Modulators

ADSR

The ADSR envelope generator - Attack, Decay, Sustain, Release: the four-stage shape behind every classic synth note.

What is a ADSR?

An ADSR envelope is a four-stage contour generator: Attack, Decay, Sustain, Release. It does not make sound on its own - instead it produces a control voltage that rises and falls in a specific shape every time a gate arrives. That shape is the difference between a piano hit, a swelling string, an organ chord, and a plucked staccato note. In a synthesizer, the envelope is *time itself*: it tells the rest of the patch when to be loud, bright, sharp, or soft.

The four-stage ADSR was formalized in the late 1960s by Vladimir Ussachevsky at the Columbia-Princeton Electronic Music Center and built into hardware by Robert Moog and the team at ARP. Before that, envelopes were ad-hoc: a few knobs that vaguely shaped a sound. ADSR introduced a clear vocabulary - *how fast it rises, how fast it falls back, how loudly it holds, how slowly it releases* - that turned envelopes into a language every synth manufacturer agreed on. Open any synth from 1972 onward and you will find these four letters somewhere on the panel.

The envelope is triggered by a gate signal: a logic-level voltage (typically 5V or 10V) that says *a note is happening*. While the gate is held high the envelope walks through Attack, then Decay, and settles on the Sustain level. The instant the gate drops, the envelope enters the Release phase and decays back to zero - even if it was still in the middle of the Attack. Patch the envelope's output into a VCA, a filter cutoff, or a VCO pitch input, and that voltage shape becomes a musical gesture in time.

Most analog ADSRs use exponential curves rather than linear ramps for the Attack, Decay, and Release stages. This is not a quirk - it matches both the physics of charging capacitors in the original circuits and the way human hearing perceives loudness. A linearly decaying envelope sounds like it cuts off abruptly; an exponential one sounds natural, like a struck string fading into silence.

Our ADSR

Our ADSR has full CV control over every stage - Attack, Decay, Sustain, and Release each have a dedicated CV input with an attenuverter, so any of them can be modulated in real time. Patch an LFO into Decay CV and watch each note breathe; patch a random source into Sustain CV for evolving dynamics. This is rarer than it sounds - most envelopes only let you CV one or two stages.

The curves are exponential on Attack, Decay, and Release for the natural shape capacitor-based hardware envelopes have always had. The output is 0-10V unipolar (the Eurorack standard for envelopes), with a matching inverted output for ducking patches, plus an EOC trigger that fires a 1ms pulse the moment Release completes - perfect for chaining envelopes or triggering sample-and-hold. The retrigger input restarts the Attack while the main gate is still high, the way a sequencer's per-step trigger should work.

Time ranges run from 1ms to 10s per stage on an exponential scale, so the same module gives you snappy plucks, slow pads, and minutes-long drones from one pair of knobs. We use a MIN_LEVEL epsilon to detect when each exponential asymptote has effectively settled - without this guard the stage would mathematically never end. It is a small thing, but it is the kind of detail that separates a working envelope from a usable one.

In a patch

An ADSR almost always feeds a VCA. The signal chain is canonical: VCO into filter into VCA, with the envelope plugged into the VCA's CV input to shape amplitude over time. A second envelope (or the same one) commonly drives the filter cutoff so brightness rises and falls with the note - the staple subtractive synthesis move.

Beyond amplitude, envelopes go anywhere voltage goes. Patch an envelope into a VCO pitch input for a kick drum's downward sweep, or into an LFO rate for accelerating tremolo. Patch the inverted output for ducking effects. Use the EOC (end-of-cycle) trigger to fire a second envelope, a sample-and-hold, or even another ADSR for cascading shapes - the kind of generative chains a function generator is also designed for.

The gate versus trigger distinction matters. A gate is a sustained voltage held for the duration of a note (Attack to Decay to Sustain holds, then Release on the falling edge). A trigger is an instantaneous pulse that fires the envelope but does not hold it - useful for percussive AD-style behavior. A separate retrigger input restarts the Attack while the gate is still held, which is how legato and re-articulated notes work.

Inputs

Gate (gate) — Main trigger input. The envelope starts its Attack phase on the rising edge and enters Release on the falling edge. Connect a clock, sequencer gate, or keyboard gate here.
Retrigger (gate) — Restarts the Attack phase while the gate is still high. Useful for sequencer patterns where you want repeated strikes within a held note.
Attack CV (cv) — Voltage control over the Attack time. Scaled by the Attack attenuverter knob.
Decay CV (cv) — Voltage control over the Decay time. Scaled by the Decay attenuverter knob.
Sustain CV (cv) — Voltage control over the Sustain level. Scaled by the Sustain attenuverter knob.
Release CV (cv) — Voltage control over the Release time. Scaled by the Release attenuverter knob.

Outputs

ENV (cv) — Main envelope output (0-10V). This is what you patch to VCAs, filters, or anything you want shaped by the envelope.
INV (cv) — Inverted envelope output (10V when idle, 0V at peak). Useful for ducking, sidechain-style effects, or subtractive modulation.
EOC (gate) — End-of-cycle trigger. Fires a short pulse when the Release phase finishes. Great for chaining envelopes or triggering other events after a note ends.

Controls

Attack — Attack time (1ms to 10s). How quickly the envelope rises from zero to full level. Short values give percussive hits; long values create slow swells.
Decay — Decay time (1ms to 10s). How quickly the envelope falls from peak to the Sustain level. Short decay with low sustain gives plucky sounds.
Sustain — Sustain level (0-100%). The level held while the gate is high, after Attack and Decay complete. At 0% you get a percussive AD envelope; at 100% the Decay phase is skipped.
Release — Release time (1ms to 10s). How quickly the envelope fades to zero after the gate drops. Short release for tight sounds, long release for pads and reverb-like tails.
Attack Atten — Attenuverter for the Attack CV input (-1 to +1). Scales and optionally inverts the incoming CV before it modulates the attack time.
Decay Atten — Attenuverter for the Decay CV input (-1 to +1). Scales and optionally inverts the incoming CV before it modulates the decay time.
Sustain Atten — Attenuverter for the Sustain CV input (-1 to +1). Scales and optionally inverts the incoming CV before it modulates the sustain level.
Release Atten — Attenuverter for the Release CV input (-1 to +1). Scales and optionally inverts the incoming CV before it modulates the release time.

Inspired by

The canonical four-stage envelope generator topology, dating back to the ADSR pioneered by Vladimir Ussachevsky and refined into hardware by ARP, Moog, and the Eurorack standards that followed. Exponential curves on attack/decay/release for natural perceived shape; per-stage CV with attenuverters; gate plus dedicated retrigger; EOC pulse for chaining.

Doepfer A-140
Moog 911
ARP 2600 envelope

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