LFO / THE SLOW OSCILLATOR

An LFO is an oscillator, full stop. The same brick that, sped up, would produce a sound. The only difference is frequency: below the human hearing threshold (~20 Hz), an oscillation is no longer heard as a pitch — it is perceived as a variation, a movement.

Its output is a control signal, not an audio signal. You don't listen to it: you observe it through the parameter it modulates. Routed to volume, it pulses; to pitch, it makes things vibrate; to a filter cutoff, it makes the timbre breathe. The LFO thus belongs to the same family as the envelope: a modulation source, transversal, indifferent to its destination.

But one property clearly separates it from the envelope: the LFO is bipolar by nature. It oscillates around a centre point, alternately going above and below — positive then negative. The envelope is unipolar: it starts from a floor and rises. This difference is not a detail. It explains why an LFO applied to pitch makes the note oscillate on both sides (vibrato), where an envelope would only push it one way.

The LFO's waveform decides how the modulation unfolds over time. The same parameter, modulated at the same speed and depth, will behave very differently depending on the profile chosen.

- Sine — rise and fall are smooth, continuous, angle-free. The default undulation: a natural vibrato, a breathing tremolo.
- Triangle — close to the sine but with linear slopes and sharp peaks. Slightly more marked at the extremes.
- Square / pulse — no transition: the value jumps between two states. On a volume, it is a rhythmic gate; on a pitch, a two-note trill. The high/low ratio (pulse width) is sometimes adjustable.
- Sawtooth (ramp) — gradual rise then abrupt fall (or the reverse). Asymmetric: useful for repeated sweeps always in the same direction.
- Random / sample & hold — the value changes in unpredictable steps, at a regular interval. No longer a periodic shape in the strict sense, but a modulation driven by chance. (The detail of this mode belongs to the modulation / sample & hold sheet.)

Each shape is a different temporal narrative imposed on the destination. Choosing the shape means choosing the gesture: undulating, switching, sweeping, or unpredictable.

Frequency sets the speed of the oscillation — how many cycles per second. Two ways of setting it coexist, and the choice between them structures the LFO's entire musical use.

In free mode, frequency is expressed in hertz: 2 Hz, 0.5 Hz, regardless of the track's tempo. The LFO runs at its own speed, independent of the music. Useful for a modulation that should not "lock" to the rhythm — a slow drift, an organic movement.

In synced mode, frequency is set in divisions of the tempo: a quarter note, an eighth, two bars. The parameter is often called Rate and is expressed in rhythmic values rather than Hz. The LFO then locks to the track's clock: its period follows the tempo, and the modulation stays aligned to the grid. This is the mode of rhythmic effects — a filter opening on each beat, a tremolo in eighths.

A further distinction concerns the starting point. In free-run, the LFO oscillates permanently from its activation, ignoring the playback position. Synced to the transport, it can instead lock its phase to the track's start, so the modulation always falls at the same point of the cycle on each playback.

Beyond shape and speed, three settings specify how the cycle is positioned.

Phase defines the starting point within the cycle. A sine LFO can start at the centre rising, at the peak, at the centre falling, at the trough. Offsetting the phase of two LFOs lets them oscillate out of step — a stereo modulation, for instance, where left and right don't move together.

Retrig (or key-sync) ties the cycle to a note's trigger. In a free LFO, the oscillation runs continuously: when a note arrives, it lands at some arbitrary point of the cycle, unpredictable. With retrig on, each Note On resets the phase to zero: the modulation always restarts from the same point for each note played. This is exactly the envelope's trigger mechanism — and it is what brings the LFO closest to an envelope's behaviour.

Polarity determines what the oscillation develops around. In bipolar (the default), the LFO goes either side of a centre: applied to pitch, the note rises then falls around its value. In unipolar, the oscillation happens on one side only — it modulates between zero and a maximum, like a repeated envelope. An offset setting shifts that centre. Choosing polarity means deciding whether the destination oscillates around its value, or only above it.

Depth (amount) sets the extent of the oscillation: how much the destination moves around its value. At zero depth, the LFO runs but has no effect; at maximum, it sweeps the parameter's whole range. It is the exact counterpart of the envelope's depth.

Like the envelope, the LFO is transversal: it patches into any parameter. But usage has given proper names to the most common destinations, and these names are worth more than a long definition:

- Pitch → vibrato. The LFO oscillates the frequency around the note. Slow and shallow, it is a singer's expressivity; fast and deep, a marked effect.
- Amplitude → tremolo. The volume pulses. The effect of vintage amps, the steady throb of a sound coming and going.
- Filter cutoff → wah, sweep. The timbre opens and closes cyclically. Synced to tempo, it is the rhythmic filter typical of electronic music.
- Pan → auto-pan. The sound moves left to right in the stereo image.

An LFO can drive several destinations at once, or even modulate another modulator's parameters. And when each note of a chord carries its own LFO — as MPE allows (see the protocols / MIDI sheet) — the modulation becomes polyphonic: each voice vibrates independently.

The "low" in Low Frequency Oscillator is not a physical boundary: it is a zone of use. Nothing prevents speeding up an LFO. And when its frequency is pushed past the hearing threshold — some twenty hertz — something tips.

The oscillation becomes fast enough to be heard. The LFO stops being perceived as a movement and becomes what it always was: an oscillator, now audible. Its modulation, until then an effect (vibrato, tremolo), becomes a synthesis phenomenon.

Two classic cases:

- Applied to the frequency of another audio-rate oscillator, it produces frequency modulation — FM synthesis, with its rich, metallic spectra.
- Applied to the amplitude of an audio-rate signal, it produces amplitude modulation — AM synthesis, and its sidebands.

This is exactly the threshold a looping envelope crosses, the other way, when accelerated (see modulation / envelope). On either side of this ~20 Hz boundary, the same object changes function: below the threshold it modulates; above, it synthesises. The LFO and the audio oscillator are one and the same tool, separated by a matter of speed.

The envelope sheet ended on a switch: a looping envelope becomes an LFO. Seen from the LFO, the reverse trip is just as direct.

An LFO, by definition, oscillates continuously. But many generators offer a one-shot mode (sometimes named envelope, one-cycle, or coupled to retrig): the LFO then plays only a single cycle on trigger, then stops. A single sine cycle is a rise then a fall — exactly the contour of an AR envelope. Set this way, the LFO no longer modulates periodically: it traces a one-time gesture, triggered by a note. It is an envelope.

The two objects therefore meet at both ends. The envelope you loop becomes cyclic and joins the LFO; the LFO you reduce to one cycle becomes one-time and joins the envelope. Between them, a single variable: does it repeat?

This is the heart of the modulation sub-series. Envelope, LFO, and later follower, randomness, sequencer: not separate tools to memorise one by one, but configurations of one mechanism — a source producing a control signal, patched to a destination with a depth and a polarity. Understand this mechanism once, and you understand them all.