The LFO

The chapter on control and audio signals showed that a slow signal sent to a setting is heard as movement. An LFO is an oscillator built for that job. It has the same waveshapes as an audio oscillator, but its range is usually from several seconds per cycle up to about 20 Hz, and it has a depth control that sets how far it moves the setting. On a modular synthesiser the LFO is a module patched to an input. On most other synthesisers it is part of the voice, and its destination and depth are chosen on the panel or in a modulation matrix.

The tool below plays a sawtooth through a lowpass filter and an amplifier, with an LFO that can be sent to any of the three. The plot shows the LFO over the last three seconds and the range it moves the setting through.

Rate and depth

Rate is the LFO's frequency, and depth is how far it moves the setting. Vibrato from singers and string players typically runs at around 5 to 7 cycles per second and moves the pitch by a fraction of a semitone either side of the note, so an LFO at that rate and depth on pitch sounds like a performer's vibrato. The same LFO at 1 Hz and a full semitone sounds like a warped record. On cutoff, a rate of one cycle every few seconds gives a slow sweep, and a few cycles a second give a rhythmic wah. Above about 20 Hz the ear can no longer follow the movement as movement, and it is heard as a change of timbre instead. The Rate slider here stops at 20 Hz for that reason, and audio-rate modulation continues from there.

Shape

The shape sets how the setting moves through each cycle. A sine or a triangle moves it smoothly up and down. A square jumps between two values, which on pitch is a trill. A rising ramp moves the setting steadily up and then drops it back, and a falling ramp does the opposite, which on cutoff or level gives a repeated swell or a repeated pluck. The random shape picks a new value at the start of every cycle and holds it, which produces the same stepped signal that a sample and hold produces from noise.

Bipolar and unipolar

A bipolar LFO swings either side of zero, so it moves the setting both above and below where you have set it. That suits vibrato, where the pitch moves either side of the note. A unipolar LFO runs from zero up to its peak, so it moves the setting in one direction only. On pitch, a unipolar LFO bends the note up and back down to where it started, so the note you play is the lowest pitch you hear. On cutoff it opens the filter from where it is set and closes it back to that point. The difference between the two is an offset: a unipolar signal is a bipolar one scaled to half its size and raised by the same amount, which is how the tool above produces it. On a modular synthesiser you can do the same with an attenuator and an offset.

Other destinations

Pitch, cutoff and level are the destinations in the tool above, and they produce vibrato, a filter sweep and tremolo, but an LFO can be sent to any setting that accepts modulation. Sent to the width of a pulse wave it gives pulse-width modulation, a slowly shifting, chorus-like tone. Sent to a wavetable's position it moves through the frames, sent to pan it moves the sound between the speakers, and sent to the time of a delay it bends the pitch of the echoes. An LFO can also modulate the rate or depth of another LFO, or the decay of an envelope, which is the subject of modulating the modulation.

Sync and retrigger

Many LFOs can be synchronised to a tempo, so that a rate is set as a note length, such as one cycle per bar or per dotted eighth note, and follows the song when the tempo changes. Many can also be retriggered by each new note, so that every note starts at the same point in the LFO's cycle rather than wherever the free-running LFO happens to be. A delay or fade-in setting, common on keyboard synthesisers, holds the LFO off for a moment at the start of each note and then brings it in. That copies the way singers and string players often start a held note without vibrato and add it a moment later.