The name is drawn from the two main components that define the function and characteristics of the module. One is the vactrol which is a light controlled resistor, these have an interesting effect whereas there is always a time delay whilst the resistor changes its value – this is a good thing and has been exploited by many synth designers, notably Don Buchla. The PiLL is from a PLL – phase locked loop, this is a special purpose IC which contains number of subcircuits and usually used in radio and digital control circuits. It contains a VCO, logic gate and a phase detector = all fun stuff!
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