An envelope detector is sometimes called an envelope follower in musical settings. It detects the amplitude variations of an incoming signal in order to produce a control signal based on those variations. In this instance, the input signal is made up of audible frequencies.
You’ll often find that envelope detectors are a component of other circuits, such as an auto-wah, a compressor, or an envelope-followed filter. In these circuits, the envelope follower is part of the “side chain.” This is a circuit that describes a particular characteristic of the input, which in this case, is its volume.
The expanders and compressors both use the envelope’s output voltage to manage the gain of an amplifier. Auto-wah uses the voltage to regulate the cutoff frequency of a filter. An analog synthesizer’s voltage-controlled filter is a similar circuit.
Modern envelope followers can be used in the following ways:
- Virtually in software
- Directly as hardware
- Indirectly through the use of DSPs