Envelope Generators

Because synthesizers create sounds artificially, we need a device to give a shape to each note. That device is the envelope generator. An envelope generator makes no sound. Instead, it sends out a control signal (called the envelope) that is applied to some aspect of the sound.

When an envelope is used to control the loudness of the note, it’s called an amplitude envelope. When it’s controlling the cutoff frequency of a filter, thereby making the sound brighter or more muted, it’s called a filter envelope. Envelope generators sometimes control other sound parameters too, but amplitude and filter cutoff are the most important destinations for envelope control. Most synths have one envelope generator (EG) for each of these functions. Each voice channel in a polyphonic synth has its own EGs.

GATE

When you press a key on a synth keyboard, the envelope generators start work. The envelope typically starts at zero and rises to some level, after which it falls back toward zero. But it may stop somewhere along the way and sustain a constant level until the key is released. When you release the key, the envelope will definitely fall back to zero, though it may not fall instantly.

The signal coming from the keyboard, which tells the EG when the note starts and when it ends, is called a gate. When the gate opens, the note is heard. When the gate closes, the note dies away.

The gate has an extremely simple on/off shape. The shape of the envelope, however, can be much more complex. An EG provides a set of parameters with which you can create various envelope shapes. By adjusting these to different values, you can use an EG to create many different musical effects. For instance, you can emulate the natural envelope produced by a piano, a marimba, an accordion, or an orchestral string section in a large concert hall.

ADSR

The basic envelope parameters come in two flavors: Some are level settings: These determine how high the envelope rises or how low it falls. Others are time settings: These determine how quickly or slowly the envelope rises or falls from one level to the next.

In the early 1960s, when envelopes were still generated with hardware, experimental composer Vladimir Ussachevsky suggested to Bob Moog a type of module that could create a wide variety of musically useful envelopes. Ussachevsky’s design had four knobs — attack, decay, sustain, and release. The name of the module was quickly shortened to “ADSR.” Today, envelope generators are often far more complex, but basic ADSR types are still seen on newly designed instruments (see Figure 1).

The attack, decay, and release parameters govern time. The sustain parameter governs level.

During the first part of the envelope, it rises from zero to its maximum level. The rate at which it rises is controlled by the attack time parameter. At the end of the attack, the envelope falls back to the sustain level. The rate at which it falls is controlled by the decay time parameter. It then stays at the sustain level for as long as the key is held down. When the key is released, the envelope falls back to zero at a rate determined by the release time parameter.

ENVELOPE WRINKLES

The envelopes in many software-based synths can be edited graphically. Some EGs can have up to 64 segments; you can add new ones by clicking with the mouse. Multisegment EGs can often loop, producing rhythmic patterns that can be synced to a master clock.

Many EGs include modulation inputs. By controlling the attack time from MIDI velocity, for instance, you can give some notes a faster attack than others. A few synths even include two-dimensional “vector” envelopes (see Figure 2), with which you can produce moving tones. Without envelope generators, synthesizers would sound very boring.