Most oscillators that output a square waveform also have an additional control voltage input that sets the width of the top portion of the “square” wave (obviously, making the top portion wider makes the bottom portion narrower and vice versa). The act of varying the width of the resulting pulse wave creates a sort of Doppler shift; varying the width back and forth – for example, by modulating the pulse width with a low frequency oscillator – creates a chorusing effect that can sound like a detuned pair of oscillators. The resulting effect is referred to as pulse width modulation.
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pulse width modulation