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  Computer Music Software and Hardware

Quick Guide To Envelopes

Envelopes play an essential part in all types of synthesis. We have the details wrapped up and stuck down...

Our ears are amazing yet incongruous organs. We can listen to a symphony orchestra and pick out individual instruments within the melee - something no computer software can do successfully - yet they can be fooled by simple alternatives to complex acoustic sounds. Which is fortunate for us otherwise the world of synthesis would be limited to drones and warbles, totally unable to create any realistic sounds.

The ability to fool the ear is particularly important for the third part of our synthesiser building block: tone generation > tone shaping > volume shaping. Tones are generated with Oscillators and shaped with EQ and filters and now we look at volume shaping which is performed by an envelope generator, also called a contour generator, a transient generator or simply an ADSR after the four phases it produces.

Just a phase

The main function of an envelope generator is to determine how the volume of a sound should change during its production although it has other uses, too, which we'll look at in a moment.

The typical envelope generator has four phases - Attack, Decay, Sustain and Release - which, between them, can mimic the sound envelopes of most acoustic instruments.

Note that the Attack, Decay and Release phases are a measurement of time while Sustain is a volume level. So we talk about Attack, Decay and Release times but a Sustain level.

The envelope generator is normally triggered when a key is pressed on a synth keyboard and we can get see how the phases fit together by relating them to a keypress.

	Attack This is the time it takes for the sound to reach a certain volume, usually the maximum volume. It begins as soon as you press a key or a note is triggered. Fast Attack times are used for percussive sounds such as drums, piano, guitar and so on, while slower Attack times would be used for strings, brass and a soft flute. Long Attack times are not found on acoustic instruments and tend to be used for electronic sounds and specials effects.
	Decay The Decay phase starts as soon as the Attack phase ends. It's the time it takes the sound to reach the Sustain level which is normally lower than the volume the Attack phase hits (we'll look at exceptions in a moment) so the sound usually drops in volume during the Decay phase. For acoustic instruments, the Decay phase is usually longer than the Attack phase.
	Sustain This is the volume level the sound reaches at the end of the Decay phase. There is no specific time period attached to it; it ends when the key is released or the note ends.
	Release When the key is released, the envelope enters the Release phase and this is the time it takes for the sound to die away to silence. Instruments such as a vibraphone and gong have long Release times.

Note that, as its name suggests, the Release phase starts when the key is released. If the envelope is in the middle of the Attack or Decay phase when the key is released, the envelope jumps directly to the Release phase. It does not pass Go or collect £200.

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