This module follows the loudness contour of a sound, and outputs a voltage that corresponds to how that loudness changes. For example, if you send the sound of a struck drum or plucked guitar into an envelope follower, its output voltage would immediately jump to a high level that corresponds to the loudness of the note (how hard it was struck or plucked), and that voltage would fall back to zero as the sound decayed into silence. They tend to perform some smoothing on this signal so that it’s not too nervous or jumpy in nature.
Envelope followers often also have a gate output that goes high when the loudness of the input signal went over a certain level, and low when it falls back below that level. Really nice ones also have an equalizer or filter stage so that they follow only a certain band of frequencies (helpful for picking a single instrument or note out of a mixed signal).
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