LFOs and Modulation

An envelope happens once per note. An LFO, or low-frequency oscillator, repeats. It is an oscillator running below the range of hearing, usually from about 0.05 to 20 Hz, and instead of making sound it moves a parameter up and down. Almost every sound that seems to "breathe" or "wobble" has an LFO on it.

The four LFO controls

Control	What it sets
Rate	How fast it cycles, in Hz or as a note value when synced
Shape	The motion: sine, triangle, saw, square, random (sample and hold)
Depth / amount	How far it moves the target
Destination	Which parameter it moves

Free rate versus tempo sync

A free LFO runs in Hz, independent of the song. Good for vibrato and slow drifting movement that should not feel locked to the beat.

A synced LFO runs in note values: 1/4, 1/8, 1/16, dotted and triplet values, or longer like 4 bars. Use sync whenever the movement should groove with the track, such as wobble bass, rhythmic filter gating or tremolo.

At 120 BPM, one beat lasts 500 ms, so:

Synced value	Rate at 120 BPM
1 bar	0.5 Hz
1/4	2 Hz
1/8	4 Hz
1/8 triplet	6 Hz
1/16	8 Hz

Retrigger

With retrigger (key sync) on, the LFO restarts from the same point at every new note, so every note moves the same way. With it off, the LFO free-runs and each note catches it at a different point. Wobble basses want retrigger; slow pad movement usually does not.

Classic routings

Effect	Destination	Shape	Rate	Depth
Vibrato	Pitch	Sine / triangle	5–7 Hz	10–30 cents
Tremolo	Amplitude	Sine or square	3–10 Hz or synced	Moderate
Wobble	Filter cutoff	Sine, saw or square	1/4–1/16, synced	Large
Auto-pan	Pan	Sine	0.1–2 Hz or 1–4 bars	30–80%
PWM	Pulse width	Triangle	0.3–2 Hz	Moderate
Slow movement	Cutoff or wavetable position	Sine or random	0.05–0.3 Hz	Small

Vibrato that sounds played

Real singers and string players do not start a note with vibrato. They hold it straight and let vibrato come in after a few hundred milliseconds. Most synths offer an LFO delay or fade-in. Set 200–500 ms and the vibrato will feel natural rather than seasick. Keep the depth small; anything past about 50 cents sounds like a siren.

The wobble bass

The dubstep wobble is a filter cutoff modulated by a synced LFO:

1. Saw or square bass, low-pass at 24 dB/oct, base cutoff around 150 Hz, resonance 20–30%.
2. LFO to cutoff, sine shape, depth large enough to open the filter to 2–4 kHz.
3. Sync the rate and change it per section, or automate the rate, moving between 1/4, 1/8 and 1/8 triplet to make rhythms.
4. Keep the sub as a separate sine layer that does not wobble, so the low end stays steady.

Modulation beyond the LFO

Modern synths route any source to any destination through a modulation matrix. Useful sources besides LFOs and envelopes:

• Velocity: brighter or louder when played harder.
• Mod wheel: live control of vibrato depth or filter, ideal for leads.
• Aftertouch: pressure after the key is down, often routed to vibrato or brightness.
• Key tracking: parameters that follow pitch, like the filter key tracking from the filters lesson.
• Random / sample and hold: a new random value each cycle, the source of computer-burble effects.

Modulators can also modulate each other. An envelope controlling an LFO's depth makes vibrato swell in; one LFO slowly modulating another's rate makes movement that never quite repeats.

Restraint

Movement is what keeps a held sound interesting, but every modulated parameter is a parameter that is not stable. Start with one LFO on one destination, set it until you can just hear it, and add more only when the sound needs it. In the recipes lesson you will see how little modulation most staple sounds actually use.