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This is a common waveform produced by a synthesizer’s oscillator. It alternates between a high and low voltage (typically +/-5 or 8 volts for an audio oscillator; sometimes low frequency oscillators go between 0v and a positive voltage). Aside from being a really easy waveshape to generate with analog circuitry, it has an interesting harmonic series: it has a strong fundamental, then gradually weaker odd harmonics: a component at three times the fundamental frequency, one at fives time the fundamental, and so forth. The result is a more open, hollow sound, especially when compared to a sawtooth (ramp) wave that has both odd and even harmonics present.

Technically, a square wave is high for the exact same duration as it is low. When you vary the relationship between the high and low portions, the result is referred to as a pulse wave. Pulse waves have weaker lower harmonics and more complex upper harmonics, resulting in a more nasal sound that is good for imitating woodwind-like sounds. Pulse width modulation, where you vary this high/low duration balance with a modulating source like an LFO, creates a cool chorus-like result that sounds more like two oscillators in unison that have been detuned slightly.

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