The most common type of envelope generator, producing a four-segment signal corresponding to four phases of the dynamics of a note played on most types of acoustical instruments. Each letter of the acronym stands for one of the four phases, in time order: attack, decay, sustain, and release. A typical ADSR allows the performer to set rise/fall rates for attack, decay, and release, and a level for the sustain phase.(Fancier envelope generators may have more settings or more operational phases.) Like other kinds of envelope generators, the ADSR is typically used as the control input to a voltage controlled amplifier, to control the dynamics of a played note (and silence it when nothing is being played), although it certainly can be used to control other functions.
Conventionally, an ADSR is triggered by the leading edge of a gate signal which typically is generated by a keybaord key; the performer pressing the key causes the gate to go high, and the gate goes low when the performer releases the key. The gate going high initiates the attack phase, in which the ADSR’s output rises from the zero level to a set maximum level at the chosen rate.This is immediately followed by the decay phase, in which the output falls from the maximum level to the sustain level. The envelope generator pauses at the sustain level, and remains there until the gate signal goes back low. At that point the release phase executes, and the ADSR output falls from the sustain level to its zero level at the chosen rate. Options often found on ADSRs are an input that allows all of the rates to be scaled (a function often tied to keyboard velocity, for better acoustic instrument simulation), and a switch that allows making the rise/fall rates exponential with time instead of constant. (Such is often useful for percussion sounds.)