The ADR envelope uses long decay times to simulate a high sustain level, in which case the resulting envelope is very much like an AR envelope, or else a percussive AD envelope by using shorter decay times (Figure 3.3.21).
FIGURE 3.3.21 The ADR envelope provides control over separate decay and release segments. This allows more complex envelope shapes to be produced than is possible with AR or AD EGs. If the key or gate is released during the attack segment, then the envelope moves to the release segment and ignores the decay segment.
Attack decay sustain
If a sustain level is added to an AD envelope, then the attack decay sustain (ADS) EG is the result (Figure 3.3.22). The attack segment reaches a maximum value and the decay time then sets how long it takes for the envelope to reach the sustain level. Some ADS EGs have switches that make the release time the same as the decay time or else have a very short release time. The type of envelope that is produced depends on the sustain level. If the sustain level is set to the maximum level (the same as the attack reaches), then two-segment ARtype envelopes are produced. If the sustain level is set to zero, then only twosegment AD envelopes are produced. With the sustain level set mid-way, then four-segment ADSR-type envelopes can be produced. If these have an initial attack and decay portion, then the sustain portion whilst the key is held down and then a release portion when the key is released.
FIGURE 3.3.22 An ADS envelope adds a sustain segment at the end of the decay segment. The 'release' time is normally set to the same as the decay time, although some synthesizers provide a switch which forces a fast release time regardless of the setting of the decay time. An ADS EG can be used to produce a wide variety of envelopes, including the ones which have many of the characteristics of ADSR (see later), AR and AD envelopes.
Attack decay sustain release
The most widely adopted EG is probably the ADSR (Figure 3.3.23). With just four controls, it is capable of producing a wide variety of envelope shapes; with only the attack decay 1 break decay 2 release (ADBDR) dual-decay variant offering superior flexibility at the cost of one extra control. The ADSR EG's main weakness is that the sustain segment is static, it is a fixed level. For this reason, ADSR-type envelopes are not particularly well suited in producing percussive piano-type envelopes, where the 'sustain' portion of the sound gradually decays to zero. See ADBDR envelope later for a better alternative.
FIGURE 3.3.23 The ADSR envelope adds a separate control for the release time. This provides enough flexibility to produce a large number of envelopes with a small number of controls and the ADSR envelope is widely used in synthesizers.
Attack hold decay sustain release
Some envelopes force the envelope to stay at the maximum or peak level for a fixed time when the attack segment has finished and before the decay segment can start (Figure 3.3.24). These are called attack hold decay sustain release (AHDSR) envelopes. This is useful when a percussive envelope is set with very rapid attack and decay times, and the minimum length of the envelope needs to be controlled. For some sounds, an AD envelope with fast times (less than 10 ms) can be too short to be audible.
FIGURE 3.3.24 An AHDSR envelope adds a 'hold' segment at the end of the attack segment, rather like the sustain segment, but the length is set by a time rather than when the key or gate is released. As with other envelope shapes, if the key is released before the sustain segment, then the envelope moves to the release segment.
A variation on the hold segment being after the 'attack' segment of the envelope is the attack decay hold release (ADHR) envelope, where the 'sustain' segment is only held up to a specific time, after which it begins to decay. This is arguably better suited to percussive and piano sounds than the ADSR.
Attack decay 1 break decay 2 release
By splitting the decay segment into two portions, with a 'break-point' level controlling when one decay portion finishes and the other starts, a wide range of envelope shapes can be produced (Figure 3.3.25). By setting the second decay to a very long time, it can be used in much the same way as a sustain segment, although it has the advantage that it can still decay away slowly. This is arguably a better emulation of real-world envelopes for instruments such as pianos, where the sustain segment is actually a long decay time. In some implementations of ADBDR envelopes, this second decay is called the 'slope' segment to distinguish it from the decay segment.
FIGURE 3.3.25 The ADBDR envelope has two decay segments and the transition from one decay is set by a variable level control, rather like a sustain level control. By setting the decay time to a long value, they can be used as pseudo-sustain segments, and so an ADBDR envelope can produce similar envelopes to an ADSR type.