“AHDSR” stands for Attack, Hold, Decay, Sustain, and Release, which are the five stages of an extended envelope generator used to shape the amplitude or other parameters of a sound over time. The AHDSR envelope generator provides a control voltage (CV) signal that can be used to modulate various aspects of a synthesizer’s sound, such as the amplitude, filter cutoff frequency, or modulation depth. The five stages of the AHDSR envelope are as follows:
- Attack: This is the first stage, during which the control voltage rises from its initial value (usually zero) to its peak value. The attack time determines how quickly this rise occurs, with shorter attack times producing a faster, more immediate onset of the sound, and longer attack times creating a slower, more gradual increase in amplitude or other parameter.
- Hold: This is the second stage, during which the control voltage remains at its peak value for a specified period. The hold time determines the duration of this constant level, allowing for a sustained peak before the envelope proceeds to the decay stage.
- Decay: This is the third stage, during which the control voltage falls from its peak value to the sustain level. The decay time determines how quickly this decrease occurs, with shorter decay times producing a faster transition, and longer decay times resulting in a slower, more gradual change.
- Sustain: This is the fourth stage, during which the control voltage remains at a constant level while a note is held or a gate signal is active. The sustain level determines the amplitude or intensity of the sound during this stage, with higher sustain levels resulting in louder or more pronounced sounds, and lower sustain levels producing quieter or more subdued sounds.
- Release: This is the final stage, during which the control voltage falls from the sustain level back to its initial value after a note is released or the gate signal is deactivated. The release time determines how quickly this decrease occurs, with shorter release times producing a faster, more abrupt fade-out of the sound, and longer release times resulting in a slower, more gradual decay.
By adjusting the attack, hold, decay, sustain, and release parameters, a musician can shape the dynamic characteristics and articulation of a sound, creating a wide range of expressive possibilities. AHDSR envelope generators are less common than the simpler ADSR type but can be found in some hardware and software synthesizers, as well as in modular synthesizer systems.