low frequency modulates one or more sounds over time, creating a dynamically changing sound.