This module controls the amplitude of a signal passing through it by scaling or attenuating it in response to a separate control voltage input. It is most commonly used as the last stage in a patch, articulating when we can hear or not hear a sound the synthesizer is producing.
VCAs have many other uses in a patch, as they allow you to control the level of virtually signal moving around inside a patch – for example, you could route a LFO through a VCA, and then control the VCA with an envelope, allowing the LFO depth to rise and fall along with the envelope. Indeed, you will often hear advanced modular uses say you can never have too many VCAs in a system.
VCAs tend to come in two flavors: linear and exponential. Exponential VCAs in theory respond to the way we hear loudness, and therefore might be good for voltage controlled mixing, panning, etc. Some also prefer to use them for percussive sounds, as the decay is steeper given the same envelope. However, they can be harder to control for more sustained sounds – especially those with slow attacks, as the exponential response can create an undesired delayed swell in the sound. Therefore, it’s often useful to have linear VCAs, and envelopes with a combination of logarithmic and exponential outputs. (Actually, best of all is to have envelopes and VCAs both that can be switched or crossfaded between linear and exponential response, so you can dial in the exact response you desire.)
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