From ADSR to Rise/Fall
In Part 7, you learned ADSR: four stages, each with one job. West Coast designers asked: what if two stages were enough? Rise goes up. Fall comes down. That's it.
From Four Stages to Two
Attack ramps up, Decay falls to Sustain, Release fades out. West Coast designers asked: what if two stages were enough? Rise goes up. Fall comes down. That's it.
In West Coast synthesis, this two-stage envelope is called a function generator. Unlike an ADSR, a function generator just goes up, then down. Rise, then Fall. The name sounds technical, but the idea is simple: it generates a shape, a function of time.
An ADSR envelope assumes you will hold a key down. That is what Sustain is for. But a function generator does not wait. Once Rise finishes, Fall begins immediately. The result is a shape with a single peak: up, then down. No plateau, no waiting.
The comparison canvas below shows both shapes side by side. The ADSR on the left has four distinct segments; the Rise/Fall on the right has two. Fewer controls, but the same fundamental job: shaping a control signal over time.
Linear vs Exponential
A linear rise is a straight ramp. An exponential rise curves, fast at first, then slowing as it approaches the peak. Your ear prefers exponential shapes because that's how real acoustic instruments behave.
A struck drum decays exponentially, loud at first, then fading gradually. A bowed string swells with a curve, not a straight line. Toggle between Linear and Exponential in the demo below and listen to the difference: linear sounds mechanical, exponential sounds natural.
The Retrigger Connection
Remember retrigger vs legato from Part 7? A Rise/Fall envelope naturally retriggers because there is no sustain stage to hold. Every trigger starts a fresh Rise. Play notes rapidly on the keyboard and you will hear each note cut into the previous one, restarting from zero every time. This is the behaviour that makes West Coast percussion so snappy. Each ping is a complete, self-contained event.
References
- Part 7: Envelopes (ADSR), the four-stage envelope this lesson builds on
- Make Noise: MATHS, the most famous function generator module
- Wikipedia: Buchla Electronic Musical Instruments
- Learning Modular: West Coast Synthesis