A device which produces a control signal which varies in level with time. The simplest of these (sometimes called ADSR) has four parameters: Attack Time, Decay Time, Sustain Level & Release Time. Thus it has three time constants and one level constant. Such a device is often found on synthesisers, where it is usually used to control the volume of an amplifier, the cut off frequency of a filter, or the frequency of an oscillator. More complex arrangements involving a series of time and level constants are available, particularly on digital synthesisers. These are called multistage envelopes and may involve eight or more parameters usually in time and level pairs.
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