ADSR

Tier: Primitives | ComponentType: 1 | Params: 5

Envelope generator with configurable curve shape for attack, decay, sustain, and release stages.

Overview

The ADSR generates a control signal envelope triggered by gate on/off events. It progresses through four stages: Attack (ramp to 1.0), Decay (ramp to sustain level), Sustain (hold), and Release (ramp to 0.0). A curve parameter controls whether transitions are linear, exponential (fast start), or logarithmic (slow start).

Used to shape the amplitude, filter cutoff, or any time-varying parameter of a sound. The envelope is a modulation source — it ignores audio input and generates its own signal from 0 to 1.

File Locations

	Path
Header	`Sources/FolioDSP/include/FolioDSP/Primitives/ADSR.h`
Implementation	`Sources/FolioDSP/src/Primitives/ADSR.cpp`
Tests	`Tests/FolioDSPTests/ADSRTests.swift`
Bridge	`Sources/FolioDSPBridge/src/FolioDSPBridge.mm` (ADSRBridge)

Parameters

Index	Name	Description	Min	Max	Default Min	Default Max	Default	Unit
0	Attack	Time to ramp from 0 to 1	0.1	30000.0	1.0	2000.0	10.0	ms
1	Decay	Time to ramp from 1 to sustain level	0.1	30000.0	1.0	5000.0	100.0	ms
2	Sustain	Level held while gate is on (after decay)	0.0	1.0	0.0	1.0	0.7
3	Release	Time to ramp from current level to 0 after gate off	0.1	60000.0	10.0	10000.0	200.0	ms
4	Curve	Shape of transitions: -1=log, 0=linear, +1=exponential	-1.0	1.0	-1.0	1.0	0.0

Processing Algorithm

1. Stage State Machine

The envelope progresses through stages: Idle(0) → Attack(1) → Decay(2) → Sustain(3) → Release(4) → Idle.

2. Time Normalization

\[t = \frac{\text{stageTime}}{\text{stageDuration}}\]

where stageDuration = milliseconds × sampleRate × 0.001.

3. Curve Shaping

\[\text{shaped} = \begin{cases} t & |\text{curve}| < 0.001 \\ t^{1 + 4 \cdot \text{curve}} & \text{curve} > 0 \\ t^{1/(1 - 4 \cdot \text{curve})} & \text{curve} < 0 \end{cases}\]

4. Level Interpolation

\[y = \text{start} + (\text{end} - \text{start}) \cdot \text{shaped}\]

Snapshot Fields

Field	Type	Range	Description
Stage	Uint8	0–4	Current envelope stage
Output	Float	0–1	Current envelope value
Progress	Float	0–1	Progress through current stage
Gate	Bool	0–1	Whether gate is currently on

Implementation Notes

gate(true) triggers Attack; gate(false) triggers Release
retrigger() forces immediate re-entry to Attack
Stage transitions are seamless — interpolation maintains level continuity
No DC blocker needed — output is always in [0, 1]

Equation Summary

y = start + (end-start)·curve(t/dur)