Subtractive synthesis
Subtractive synthesis is a technique used in some synthesizers which creates musical timbres by filtering simple waveforms generated by oscillators. Subtractive synthesis is usually (but not exclusively) associated with analogue voltage controlled synthesizers such as the moog or the minimoog. It can produce very natural changes in a sound, owing to the intuitive way in which it works.
The starting waveforms are often square waves, pulse waves, saw waves or triangle waves. Digital and software synthesizers may include other, more complex waveforms or allow the user to upload arbitrary waveforms. Some synthesizers use a form of pulse width modulation to dynamically alter the waveform for a more organic tone.
Typically a lowpass filter is applied to the initial tone, because natural musical sounds generally include less of the harsh high frequencies found in square or sawtooth waves. Often the user has a choice of steeper and shallower filters. The cut-off frequency and resonance of the filter may be modulated by an envelope to create a more dynamic sound.
Example 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. We will attempt to imitate the sound of a plucked string.
- First, two oscillators produce relatively complex and harmonic-rich waveforms:
- In this case we will use pulse width modulation 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, its 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 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 parameters over time, creating a dynamically changing sound. This example modulates the cutoff frequency of the filter over several bars of music:
See also
