Envelopes & VCAs: how notes start and stop

In the oscillators guide we found out that an oscillator never stops. It just hums, forever. So how does a synth play notes, with a beginning and an end? And why do some notes snap in like a pluck while others swell in slowly like strings?

The answer is a little team of three: a gate that says when, an envelope that says how, and a VCA that does the actual turning up and down.

The VCA: a volume knob that listens

A VCA (voltage-controlled amplifier) is a volume control, but instead of you turning it, a signal does.

On a hardware modular, every cable carries a voltage that changes over time. Some of those voltages wiggle hundreds of times a second, and you hear them: that's audio. Others move slowly, or just jump between off and on, and you don't hear them directly. You hear what they do, because they're turning a knob for you. Those are called control voltage, or CV for short, and a jack labelled CV is a place to plug one in. Kables works the same way, with numbers instead of volts.

So a VCA has two inputs. Sound goes into its IN, and its CV input decides how much of that sound gets through: 0 is silence and 1 is full volume. Nothing on the CV? Then the door stays shut and you hear nothing.

That's why "I patched everything and there's no sound" is so often a VCA with nothing opening it. The VCA's Bias knob opens it a little on its own, which is handy for a drone, but most of the time you want something to open and close it for each note.

The gate: the simplest "when"

When you hold a key, the Keyboard sends a gate: 1 while the key is down, 0 when you let go. Patch that straight into the VCA's CV and the door swings fully open while you hold a key, and slams shut when you release it.

That click is the clue. The gate jumps from 0 to 1 instantly, so the volume does too, and an instant jump in volume sounds like a click. Real instruments never do that: even the fastest drum hit takes a moment to arrive and a while to die away. We need a smoother shape.

The envelope: a shape for every note

An envelope listens to the gate and draws a smooth curve each time a note is played. The classic one has four stages, which give it its name, ADSR:

• Attack: how long it takes to rise when the key goes down.
• Decay: how long it takes to fall from the peak…
• Sustain: …to the level it holds while you keep the key down.
• Release: how long it takes to fade to nothing after you let go.

Patch the envelope between the gate and the VCA (Keyboard GATE → ADSR → VCA CV) and the VCA now follows the curve instead of the on/off gate. The click is gone, and you can shape every note.

Why four stages are enough

Think of real instruments and their shapes:

Almost every instrument fits one of those, which is why ADSR has been the standard synth envelope for decades.

An envelope is just a signal

The envelope's output is an ordinary signal, so it doesn't have to go to a VCA. Patch it into a filter and each note gets brighter then darker (filters guide). Patch it into an oscillator's pitch and each note swoops. And the envelope also has an INV output: the same shape, upside down.

Low-pass gates: a VCA and a filter in one

Hit a piece of wood or a marimba bar and the sound doesn't just get quieter as it fades, it also gets darker. The bright, clicky part dies first and the warm low part lasts longer. An ordinary VCA can't do that, because it only changes the volume.

A Low-Pass Gate is a filter and a VCA that open and close together. Strike it with a trigger and it opens bright and loud, then closes into dark and quiet. It even closes fast at first and slowly at the end, just like a real object, which is why it sounds so natural with no envelope at all.

Slew: shaping the moves between notes

Envelopes shape a note's volume over time. A Slew shapes something else: how fast any signal is allowed to change. Put it between a keyboard's pitch and an oscillator, and instead of jumping from note to note, the pitch slides. That slide is called portamento or glide.

Rise sets how slowly it climbs, Fall how slowly it drops, so you can make notes scoop up gently but fall instantly.

A slew works on any signal, not just pitch. Lesson 23 uses a second one to smooth random steps into a gentle wander on a filter; it makes more sense once you've met the clock and the Sample & Hold in later guides.

What to remember

• A VCA is a volume control that a signal turns. With nothing on its CV, it stays shut.
• A gate says when a note is held: 1 while the key is down, 0 after.
• An envelope turns the gate into a smooth shape: Attack, Decay, Sustain, Release.
• Gate → envelope → VCA is the standard way to play notes. Change the envelope and you change the instrument.
• A low-pass gate closes volume and brightness together, like a real struck object.
• A slew turns jumps into slides.