Envelopes

In the chapter on gates and triggers the gate switched an amplifier straight on and off, and every note began and ended with a click. An envelope generator sits between the gate and the amplifier. It receives the gate and produces a smooth control signal, which rises over a time you choose when the gate goes high and falls over a time you choose when the gate goes low.

The most common kind is the ADSR envelope, named after the initials of its four stages. Attack is the time the envelope takes to rise from zero to its peak once the gate goes high. Decay is the time it then takes to fall from the peak to the sustain level. Sustain is that level, which the envelope holds for as long as the gate stays high. Release is the time it takes to fall back to zero once the gate goes low. Attack, decay and release are lengths of time, set in milliseconds. Sustain is a level, given as a fraction of the peak, and it has no length of its own: the envelope stays there until the gate ends, whether that takes a tenth of a second or a minute.

Where the shape came from

The four stages were settled in the mid-1960s. The composer Vladimir Ussachevsky, then head of the Columbia-Princeton Electronic Music Center in New York, asked Robert Moog, who was building some of the first commercial modular synthesisers, for an envelope with these four parts, and Moog built it as a module. Most synthesiser envelopes since have used the same four stages, sometimes with extra ones added.

AD envelopes

An AD envelope has only attack and decay. It runs both as soon as it receives a gate or a trigger and falls to zero at the end of the decay, whether the gate is still high or not. That suits sounds that die away on their own, such as drums and plucked strings, whose length does not depend on how long the gate stays high. Switch the tool to AD and the button becomes a trigger: the envelope runs the same attack and decay however long you hold it.

Straight lines and curves

The envelope here moves in straight lines. Most analogue envelope generators move in curves instead, because each stage charges or discharges a capacitor through a resistor, which produces a curve that moves fast at first and slows as it nears its target. The difference is easiest to hear in the release. The ear judges loudness on a roughly logarithmic scale, so a release that falls in a straight line sounds as if it stays loud for most of its length and then drops away quickly at the end. A curved release sounds like an even fade.

An envelope is a control signal

An envelope generator produces a control signal, and it changes the sound only when it is sent to a control input. In the tool above it is already connected to the control input of an amplifier, which raises and lowers the level of the sawtooth so that you can hear the shape. That amplifier is the subject of the chapter on the VCA.

Because the output is a control signal, it can go to any other setting that accepts one. Sent to a filter's cutoff it makes each note start bright and grow darker as the cutoff falls. Sent to an oscillator's pitch it makes the pitch of each note rise and fall in the shape of the envelope. Those uses are covered in envelopes as modulation.