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A Four-Quadrant Multiplier is a special case of Amplitude Modulation (AM). It is also referred to as ring or balanced modulation. One signal changes the level of – “multiplies” – the level of a second signal. So if the first signal currently has a level of +50%, and the second signal also has a level of +50%, then the output is +25%. If the first signal has a level of –100%, and the second signal has a level of +30%, then the output is –30%, as a four-quadrant multiplier honors both positive and negative signals.

A typical use is two VCOs running at audio rates fed into a ring modulator (a four-quadrant multiplier). The output is a complex set of component tones that don’t follow typical “musical” spacing based on octaves above the fundamental that harmonics usually follow. Namely, the modulation frequency is both added to and subtracted from the carrier’s frequency; the resulting harmonics replace the original carrier and modulator. Say the carrier was a sine wave (only the fundamental harmonic present) at 600Hz, and the modulator was a sine wave at 100Hz. The result would be a tone that had frequency components at 500 and 700Hz. Click here for more details of exploring “modulation” synthesis.

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