Whereas additive synthesis is the process of combining individual sinusoidal partials to construct a complex sound, subtractive synthesis is essentially the reverse of this process. By starting with a harmonically (or partially) rich sound, a subtractive system will filter and modify the signal to reduce it to a desired form. By doing this, one can use one or two oscillators instead of a bank of 10 to achieve similar sonic results. Subtractive synthesis is an extremely popular method of synthesis that has been employed in hardware and software synthesizers since their popularity skyrocketed in the 1970s. This method of synthesis can be found traditionally in old modular or compact analogue synthesizers as well as modern virtual analogue models and software synthesizers. Fig. 7.1 illustrates a simple block diagram form of a subtractive system.
The oscillator will typically produce a waveform with a wide frequency spectrum (square wave, noise) at a frequency, and an amplitude , although there can be two or perhaps even three oscillators in the design; any more begin to distract from the purpose of a subtractive synthesis technique although there are designs which exist. The filter block, which can be comprised out of many different types of filter (lowpass, bandpass), changes the character of the oscillator by controlling the balance of the frequency spectrum, before being modified further by additional effect modules built into the system. These elements can be user specified or automated using other oscillators or value envelopes. The subtractive approach is favoured by many musicians due to the fact that it is comparatively imprecise and therefore requires tweaking and experimentation of control parameters to get a desired sound.