An envelope detector is sometimes referred to as an envelope follower in musical environments. It is used to detect the amplitude variations of an incoming signal to produce a control signal that resembles those variations. However, in this case the input signal is made up of audible frequencies.
Envelope detectors are often a component of other circuits, such as a compressor or an auto-wah or envelope-followed filter. In these circuits, the envelope follower is part of what is known as the "side chain", a circuit which describes some characteristic of the input, in this case its volume.
Both expanders and compressors use the envelope's output voltage to control of the gain of an amplifier. Auto-wah uses the voltage to control the cutoff frequency of a filter. The voltage-controlled filter of an analog synthesizer is a similar circuit.
Modern envelope followers can be implemented:
- directly as electronic hardware,
- indirectly using DSPs
- completely virtually in software.