A voltage controlled amplifier, or VCA, is an amplifier whose gain is set by the voltage level of a control signal. (The word "amplifier" is somewhat misleading in this usage; most VCA circuits do not exceed a gain ratio of 1, meaning that they attentuate rather than amplifying.) In many synthesizers, a VCA (or a digital equivalent) is the last functional block that a signal goes through before being sent to the synth's output; the VCA determines the instantaneous volume level of a played note, and it quiets the output at the end of the note. (Most synthesizer VCO circuits do not have amplitude control over their output or the ability to stop oscillating; if the synth did not contain a VCA, its outputs would sound continuously!) A VCA may be referred to as being "two quadrant" or "four quadrant" in operation. In a two quadrant VCA, if the control voltage input drops to less than or equal to zero, the VCA produces no output. In a four quadrant VCA, once the control voltage drops below zero, the output gain rises according to the absolute value of the control voltage, but the output is inverted in phase from the input. A four quadrant VCA is used to produce amplitude modulation and ring modulation effects.

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