Subtractive synthesis Totally Explained

Everything about Subtractive Synthesis totally explained

Subtractive synthesis is a method of subtracting harmonic content from a sound via sound synthesis, 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 synthesisers 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 synthesisers may include other, more complex waveforms or allow the user to upload arbitrary waveforms. Some synthesisers 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 and familiar way to understand the basis of subtractive synthesis is to consider our own personal "synthesiser" - when we speak, sing or make 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 which is rich in harmonics. The difference between the two comes from the filtering which we apply with the mouth and throat. By changing the shape of the mouth, we vary 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, you can simulate the "sweeping filter" effect which is widely used in electronic music and which is the basis of the "wahwah" guitar effect (which is so named for its similarity to this vocal "filter.")
We are also capable of generating something approximating white noise with the mouth by making a "sshh" sound. If you "synthesise" a "jet plane landing" sound you're doing so mostly by using the shape of your mouth to filter the white noise into pink noise by removing the higher frequencies. The same technique (filtered white noise) can be used to electronically synthesise the sound of ocean waves and wind, and was used in early drum machines to create snare drum and other percussion sounds.

An electronic example

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 synthesiser. We will attempt to imitate the sound of a plucked string. Whilst the following example illustrates how a desired sound might be achieved in practise, 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:

In this case we'll use pulse-width modulation for a dynamically changing tone:

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

The combined wave is passed through a voltage controlled amplifier connected to an ADSR envelope. In plain language, it's changed according to a pre-set pattern. In this case we attempt to emulate the envelope of a plucked string:

We then pass the sound through a shallow low-pass filter:

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

In real music production, there's often an additional step. An oscillator with a very low frequency modulates one or more sounds over time, creating a dynamically changing sound. This example modulates the cutoff frequency of the filter over several bars of music:

Further Information
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