There are many sophisticated enhancements of the basic analogue ADSR EG (Figure 3.3.26). Most of these are ADSRs with the addition of initial time delay, break-points in the attack or decay segments and times for the peak and sustain levels. Although the extra controls provide more possibilities for envelope shapes, they also greatly increase the complexity of the user interface. Delayed envelopes (denoted by an initial 'D' in the abbreviation: DADSR for delayed ADSR) are used when the start of the envelope needs to be delayed in time without the need for using a long attack time, or where the attack needs to be rapid after the delay time.
FIGURE 3.3.26 Multi-segment envelopes can have several attack, decay and release segments, as well as hold and sustain segments. Break-points can also be used to split a segment into smaller segments.
Some of these EGs provide a break-point in the attack segment, so that two different attack times can be controlled. This is especially useful for long attack times, where the start of the audio signal is too quiet to be heard, and the initial portion of the attack segment is heard as a delay. By having a rapid rise to a level where the audio signal is audible, followed by a slower second attack portion, this unwanted apparent delay can be avoided. This extra break-point is also useful for simulating more complicated attack curves.
Break-points are not always explicitly named as such. The interaction between the gate signal and the envelope often has implied break-points at the transitions between attack, decay, sustain and release. These are frequently not documented in the manufacturer's product information. The usual method of operation is shown in Figure 3.3.27.
FIGURE 3.3.27 The transition from the attack segment to the release segment when the key or gate is released can be thought of as adding in a break-point to the attack segment.
If the key is only held down for a short time, and the envelope is still in the attack segment when the key is released, then the envelope will go into the release segment. In this case the envelope may not reach the maximum level, although some EGs always rise to the maximum level. If there is a hold time associated with the maximum level, then this is usually not affected by the key being released. If the envelope has reached the decay segment, then when the key is released, the envelope will go into the release segment.
If the initial, final, peak and sustain levels are all controllable, then the envelope flexibility can become approximately equivalent to the multi-segment envelopes often found in digital synthesizers, although the terminology is normally very different. See Chapter 5 for more details on digital envelopes.
Some analogue synthesizers only have one EG, which is then used to control both the VCF and VCA. If two envelopes are available, then patching one to the filter and the other to the amplifier provides independent control over the volume and timbre. A third envelope could be used to control the pitch of the VCOs or perhaps the stereo position of the sound using two VCAs arranged as a pan control.