The lopass gate module is unique in the way that it simulates the characteristics of natural instruments. When used in lopass gate mode, louder sounds contain more harmonic content and quieter sounds contain less harmonic content. This is not the case when using a standard VCA such as the Pittsburgh Modular Dual VCA module. A standard VCA simply changes the loudness of the sound without changing the harmonic content. There is no reason that synthesizers need to simulate how acoustic instruments function but because of how it affects the harmonic content of sound, a lowpass gate sounds more organic or natural compared to a standard filter/vca chain
A lot of the characteristics associated with the lopass gate circuit come from the use of vactrols. A vactrol is an optoelectronic device consisting of an LED and light detector enclosed in a light tight package. The elements of the vactrol are optically coupled and electrically isolated from each other. When the LED within the vactrol turns on, the light detector reacts very quickly creating a very sharp attack, however, when the LED turns off, the light detector within the vactrol does not react as quickly and closes more slowly. That slow decay or ringing is the quality most associated with the lopass gate. Therefore, using a very short trigger to ping the vactrol will not result in a very short sound, instead the result is an organic percussive sound with an natural sounding decay.
https://pittsburghmodular.com/lpg
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