Insound andmusic, anenvelope describes how a sound changes over time. For example, a piano key, when struck and held, creates a near-immediate initial sound which gradually decreases in volume to zero. An envelope may relate to elements such asamplitude (volume), frequency (with the use offilters) orpitch.
Envelope generators, which allow users to control the different stages of a sound, are common features ofsynthesizers,samplers, and otherelectronic musical instruments. The most common envelope generator is controlled with four parameters:attack,decay,sustain andrelease (ADSR).
The envelope generator was created by the American engineerRobert Moog, the creator of theMoog synthesizer, in the 1960s. The composerHerbert Deutsch suggested Moog find a way to articulate his synthesizer so notes did not simply trigger on and off. Moog wired adoorbell button to the synthesizer and used acapacitor to store and slowly releasevoltage produced from hitting a key. He refined the design to remove the need to push a separate button with every keypress, with two switches on every key: one to produce the control voltage determining pitch and the other to trigger the envelope generator. The envelope generator became a standard feature of synthesizers.[1]
Following discussions with the engineer and composerVladimir Ussachevsky, the head of theColumbia-Princeton Electronic Music Center, in 1965, Moog developed a new envelope module whose functions were described in f T1 (attack time), T2 (initial decay time), ESUS (sustain level), and T3 (final decay time). These were later simplified to the modern ADSR form (attack time, decay time, sustain level, release time) byARP.[1]
The most common kind of envelope generator has four stages: attack, decay, sustain, and release (ADSR).[2]
While attack, decay, and release refer to time, sustain refers to level.[2]
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Some electronic musical instruments can invert the ADSR envelope, reversing the behavior of the normal ADSR envelope. During the attack phase, the modulated sound parameter fades from the maximumamplitude to zero then, during the decay phase, rises to the value specified by the sustain parameter. After the key has been released the sound parameter rises from sustain amplitude back to maximum amplitude.
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Some envelopes, such as that of theKorg MS-20, have an extra parameter, hold. This holds notes at the sustain level for a fixed length of time before decaying. TheGeneral Instrument AY-3-8910sound chip includes only a hold time parameter; the sustain level is not programmable.
Another common variation in the same vein is the AHDSR (attack, hold, decay, sustain, release) envelope, in which thehold parameter controls how long the envelope stays at full volume before entering the decay phase. Multiple attack, decay and release settings may be found on more sophisticated models.
Certain synthesizers also allow for adelay parameter before theattack. Modern synthesizers, such as theProphet '08, have DADSR (delay, attack, decay, sustain, release) envelopes. The delay setting determines the length of silence between hitting a note and the attack. Somesoftware synthesizers, such as Image-Line's 3xOSC (included with theirDAWFL Studio) have DAHDSR (delay, attack, hold, decay, sustain, release) envelopes.
A common feature on many synthesizers is an AD envelope (attack and decay only). This can be used to control, for example, the pitch of one oscillator,[4] which in turn may be synchronized with another oscillator byoscillator sync.