Subtractive synthesis
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Subtractive synthesis is a method of sound synthesis characterised by the application of an audio filter to a source signal. For example, taking the output of a sawtooth oscillator 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 oscillator 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 or the minimoog 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.
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, 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 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
External links
- Buzz Tracker (http://www.buzzmachines.com) - A freeware electronic music program which could be used to explore this topic further. The example here was generated in Buzz.
- AMS (http://alsamodular.sourceforge.net) - A free software synthesis program for ALSA.