Subtractive synthesis

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 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’).

Humans are also capable of generating something approximating white noise by making a “sshh” sound. If a person ‘synthesizes’ a ‘jet plane landing’ sound, this is achieved mostly by altering the shape of the 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 synthesize 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 synthesizer. 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:
#*Audio|subsynth-wave1.ogg|Waveform #1
#*Audio|subsynth-wave2.ogg|Waveform #2
#In this case we will use pulse-width modulation for a dynamically changing tone:
#*Audio|subsynth-wave1pwm.ogg|PWM waveform #1
#*Audio|subsynth-wave2pwm.ogg|PWM waveform #2
#The two sounds are mixed. In this case they are combined at equal volume, but any ratio could be used.
#*Audio|subsynth-wavemix.ogg|Combined waveforms
#The combined wave is passed through a voltage controlled amplifier connected to an ADSR envelope. In plain language, it is changed according to a pre-set pattern. In this case we attempt to emulate the envelope of a plucked string:
#*Audio|subsynth-mixenv.ogg|Enveloped sound
#We then pass the sound through a shallow low-pass filter:
#*Audio|subsynth-nomodfilter.ogg|Low-passed sound
#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.
#*Audio|subsynth-modfilter.ogg|Final sound
#*Audio|subsynth-arpeggios.ogg|Some arpeggios
#*Audio|subsynth-internationale.ogg|A patriotic tune

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. This example modulates the cutoff frequency of the filter over several bars of music:

*Audio|subsynth-lfomod.ogg|Arpeggios, with filter modulation

External links

* [http://www.buzzmachines.com Buzz Tracker] - A freeware electronic music program which could be used to explore this topic further. The example here was generated in Buzz.
* [http://alsamodular.sourceforge.net AMS] - A free software synthesis program for ALSA.