When you vary a parameter of a synthesizer module using voltage control, it is said that you’re modulating that parameter. For example, when a low frequency oscillator (LFO) varies the cutoff frequency of a filter to create a wah-wah effect, it is said that the LFO is modulating the cutoff. When an envelope generator causes a voltage controlled amplifier (VCA) to open up to allow a sound to become suddenly loud, and then fades it back down to silence, you can also say the envelope is modulating the amp (although some like to restrict the term “modulate” to a repetitive action). Therefore, we call the sources of these changes modulators.
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