These are the four parameters found on a basic synthesizer envelope generator. An envelope generator is sometimes called a transient generator and is traditionally used to control the loudness envelope of sounds, though some modern designs allow for far greater flexibility. The Attack, Decay, and Release parameters are rate or time controls. Sustain is a level control. When a key is pressed, the envelope generator will begin to rise to its full level at the rate set by the attack parameter; upon reaching peak level it will begin to fall at the rate set by the decay parameters to the level set by the sustain control. The envelope will remain at the sustain level as long a the key is held down. Whenever a key is released, it will return to zero at the rate set by the release parameters.