The Yamaha DX7 synthesizer introduced a new type of envelope generator, one which had four rate control and four level control parameters, for a total of eight parameters. Each rate parameter controls how long it takes the envelope to move from one level to the next. This is referred to as a four-stage envelope because each rate/level pair is considered a stage. Technically, an ADSR envelope only has one true stage, the decay/sustain stage and two more partially controllable stages because the attack and release levels were fixed at 100% and zero, respectively.