A circuit which generates a signal, usually meant to control a voltage controlled amplifier for the purpose of giving dynamic contour to a played note (in other words, providing a rise and fall of the volume of the note). A typical envelope generator produces a signal which, as seen on an oscilloscope, appears as a series of line or curve segments. Controls are provided for setting the shape of the resulting curve.
The most common use of an envelope generator is to control the output level of a voltage controlled amplifier. The shapes possible with typical envelope generators designed for this purpose are generally designed to allow the user to mimic the note dynamics of various acoustic instruments. A minimal envelope generator has an attack, in which the level of the envelope rises from an initial zero level to a maximum level, followed by a release, in which the level falls back to zero. This simple type of envelope generator is known as an "attack-release", or AR, and it is sufficient for producing the dynamics of a simple non-sustaining percussion sound. The AR envelope generator will have two controls, one of which sets the time it takes for the attack to reach the maximum level, and another which sets the amount of time it takes for the release to fall back to the zero level. The AR starts its cycle when it receives a trigger signal, which typically would come from a key on a keyboard being pressed.
A more sophisticated envelope generator adds a sustain phase, in between the attack and the release. The sustain phase is usually "flat", in that the output level does not change during the sustain phase. The sustain time might be settable by a control, but typically this type of envelope generator takes a gate signal from a keyboard or other source. When the gate goes high (as from a key on the keyboard being pressed), the envelope generator begins its cycle. When it reaches the sustain phase, it pauses and remains there until the gate goes low (e.g., from the keyboard key being let up). When this occurs, the envelope generator goes through its release phase and finishes the cycle. This type of envelope generator is known as an ASR, or "attack-sustain-release". A further improvement is to add a decay phase in between the attack and sustain phases. In the decay phase, the level drops from the maximum level to the "sustain level" (which is now controllable). On gate high, the generator goes through attack and decay to sustain, where it pauses until gate low. This is the very common "attack-decay-sustain-release", or ADSR envelope generator. When used to drive a VCA, it mimics the dynamics of instruments that can sustain a note but have an initial attack transient, such as brass and bowed string instruments. There are four controls: the attack time, the decay time, the sustain level, and the release time.
The commonly found ADSR is one type of envelope generator. More advanced envelope generators provide more segments, allowing more complicated volume envelopes to be produced. A few types that have appeared on synths in the past include:
- DADSR: This has an initial delay between gate-on and the start of the attack phase. Seen on some Oberheim synths, and many modular envelope generators.
- ADSDR: This envelope has a non-constant sustain. The second 'D' is the "sustain decay", the rate at which the envelope level drops during the sustain phase. The release phase begins from whatever level the sustain has decayed to at that point. A common use of this is to mimic the way that a note played on a piano gradually dies out if the key or the damper pedal is held down. Commonly seen on Yamaha synths of the 1980s, particularly the DX7.
- AHDSR: The 'H' is a hold phase where the envelope output remains at the maximum level between the end of the attack and the beginning of the decay. This emulates an inadvertent feature of the early Minimoog envelope generators, which due to a design issue, had a short pause between the end of the attack and the beginning of the decay. It was found that this was particularly effective at producing "punchy" bass sounds, and accounts for some of the Minimoog's reputation as being especially good for bass patches.
More sophisticated envelope generators may have additional segments, before or after the sustain phase. The ultimate in this area can be seen in some soft synths which provide envelope generators with hundreds of segments. Another feature sometimes seen, for envelope generators meant to control a a voltage controlled filter to produce a timbre contour, or mixed with the control voltage input to a voltage controlled oscillator to produce a frequency contour, is the ability to have the release end at an arbitrary level, instead of returning to zero. A related device called a universal event generator provides multiple segments and individual control over both the time and ending level for each segment.