Filter Envelopes

We now combine the ideas from the previous lessons into one playable synth voice: keyboard → oscillator → filter → amplifier.

This instrument is deliberately monophonic: one synth voice, one note at a time. Polyphony comes later, after the complete voice is understood.

1. The signal path

Oscillator
creates the waveform
→
Filter
shapes the spectrum
→
Amplifier
controls output level

The keyboard supplies pitch and note-on / note-off. Those note events trigger two separate envelopes:

Filter ADSR → cutoff frequency
Amp ADSR → output gain

An envelope is therefore a control signal. It does not have to control loudness — it can move any parameter we choose.

2. Why a filter envelope?

A static low-pass filter gives one fixed amount of brightness. A filter envelope lets the cutoff move during the note.

cutoff(t) = base cutoff + envelope(t) × envelope amount

With a low starting cutoff and a positive envelope amount, a note can begin bright and then close down as the envelope decays. This creates the familiar “wah / opening filter” movement heard throughout subtractive synthesis.

Important distinction: the filter envelope changes spectral brightness through time; the amp envelope changes loudness through time.

3. Interactive monophonic synth

—

Presets use the same monophonic voice. Only oscillator, filter and envelope settings change.

One-octave keyboard

Click and hold a key, or use the computer keys shown on the keyboard. Only one note sounds at a time.

Oscillator + Filter

Filter ADSR

Amp ADSR

Filter movement

Ready: play a note and watch the filter cutoff move independently from the amplifier envelope.

4. What to listen for

Filter Attack

How quickly the filter moves toward its brightest/openest point after note-on.

Filter Decay

How quickly the cutoff settles from the peak toward its sustain position.

Filter Sustain

The cutoff position maintained while the key remains held.

Filter Release

How the cutoff returns toward its base value after note-off.

Try keeping the amp envelope almost unchanged while dramatically altering the filter envelope. The note can have almost the same loudness shape while its brightness and apparent articulation change completely.

5. Monophonic for a reason

This lesson deliberately uses one voice so the complete path is easy to follow:

note → oscillator → filter → amplifier → output

Polyphony requires multiple independent copies of this voice plus a system for deciding which voice plays each incoming note. That is a separate topic and is best added only after the single voice is fully understood.