ENVELOPE / THE CONTOUR OF TIME

The envelope belongs to the modulator family: a source that drives a destination. It is neither an audio oscillator nor an effect. It is not heard directly — what is heard is its effect on the parameter it modulates.

The term comes from observing a sound waveform: connect the peaks of a sound's oscillations and you draw its "amplitude envelope", the global contour of its volume over time. The envelope generator artificially reproduces that contour and imposes it on a parameter.

The model crystallised on the first voltage-controlled synthesizers in the late 1960s (Moog, ARP, EMS). The need was precise: to give electronic sounds the temporal behaviour of acoustic instruments. A plucked string attacks sharply and decays on its own; a bowed note swells then holds. The envelope encodes that profile.

Two points structure everything else. First, the envelope is transversal: nothing binds it to amplitude in particular. It is an abstract time profile — start here, rise there, fall back, hold, release — connecting indifferently to volume, filter cutoff, pitch, pan, an effect's amount. The modulated parameter is a wiring choice, not a property of the envelope. Second, it is unipolar by nature: it starts from a floor and rises toward a ceiling, never going negative — which sets it apart from the LFO, usually bipolar, from the outset.

An envelope does not run on its own: it waits for an event. That event most often comes from the keyboard, in two distinct forms that must be told apart clearly.

The trigger is an instantaneous impulse. It says "go now" and nothing more. The envelope starts its cycle, full stop.

The gate is a sustained signal: it stays "high" as long as the key is held, and drops on release. It carries the notion of playing duration. In MIDI terms, Note On opens the gate, Note Off closes it (see the protocols / MIDI sheet).

The ADSR envelope reads both. The trigger launches the Attack. The gate governs the passage between the held phase and the extinction phase: as long as the gate is high, the envelope stays at Sustain; on its closing, Release begins. "Holding a note" therefore means, mechanically, keeping the gate open — and that is exactly what separates a long held note from a brief staccato.

In monophonic playing, retrigger behaviour becomes a parameter in its own right. In retrigger mode, each new note relaunches the envelope from zero. In legato (or single-trigger) mode, as long as a key stays held, playing a new note does not relaunch the envelope: only the pitch changes, the contour continues. This setting decides the grain of a bass line played legato — each note re-strikes, or the phrase flows in one breath.

The gate carries one more piece of information at the moment it opens: the Note On velocity. This data, distinct from the gate duration, feeds the dynamic links described later.

The ADSR shape is the canonical model: four chained segments describing a parameter's evolution from trigger to extinction.

- Attack — the time to reach the maximum value after the trigger. A short attack gives a percussive, sharp sound; a long attack, a gradual fade-in.
- Decay — the time to fall from the maximum down to the Sustain level.
- Sustain — the stable level held as long as the gate stays high.
- Release — the time to reach the initial zero after the gate closes.

A structural imbalance hides here, and it is the central trap: three of these parameters are durations, only one is a level. Attack, Decay and Release are set in time (ms or s). Sustain is expressed not as a duration but as an amplitude — the height of the held plateau, often as a percentage of the maximum. Setting the Sustain changes no duration; it fixes the level at which the envelope stabilises.

Segments are not necessarily straight. A linear ramp progresses at constant slope; an exponential ramp starts fast then slows (or the reverse). The ear perceives volume logarithmically, which makes exponential curves feel more "natural" on amplitude. That curve is not arbitrary: an analog circuit charges or discharges a capacitor through a resistor, and the voltage then follows an exponential set by the circuit's time constant. Digital envelopes often reproduce these curves to recover that naturalness.

A last point, often overlooked: Release does not necessarily start from the Sustain level. If the note is released during Attack or Decay — before the plateau is reached — Release starts from the current level, wherever the envelope sits at that instant. On a very short note, the Sustain phase may therefore never be reached.

ADSR is only one configuration among others. The number of segments varies with the need, and each letter added or removed names a stage.

- AR (Attack-Release) — the minimal form: it rises, it falls. No held plateau. Typical of percussion and short sounds.
- AD (Attack-Decay) — rise then fall to zero, no gate. A one-shot form by nature, independent of note duration.
- AHDSR — a Hold inserts between Attack and Decay: the maximum is held briefly before falling.
- DADSR — a Delay precedes the Attack: a waiting time between the trigger and the actual start of the rise.
- DAHDSR — the complete form gathering Delay, Attack, Hold, Decay, Sustain, Release: six stages, the skeleton the others are subsets of.
- Multi-segment (MSEG) — the generalisation: a free number of points joined by segments whose duration and curvature are set individually. ADSR becomes a special case with four points.

A neighbouring paradigm reverses the parameterisation: rate and level envelopes, common on FM synths, define not segment durations but a series of target levels, each reached at an adjustable rate. Same geometry, different handling.

At the far end of generalisation, the modular function generator — a module like Maths — melts envelope, LFO, glide and follower into a single adjustable object. Proof by tool that these families are merely configurations of one brick: a segment links two levels over a given duration. The acronyms merely name frequent assemblies of this elementary brick.

A static envelope always applies the same shape, whatever the playing. Instrumental gesture breaks this uniformity through several links.

The first ties velocity to depth. Velocity — the striking force carried by the Note On — scales the envelope's amplitude: a hard strike produces a wide contour, a soft strike a reduced one. This is what brings a synth's behaviour closer to an acoustic instrument's, where playing intensity changes both volume and timbre.

The second ties pitch to durations: key tracking (or key follow). On an acoustic instrument, high notes decay faster than low ones. To reproduce this, the envelope's durations shorten as the note climbs the keyboard. The same envelope then behaves differently depending on the key played.

The dynamic link does not stop at amplitude. On many instruments, a hard strike also shortens the attack: the sharp gesture makes the onset crisper, on top of amplifying it. And velocity is only one source among others — mod wheel, aftertouch, a second modulator can drive the same envelope parameters.

In all these cases, the envelope is no longer a frozen shape: it is a shape whose own parameters are modulated by the playing. Modulation modulates itself.

The genesis stated it: the envelope is transversal. It is in the destinations that this principle takes its full scope.

- Amplitude — the historical destination, the one that gives the object its name. The amp envelope draws the note's volume, from silence to silence. It is what decides a sound is percussive or pad-like.
- Filter cutoff — the filter envelope opens then closes the spectrum. A bright attack darkening as it holds: it is the filter envelope, not the amp one, that produces the characteristic synth pluck.
- Pitch — applied to an oscillator's frequency, the envelope creates a pitch sweep. A very short, deep pitch envelope at the very start of a sound gives that attack "click" that makes a synthetic kick or tom snap.
- Any parameter — pan, an effect's amount, a sample's playback position, another modulator's depth. Any parameter exposed to modulation can receive an envelope.

Two settings govern the application to any destination: depth (amount), which sets the extent of the effect, and polarity. An envelope is unipolar at the source, but routing can invert it: applied to pitch with a negative depth, it makes the note dive instead of climb. Unipolar at origin, bipolar through wiring.

In modular synthesis, this transversality reaches its barest form: the envelope is just a control voltage (CV), a slow signal like any other. Nothing physically distinguishes it from an LFO or a follower; it patches into any modulation input.

A classic subtractive synth offers, in fact, at least two independent envelopes — one for amp, one for filter — precisely because separating the volume contour from the timbre contour is the basis of sound shaping.

An envelope runs once, on trigger, then dies. That is the one-shot mode: a one-time gesture, tied to a note.

But most generators offer a loop mode. Instead of stopping after Release (or Decay), the envelope loops back on itself and starts again. Loop modes vary: loop the whole cycle, or repeat only a segment. Samplers know the sustain loop well, holding a looped portion as long as the note lasts, then moving to Release on release.

This switch changes the nature of the object. A looping envelope is no longer a one-time contour: it is a periodic shape repeated at regular intervals. And a low-frequency periodic shape applied to a parameter is exactly the definition of an LFO (see modulation / LFO). The border is a setting, not a category: looping envelope and LFO are two names for the same phenomenon, approached through two doors.

The crossing even has a precise bound. As long as the loop stays slow, a modulation is perceived. But accelerate it past some twenty cycles per second and the periodic shape enters the audible domain: the looping envelope ceases to be a modulator and becomes an oscillator in its own right. That is exactly the threshold an LFO crosses when its frequency climbs enough to do modulation synthesis.

The reverse trip exists too. An LFO set to play a single cycle on trigger — often via a one-shot or retrig function — behaves like an envelope. The two objects converge at the same place. This is the thread of the whole series: the envelope and the LFO are not opposites, they are two faces of the same modulation mechanism, separated by a single question — does it loop?

It is also a practical entry point: setting a looping envelope on a cutoff gives a cyclic sweep without touching any LFO at all. The LFO is the tool when periodicity is the goal; the looping envelope, when you reach it by extending a gesture. The sonic result tells no difference.