The Envelope: One Voltage, Four Decisions

What the Envelope Is Actually Doing

A gate arrives. A voltage starts moving. That movement is the envelope, and almost everything else — the timbre, the dynamics, the sense of physical attack — follows from how that voltage travels through four distinct phases. People talk about envelopes as if they were shapes drawn on a screen, but a hardware envelope generator is not drawing anything. It is charging and discharging capacitors through variable resistances, and the shape you see on an oscilloscope is the electrical consequence of those physical events, not a target the circuit is trying to match.

Understanding this changes how you use the controls. Attack, Decay, Sustain and Release are not four sliders on a single shape; they are four separate decisions about voltage behaviour, and each one operates differently. Attack and Decay are time constants. Sustain is a level. Release is a second time constant triggered by a different event. Conflating them into a single curve causes calibration confusion and bad patches.

Attack: The Capacitor Charges

When the gate goes high, the envelope circuit connects a capacitor to a voltage source through a resistor — the Attack control sets that resistance. Current flows in, the capacitor charges, and voltage rises. The shape of that rise is not a straight line; it is an exponential curve that climbs steeply at first and levels off as the capacitor approaches the supply rail. Most classic ADSR designs — those descended from the general topology codified in instruments of the 1970s — produce this natural RC exponential. Some designers add a shaping network to approximate a linear rise, which sounds less familiar but gives more predictable control over brightness: a linear attack reaches half amplitude at exactly half the time, whereas an exponential attack reaches half amplitude much earlier.

The practical consequence is that slow exponential attacks sound as if most of the change happens early, then the last stretch crawls toward the peak. If your pad sound feels like it never quite opens, look at whether your Attack is set longer than you think and whether the exponential character is compressing the audible change into the final fraction of the phase. A linear-mode attack — some modules offer both — distributes that change more evenly.

Attack ends when the capacitor voltage reaches its target. That target is the envelope's peak, usually a fixed internal reference rather than a user-set value, because the peak is typically normalised to match the Decay's starting point.

Decay: The Same Capacitor, Discharging

When the peak is reached, the circuit switches: the capacitor now discharges through the Decay resistance toward the Sustain level. This is again an RC curve, but now falling rather than rising. The asymmetry between charging and discharging curves is one reason exponential envelopes sound natural for percussive transients — the fast initial fall mimics the way a struck string or drum skin loses energy most rapidly right after the impact.

Sustain interrupts that decay. The Decay phase does not run to completion on its own; it is cut short when the falling voltage reaches the Sustain setpoint. If Sustain is set high, Decay ends quickly and may be nearly inaudible. If Sustain is set to zero, Decay runs all the way to silence — which means the Decay control is now the entire envelope for any note longer than the Attack phase. Many players discover this by accident when working with low-Sustain patches: the Decay time becomes the de facto note length, and Release becomes irrelevant because there is nothing left to release.

The Decay time constant operates from the peak level down toward Sustain, not from peak down to zero. This matters for calibration: a Decay knob that sweeps the RC time constant produces different audible results depending on where Sustain is set, because the total voltage distance being traversed changes. Longer voltage distance, more audible change for the same time constant.

Sustain: Not a Time, a Level

Sustain is the envelope's only static phase. No capacitor is charging or discharging during Sustain — the circuit holds the voltage at the setpoint for as long as the gate remains high. This is the phase most often misunderstood because its control behaves differently from Attack and Decay. Those knobs set time constants. Sustain sets a voltage. Rotating it changes the amplitude of everything that follows: the level heard during the held phase, and the starting level from which Release begins.

Sustain at maximum means the envelope holds at full peak amplitude indefinitely. Sustain at minimum means Decay has run all the way to zero and Release starts from zero — at which point Release will do nothing audible. This creates an important, practical routing consideration: if you are using the envelope to modulate filter cutoff, a Sustain level near full opens the filter just as wide as the Attack peak; a low Sustain value means the filter closes during Decay and closes the rest of the way on Release. Those are two qualitatively different filter motions, and only Sustain determines which one you get.

Release: The Gate Falls, the Capacitor Drains

When the gate goes low, the envelope enters Release. The Sustain voltage — whatever level the envelope was holding, or wherever Decay had reached if a gate ends mid-Decay — becomes the new starting point, and the capacitor discharges toward zero through the Release resistance. This is the same RC discharge physics as Decay, but the starting level is variable: it depends entirely on when the gate closed relative to the earlier phases.

A note cut short inside the Attack phase will release from a partial voltage, producing a shorter-sounding release than the same module playing the same patch with a longer note. A note cut short mid-Decay releases from wherever Decay had reached. This variability is why envelopes on hardware behave differently from DAW automation: the starting voltage for Release is not fixed, and a fast staccato passage produces subtly different release shapes for each note depending on velocity, gate length and playing position within the phase cycle. Some envelope designs retrigger by resetting the capacitor to zero before starting Attack again; others retrigger from wherever the capacitor currently sits. The second behaviour — called legato or non-zero retrigger — produces the softer attack transients associated with held playing styles.

Reading the Four Decisions Together

Treated as a system rather than four independent settings, ADSR gives you continuous control over where voltage is at any point in a note's lifetime. Attack answers: how long does it take to reach full amplitude? Decay asks: how quickly does it fall from peak once it gets there? Sustain determines: what level does it hold at? Release settles: how long does it take to die away after the key lifts?

Because the envelope output is just voltage, it modulates anything with a CV input — filter cutoff, oscillator pitch, LFO rate, VCA level. The same four decisions produce completely different results depending on the destination, the depth of the patch cable's attenuation, and whether the signal path is linear or exponential. The envelope itself is agnostic about what it controls. It makes one voltage, traces four decisions across time, and hands it off.