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Music is always changing. Even in the most repetitious groove, the individual sounds change from moment to moment. Some of the changes come in the form of chord progressions and changes in instrumentation, which are topics for a different article. In this column we'll look at envelopes, which are a primary tool for controlling changes in the loudness and tone color of electronic sound.

An envelope is a contour or curve that describes or controls the way some aspect of a sound alters over time. An envelope is simply an abstract shape. To understand what an envelope is doing, I'll explain its source (which might be a module called an envelope generator) and its effect on the sound.

Acoustic sounds have their own natural envelopes. Synthesizers and multitrack digital recorders provide envelopes that can be imposed on a preexisting sound, as well as numerous parameters and utilities for editing the envelope. I'll begin by discussing synth envelopes and the envelopes used to automate sounds in digital recorders.

The most basic type of envelope is volume (amplitude). A piano or plucked guitar string, for instance, jumps suddenly from no volume (before the start of a note) to a high volume, and then decays gradually back toward no volume. The amplitude envelope of an organ, a violin, or a trumpet sound, in contrast, jumps quickly to a high level and then sustains at that same level until the note ends, at which time it quickly drops back to zero. Each note has its own amplitude envelope. Other aspects of the sound, such as timbre (tone color), can also be described or controlled using envelopes.

The Classic ADSR

In order to give musically useful shapes to synthesizer notes, a synthesizer always includes one or more envelope generators. (The term is often abbreviated “EG.”) Like other synth features, the EGs are often user-programmable. By adjusting the EG controls, you'll be able to create notes with various shapes, suitable for different musical purposes, such as plucked basses, sustaining leads, strings that swell and die away slowly, and so on.

The classic envelope generator, found in many first-generation synthesizers and still widely used today, includes four parameters: attack, decay, sustain, and release. These four terms gave rise to the acronym “ADSR,” which is often seen on the front panels of such devices. The attack, decay, and release parameters control how quickly or slowly the envelope generator's output changes during the course of a note — they are time-based parameters. The sustain parameter is a level-based parameter.

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FIG. 1: This is an example of an ADSR (attack-decay-sustain-release) envelope. The level of the signal output from the envelope generator is shown by the height of the contour, and the time is shown on the x-y axis.

Before the beginning of a note, the ADSR's output is zero. When you press a key on the keyboard, the attack portion of the envelope begins. During the attack, the EG's output rises from zero to its maximum. During the decay, the EG's output falls from the maximum to the sustain level. When the key on the keyboard is lifted, the EG's output falls from the sustain level back to zero. That process is shown in Fig. 1.

Shape, Rattle, and Roll

An envelope generator makes no sound on its own. It outputs a control signal, which is used to control one or more aspects of the sound to which it is assigned (or mapped). In addition to changing the loudness of a synth tone, for instance, you could change the filter cutoff frequency using an envelope. Assuming the filter is in lowpass mode, raising the cutoff frequency will allow more of the overtones generated by the oscillators to pass through the filter, increasing the brightness of the tone. Using the synth's filter EG, you can raise and lower the cutoff frequency automatically during the course of each note. That is easier, though less fun, than twiddling the cutoff knob manually while you play. Filter sweeps created with an envelope generator are heard in many classic types of synth sounds.

Many other aspects of the sound — such as pitch, stereo panning, and vibrato depth or rate — can be controllable using envelopes. Each synth has a slightly different configuration of features for that purpose, so you'll have to consult your owner's manual for specifics. In some synths, each section could have its own dedicated envelope generator — a pitch envelope, a filter envelope, and so on. In other synths, a set of two or more general-purpose EGs might be used to modulate many different parameters at once.

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FIG. 2: In this multisegment envelope (from the Sytrus FM synth in Image-Line Software FL Studio 5), the end points of the segments, shown as yellow squares, align with the faintly visible rhythm grid. The red curve represents the output of the EG. By clicking and dragging on the red circles, you can change the curvature of individual segments.

Multisegment Envelopes

Software-based synthesizers, which aren't limited to a small number of physical knobs or sliders, often have EGs that don't limit you to a fixed ADSR shape. Instead, you might find multisegment envelopes, in which you can add 32 or more separate segments to create a complex, constantly evolving contour (see Fig. 2). Segments are defined by their start and end points, each of which has its own time and level values. The end point of each segment is the same as the start point of the next segment.

Generally, a multisegment envelope can be edited graphically by clicking on and dragging segment start/end points using a mouse. More points can usually be added between existing points by right-clicking (Windows) or option-clicking (Mac) on the area where you'd like to create a point. You may be able to edit other aspects of the envelope as well, such as the curvature of the segments (using a logarithmic curve rather than a linear one, for example) and the loop start and end points. Looping envelopes, in which a number of segments are cycled through over and over until you lift your finger from the key, can be used to create exciting groove-oriented patches. With that type of patch you can play and sustain one key on a MIDI keyboard and hear a propulsive rhythm complete with pitch changes and volume accents.

Multisegment envelopes with looping are usually designed to synchronize with an external clock signal. By switching on envelope sync, you can ensure that the rhythm being generated by the synth patch will stay at the tempo of your song. If the envelope can be synced, the Envelope Editing window will probably have a snap-to-grid feature, which makes it possible to align the segment start/end points of segments precisely with 16th notes or other rhythmic values.

Automation Envelopes

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FIG. 3: In Cubase SX3, volume envelopes appear below the audio track that they control, in a separate display space that includes a gray version of the waveform. Note the text at the cursor, which shows the time and the level of an envelope point that has been clicked on.

Multisegment envelopes are not only in synthesizers. Many computer-based multitrack audio recorders use similar envelopes for track automation. Such an envelope is assigned to or created within a single audio track and controls one parameter of that track, such as loudness, panning, an EQ cut/boost amount, or an effect send. You'll probably be able to edit the envelope graphically in the Track window, again by clicking to add new envelope points and then dragging individual points up and down or left and right (see Fig. 3).

Automation envelopes provide a convenient way to sculpt a multitrack mix over the course of a song. A guitar track can be boosted during the solo, for instance, and then automatically dropped into the background when the vocal enters. Before the advent of affordable automation, that type of manipulation was done by hand.

Unlike a synth's envelope generators, however, automation envelopes are always applied to one specific parameter. A single envelope can't be used to modulate several things at once. You can, however, use cut-and-paste editing to copy an envelope shape and paste it into another envelope.

The Envelope, Please

There isn't enough space here to discuss some other important envelope features — for example, many synths allow you to modulate the time or level values of an envelope segment from Key Velocity, which is important for creating realistic emulations of acoustic instruments. Understanding the fine points of envelopes is essential for any musician who uses electronics.

Jim Aikin writes, teaches, and plays music in Northern California. His most recent book is Chords & Harmony (Backbeat Books). For more on Jim's varied activities, visit him online