[Part 1 briefly reviews the differences between analogue and digital synthesis, and discusses "one of the major innovations in the development of the synthesizer" - voltage control. Part 2 begins a look at subtractive synthesis with a discussion of VCOs, waveforms, harmonic content, and filters.]
An envelope is the overall 'shape' of the volume of a sound, plotted against time (Figure 3.3.17). In an analogue synthesizer, the volume of the sound output at any time is controlled by a voltage-controlled amplifier (see VCA) and the voltage that is used is called an envelope. Envelopes are produced by 'EGs' and have many variants. EGs are categorized by the number of controls which they provide over the shape of the envelope. The simplest provide control only over the start and end of a sound, whilst the most complex may have a very large number of parameters.
FIGURE 3.3.17 The 'envelope' of a sound is the overall shape – the change in volume with time. The shape of an envelope often forms a distinctive part of a sound.
Envelopes are split into segments or parts (Figure 3.3.18). The time from silence to the initial loudest point is called the attack time, whilst the time for the envelope to decrease or decay to a steady value is called the decay time. For instruments that can produce a continuous sound, such as an organ, the decay time is defined as the time for the sound to decay to the steady-state 'sustain' level, whilst the time that it takes for the sound to decay to silence when it ends is called the release time.
FIGURE 3.3.18 Envelopes are divided into segments depending on their position. The start of the sound is called the 'attack segment'. After the loudest part of the sound, the fall to a steady 'sustain' segment is called the 'decay' segment. When the sound ends, the fall from the sustain segment is called the 'release' segment.
Bowed stringed instruments can have long attack, decay and release times, whilst plucked stringed instruments have shorter attack times and no sustain time. Pianos and percussion instruments can have very fast attack times and complex decay/sustain segments. There is an almost standardized set of names for the segments of envelopes in analogue synthesizers, which contrasts with the more diverse naming schemes used in digital synthesizers.
Envelopes are usually referred to in terms of the CV that they produce, and it is normally assumed that they are started by a key being pressed on a keyboard. Envelopes can be considered to be sophisticated time-based function generators with manual key triggering.
The following are some of the common types of EGs.
Attack release (AR) envelopes only provide control over the start and end of a sound (Figure 3.3.19). The two-segment envelope CV, which is produced, rises up to the maximum level and then falls back to the quiescent level, which is usually 0 volts. AR envelopes are often found on 1970s vintage string machines: simple polyphonic keyboards that used organ 'master oscillator and divider' technology with simple filtering and chorus effects processing to give an emulation of an orchestral string sound (see Section 3.4 for more information).
FIGURE 3.3.19 In an AR envelope the pressing down of a key (or a similar gating device on a synthesizer that does not use keys) starts the attack segment. When the peak level has been reached, then the envelope stays at this level until the key is released (of the gating signal is removed) and the envelope falls in the release segment. If the key is released whilst the envelope is in the attack segment, then the envelope normally moves to the release segment, and need not reach the peak level (see also Figure 3.3.27). Some synthesizers provide a control which forces the whole of the attack segment to be completed.
If the envelope moves into the decay segment as soon as the attack segment has reached its maximum level, then the decay time sets how long it takes for the envelope to drop to zero. This means that only percussive (non-sustaining) envelopes can be produced (unless the decay time is set to be very long, as in some attack decay release (ADR) envelopes). These two-segment attack decay (AD) envelopes (Figure 3.3.20) are often found connected to the frequency control input of VCOs, where the envelope then produces a rapid change in pitch at the start of the note, known as a 'chirp'. This can be effective for vocal and brass sounds. Inverting the envelope can produce changes downwards in pitch instead of upwards.
FIGURE 3.3.20 An AD envelope is similar to an AR envelope, except that there is no sustain segment. When the peak level is reached, the envelope decays, even if the key is held down.