Modulation
Every modulation effect is the same idea applied to a different parameter: a slow, silent oscillator (an LFO) steers a knob for you instead of your hand. This chapter covers chorus, flanger, phaser, tremolo, and vibrato as one mental model with five different targets, plus the century-old Fender naming mixup that still causes confusion between tremolo and vibrato today.
Chorus, flanger, phaser, tremolo, and vibrato sound like five unrelated effects, but they’re built from one shared mechanism applied to five different targets: a low-frequency oscillator, or LFO, that slowly and silently steers some parameter of the circuit back and forth, the way a hand would turn a knob if it moved smoothly and automatically instead of in discrete adjustments.
The mental model: an invisible hand slowly turning one knob
An LFO is an oscillator too slow to hear directly — typically well under 20Hz, often under 5Hz for the effects in this chapter — running silently in the background and outputting a smoothly rising-and-falling control voltage instead of an audible tone. Every modulation effect wires that control voltage to a different point in the signal path: what changes between chorus, flanger, phaser, tremolo, and vibrato isn’t the LFO itself, it’s what it’s steering. Rate controls how fast the invisible hand moves; depth controls how far it turns the knob each cycle.
The five effects, sorted by what the LFO actually modulates
| Effect | What the LFO modulates | Typical mechanism | Character |
|---|---|---|---|
| Tremolo | Signal amplitude (volume) | An LDR/photocell driven by a pulsing LED, or a JFET used as a voltage-controlled resistor, placed after the gain stage | Rhythmic volume pulsing — the simplest of the five, no delay line involved at all |
| Vibrato | Pitch | A short delay line (commonly BBD-based) whose delay time is modulated by the LFO, with the output taken 100% wet, no dry blend | A wavering pitch, with no comb-filtering or thickening artifact because there’s nothing dry to interfere with |
| Chorus | Delay time (short, ~5–25ms), blended with dry | A short BBD delay line, LFO-modulated, mixed back in with the unprocessed signal | Pitch-wavering copies layered under the dry signal — a thickening, “more than one player” effect |
| Flanger | Delay time (very short, ~0–10ms) with feedback, blended with dry | The same short modulated delay line as chorus, but with a feedback path added around it | A sweeping comb-filter notch effect — the “jet plane” sound, distinguished from chorus specifically by that feedback path |
| Phaser | The corner frequency of a chain of all-pass filter stages | No delay line at all — a series of op-amp or JFET-based all-pass filter stages, LFO-swept together | A sweeping notch effect from filtering alone, without any actual delay in the signal path |
Why chorus, flanger, and vibrato get confused constantly, and phaser doesn’t need a delay line at all
Chorus, flanger, and vibrato are all built from the same underlying part — a short, LFO-modulated delay line — which is exactly why they’re so often mixed up by ear alone: the difference between chorus and flanger is a feedback path, and the difference between chorus and vibrato is whether the dry signal is blended in at all (chorus) or discarded entirely (100% wet vibrato). Phaser is the odd one out in this table: it produces a superficially similar sweeping, swooshing sound using all-pass filter stages instead of any delay line, and the number of filter stages (commonly four, six, eight, or ten in different commercial designs) directly determines how many notches sweep through the spectrum and how dense the effect sounds.
The historical mixup: Fender’s “tremolo” and “vibrato” are swapped from their technical definitions
This is worth naming explicitly because it’s a genuine, widely-documented historical naming error that still causes confusion today, not just imprecise slang: vintage Fender amplifiers famously labeled their amplitude-modulation circuit “vibrato” on some models and their pitch-bending guitar hardware “tremolo arm” (the whammy bar) — backwards from the technical definitions in the table above, where tremolo is amplitude and vibrato is pitch. The mislabeling stuck hard enough in guitar culture that “tremolo arm” for a pitch-bending bridge is still the near-universal term today, even though it’s modulating pitch, which is technically vibrato. When a schematic or spec sheet uses one of these two terms, check what it actually modulates rather than assuming the word matches its technical definition.
Common mistake: assuming rate and depth are the only two things that define a modulation effect’s character
Two modulation pedals with identical rate and depth settings can sound completely different because the underlying LFO waveform shape (triangle, sine, or square) changes how the modulated parameter moves between its extremes — a triangle-wave LFO moves at a constant rate and reverses direction sharply, a sine-wave LFO eases in and out of each extreme, and a square-wave LFO snaps between two settings with no gradual sweep at all, producing a much more abrupt, choppy effect (most commonly heard in hard-edged tremolo circuits). Rate and depth control the scale of the modulation; the LFO’s waveform shape controls its feel, and it’s just as responsible for a given pedal’s character as the two knobs a player actually sees.