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Create sounds using analogue electronics (Part 3)

Posted: 06 Dec 2011     Print Version  Bookmark and Share

Keywords:voltage-controlled amplifier  digital synthesisers  EG  attack decay 

In this instalment, we comprehensively discuss envelopes. Part 1 briefly reviews the differences between analogue and digital synthesis, and discusses "one of the major innovations in the development of the synthesiser"—voltage control. Part 2 begins a look at subtractive synthesis with a discussion of VCOs, waveforms, harmonic content, and filters.

Envelopes
An envelope is the overall 'shape' of the volume of a sound, plotted against time (figure 1). In an analogue synthesiser, the volume of the sound output at any time is controlled by a voltage-controlled amplifier (VCA) and the voltage that is used is called an envelope. Envelopes are produced by 'EGs' and have many variants. EGs are categorised 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 1: 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 2). 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 2: 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 standardised set of names for the segments of envelopes in analogue synthesisers, which contrasts with the more diverse naming schemes used in digital synthesisers.

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.

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