LFO: Repeating Modulation

An LFO is a low-frequency oscillator, normally operating below the audible range (< 20 Hz) and used as a control signal. Instead of being heard directly, it repeatedly modulates another synthesiser parameter over time.

Core idea: An envelope gives us a triggered control shape. An LFO gives us a repeating control shape.

1. Envelope vs LFO

Envelope

Triggered by an event. It travels through a shape such as ADSR.

note-on → A → D → S → note-off → R

LFO

Repeats continuously. Its waveform supplies a changing control value again and again.

cycle → cycle → cycle → cycle…

This is still modulation: one signal controls another parameter. The LFO is normally slow enough that we perceive the parameter moving over time.

2. Audio path vs modulation path

OSCILLATOR
pitch
→
FILTER
cutoff
→
AMPLIFIER
level
→
PAN / OUTPUT
LFO — CONTROL PATH
Important: the top path carries audio. The LFO is a control source telling one parameter what to do.

3. Interactive monophonic synth + LFO

—

One-octave keyboard

Click and hold a key, or use the computer keys shown. This remains deliberately monophonic.

LFO waveform

What is being controlled?

Rate controls how quickly the cycle repeats. Depth controls how far the destination moves.

Basic voice

Try it: enable audio, hold a note, then change destination, rate, depth and shape.

4. What to listen for

Pitch → Vibrato

Pitch moves repeatedly above and below its centre value.

Amplitude → Tremolo

Loudness repeatedly rises and falls.

Filter → Timbral movement

Cutoff repeatedly opens and closes.

Pan → Auto-pan

The signal repeatedly moves between left and right.

5. LFO shapes are control shapes

Sine

Smooth movement.

Triangle

Linear movement up/down.

Square

Switches abruptly between two values.

Saw

Repeated ramp and reset.

Sample & Hold

Samples a random voltage at timed clock intervals, creating stepped random output.

6. Still one voice

note → oscillator → filter → amplifier → pan → output
LFO → chosen destination