Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Subtractive synthesis is a method creating a sound by removing harmonics, 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.