In easy words
A VCA or "voltage controlled Amplifier" is a module which does exactly what it name says: you can amplify signals via voltage control. You can use a VCA to control the Level of every signal in your Modular system via another signal. In radio electronics this is also known als "Amplitude Modulation" as it modulates the amplitude of the signal: AM.
Here is a typical (Quad-)VCA:
There are different ways of implementing a VCA. Eliott Sound Products has a good guide on the various techniques to achieve a VCA called VCA Techniques Investigated.
A small overview over the pros and cons of the different technologies:
|Technology||Distortion||CV Feed-Thru||Speed (Attack/ Release)||Example Parts|
|'True' VCA||low||low¹||fast||THAT2181, SSM2164, ...|
|Analogue Multipliers||low/medium¹||low¹||fast||AD633, MC1594, AD834, ...|
|OTA||high||medium/ low||fast||LM13700, NE5517,|
¹ usually adjustable using preset pots
² depends on speed and circuit topology
³ speed depends on implementation and topology