Subtractive synthesis
Encyclopedia
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 analogue synthesisers of the 1960s and 1970s, in which the harmonics of simple waveforms such sawtooth
, pulse
or square wave
s are attenuated with a voltage controlled
resonant low-pass filter
. Many digital
, virtual analogue and software
synthesisers utilise subtractive synthesis, sometimes in conjunction with other methods of sound synthesis.
act as an oscillator and the mouth
and throat
as a filter
. Consider the difference between singing “oooh” [uː] and “aaah” [ɑː], at the same pitch
. The sound generated by the vocal folds is much the same 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, the frequency response
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, a spectral glide
is created, emulating the ‘sweeping filter’ effect that is the basis of the ‘wah-wah
’ guitar effect.
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.
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 practice, only the final three stages are really subtractive synthesis and the early stages could be considered to be a form of additive synthesis
.
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.
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
synthesis in which partials of an audio signal (often one rich in harmonics) are attenuated by a filter
Audio filter
An audio filter is a frequency dependent amplifier circuit, working in the audio frequency range, 0 Hz to beyond 20 kHz. Many types of filters exist for applications including graphic equalizers, synthesizers, sound effects, CD players and virtual reality systems.Being a frequency dependent...
to alter the timbre
Timbre
In music, timbre is the quality of a musical note or sound or tone that distinguishes different types of sound production, such as voices and musical instruments, such as string instruments, wind instruments, and percussion instruments. The physical characteristics of sound that determine the...
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 analogue synthesisers of the 1960s and 1970s, in which the harmonics of simple waveforms such sawtooth
Sawtooth wave
The sawtooth wave is a kind of non-sinusoidal waveform. It is named a sawtooth based on its resemblance to the teeth on the blade of a saw....
, pulse
Pulse wave
A pulse wave or pulse train is a kind of non-sinusoidal waveform that is similar to a square wave, but does not have the symmetrical shape associated with a perfect square wave. It is a term common to synthesizer programming, and is a typical waveform available on many synths. The exact shape of...
or square wave
Square wave
A square wave is a kind of non-sinusoidal waveform, most typically encountered in electronics and signal processing. An ideal square wave alternates regularly and instantaneously between two levels...
s are attenuated with a voltage controlled
Voltage-controlled filter
A voltage-controlled filter is a filter whose operating characteristics can be controlled by means of a control voltage applied to one or more inputs...
resonant low-pass filter
Low-pass filter
A low-pass filter is an electronic filter that passes low-frequency signals but attenuates signals with frequencies higher than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter. It is sometimes called a high-cut filter, or treble cut filter...
. Many digital
Digital synthesizer
A digital synthesizer is a synthesizer that uses digital signal processing techniques to make musical sounds.Electronic keyboards make music through sound waves.-History:...
, virtual analogue and software
Software synthesizer
A software synthesizer, also known as a softsynth is a computer program or plug-in for digital audio generation. Computer software which can create sounds or music is not new, but advances in processing speed are allowing softsynths to accomplish the same tasks that previously required dedicated...
synthesisers utilise subtractive synthesis, sometimes in conjunction with other methods of sound synthesis.
A human example
The basis of subtractive synthesis can be understood by considering the human voice; when a human speaks, sings or makes other vocal noises, the vocal foldsVocal folds
The vocal folds, also known commonly as vocal cords, are composed of twin infoldings of mucous membrane stretched horizontally across the larynx...
act as an oscillator and the mouth
Mouth
The mouth is the first portion of the alimentary canal that receives food andsaliva. The oral mucosa is the mucous membrane epithelium lining the inside of the mouth....
and throat
Throat
In vertebrate anatomy, the throat is the anterior part of the neck, in front of the vertebral column. It consists of the pharynx and larynx...
as a filter
Audio filter
An audio filter is a frequency dependent amplifier circuit, working in the audio frequency range, 0 Hz to beyond 20 kHz. Many types of filters exist for applications including graphic equalizers, synthesizers, sound effects, CD players and virtual reality systems.Being a frequency dependent...
. Consider the difference between singing “oooh” [uː] and “aaah” [ɑː], at the same pitch
Pitch (music)
Pitch is an auditory perceptual property that allows the ordering of sounds on a frequency-related scale.Pitches are compared as "higher" and "lower" in the sense associated with musical melodies,...
. The sound generated by the vocal folds is much the same 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, the frequency response
Frequency response
Frequency response is the quantitative measure of the output spectrum of a system or device in response to a stimulus, and is used to characterize the dynamics of the system. It is a measure of magnitude and phase of the output as a function of frequency, in comparison to the input...
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
Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
). By gradually changing from “oooh” to “aaah” and back again, a spectral glide
Spectral glide
A spectral glide is a "modification of the vowel quality of a tone" . Since the vowel quality of a tone is determined by the overtones, spectrum, or timbre of that tone, a spectral glide is a move from one spectrum characteristic of a vowel to another...
is created, emulating the ‘sweeping filter’ effect that is the basis of the ‘wah-wah
Wah-wah pedal
A wah-wah pedal is a type of guitar effects pedal that alters the tone of the signal to create a distinctive effect, mimicking the human voice...
’ guitar effect.
Humans are also capable of generating something approximating white noise
White noise
White noise is a random signal with a flat power spectral density. In other words, the signal contains equal power within a fixed bandwidth at any center frequency...
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
Pink noise
Pink noise or 1/ƒ noise is a signal or process with a frequency spectrum such that the power spectral density is inversely proportional to the frequency. In pink noise, each octave carries an equal amount of noise power...
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
Snare drum
The snare drum or side drum is a melodic percussion instrument with strands of snares made of curled metal wire, metal cable, plastic cable, or gut cords stretched across the drumhead, typically the bottom. Pipe and tabor and some military snare drums often have a second set of snares on the bottom...
and other percussion
Percussion instrument
A percussion instrument is any object which produces a sound when hit with an implement or when it is shaken, rubbed, scraped, or otherwise acted upon in a way that sets the object into vibration...
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 computerPersonal computer
A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...
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 practice, only the final three stages are really subtractive synthesis and the early stages could be considered to be a form of additive synthesis
Additive synthesis
Additive synthesis is a technique of sound synthesis that creates musical timbre by explicitly adding sinusoidal overtones together.The timbre of an instrument is composed of multiple harmonic or inharmonic partials , of different frequencies and amplitudes, that change over time...
.
- First, two oscillators produce relatively complex and harmonic-rich waveforms:
- }
- }
- In this case we will use pulse-width modulationPulse-width modulationPulse-width modulation , or pulse-duration modulation , is a commonly used technique for controlling power to inertial electrical devices, made practical by modern electronic power switches....
for a dynamically changing tone: - }
- }
- The two sounds are mixed. In this case they are 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 is 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 filterLow-pass filterA low-pass filter is an electronic filter that passes low-frequency signals but attenuates signals with frequencies higher than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter. It is sometimes called a high-cut filter, or treble cut 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 is often an additional step. An oscillator with a very low frequency modulates one or more sounds over time, creating a dynamically changing sound.
External links
- Buzz Tracker - A freeware electronic music program which could be used to explore this topic further. The example here was generated in Buzz.
- AMS - A free software synthesis program for ALSAALSA (Linux)Advanced Linux Sound Architecture is a software framework providing an API for device drivers for sound cards. As such, it is a Linux kernel component. Some of the goals of the ALSA project at its inception were automatic configuration of sound-card hardware, and graceful handling of multiple...
.