The tremolo arm on a Fender Stratocaster produces vibrato: it changes pitch. The vibrato channel on a Fender amplifier produces tremolo: it changes volume. Leo Fender reversed both terms, and the guitar world has been using them backwards ever since. Behind the naming confusion are two distinct signal processing operations with different spectra, different perceptual qualities, and a surprising relationship when you push them to audio rates.
The Definitions
Vibrato is periodic modulation of pitch. The name comes from "vibrare," Latin for to vibrate. A violinist's vibrato is produced by oscillating a finger on the string, varying its effective length and therefore its frequency. Rate is typically 4 to 8 Hz. Depth is measured in cents or as a percentage of the fundamental frequency.
Tremolo is periodic modulation of amplitude. The name comes from "tremare," to tremble. A guitarist's right-hand tremolo technique is rapid pick alternation that varies loudness in time. In electronics, tremolo is amplitude modulation at sub-audio rates.
Leo Fender's Reversal
In 1954, Fender introduced the Stratocaster with a pitch-varying spring mechanism called the "Synchronized Tremolo." It shifts pitch, so by definition it produces vibrato. The name was wrong.
Two years later, Fender introduced the Vibrolux amplifier with an onboard effect called "vibrato." The circuit was a low-frequency oscillator modulating the bias of a preamp tube, which varied gain and therefore amplitude. It produced tremolo. The name was wrong again.
The result is a symmetrical naming disaster. A device that makes true vibrato (pitch change) was called a tremolo. A device that makes true tremolo (amplitude change) was called a vibrato. This was not inconsistency within a single product, but a paired reversal that locked in the confusion for decades. Modern guitar manufacturers still follow Fender's mislabeling because the terms are now entrenched in the market.
Tremolo as Amplitude Modulation
Tremolo is amplitude modulation (AM) at a sub-audio rate, typically 1 to 10 Hz. The mathematical form is:
output(t) = carrier(t) × (1 + d × sin(2π × fm × t))where d is depth (0 to 1) and fm is the modulation frequency.
Multiplying two sinusoids generates their sum and difference frequencies. If the carrier is a single sine at frequency fc and the LFO is at fm, the output contains three components: fc, fc + fm, and fc - fm. The sidebands are each half the depth amplitude relative to the carrier.
For a 440 Hz tone with a 5 Hz LFO at 100% depth, the spectrum shows three lines: 435 Hz, 440 Hz, 445 Hz. The carrier frequency itself remains present. The sidebands sit symmetrically around it. This is a clean, predictable result with no harmonics beyond those three.
Standard AM with the formula above always preserves the carrier component. Ring modulation differs by removing the DC offset term entirely: output(t) = carrier(t) × sin(2π × fm × t). This multiplication produces only the two sidebands with no carrier, which is why ring modulation sounds hollow and metallic. At sub-audio modulation rates with partial depth, the effect is perceived as a rhythmic volume pulsation. Raise the rate above 20 Hz and the sidebands separate audibly from the carrier, producing a distinct beating timbre.
Vibrato as Frequency Modulation
Vibrato is frequency modulation (FM) at a sub-audio rate. The frequency of the carrier oscillates rather than its amplitude:
output(t) = sin(2π × fc × t + β × sin(2π × fm × t))where β (the modulation index) is the ratio of peak frequency deviation to modulation frequency: β = Δf / fm.
The spectrum of FM is more complex than AM. The sideband amplitudes are determined by Bessel functions of the first kind: the nth sideband pair has amplitude Jn(β). At low modulation index (β < 0.2), only J0 and J1 are significant, and the spectrum looks almost identical to AM: carrier plus two sidebands. As β increases, higher-order sidebands grow and the spectrum widens. At β ≈ 2.4, J0 crosses zero, and the carrier frequency briefly disappears from the spectrum.
For a subtle violin vibrato: ±25 cents deviation at 6 Hz on A4 (440 Hz) means Δf ≈ 6.4 Hz and β = 6.4 / 6 ≈ 1.1. This generates noticeable second-order sidebands. For a wide guitar tremolo arm bend of ±1 semitone at 5 Hz, Δf ≈ 26 Hz and β ≈ 5.2, producing a rich sideband structure spread across many harmonic positions.
This is why vibrato sounds richer than tremolo on a sustained note: it spreads energy across a wider frequency range, which perceptually blends into the tonal character rather than sitting as a separate amplitude pulsation. At equal rates and shallow depths, the two effects can sound nearly identical. At deeper settings, vibrato thickens the sound while tremolo simply pulses it.
The Crossover to Synthesis
The sub-audio rate boundary (roughly 20 Hz) marks where modulation effects transition into synthesis operations.
Tremolo at audio rates is ring modulation: a fixed modulation frequency applied to a carrier, producing sum and difference sidebands that can be inharmonic if the ratio of fc to fm is not an integer. Ring modulation was used extensively in electronic music from the 1960s onward for metallic and bell-like timbres.
Vibrato at audio rates is FM synthesis in the Chowning sense: the modulation index and the carrier-to-modulator ratio determine the harmonic content of the output. The Yamaha DX7's distinctive timbres come from FM at audio rates with high modulation indices, producing complex Bessel sideband structures. A guitarist's tremolo arm operating at 5 Hz is doing the same operation as John Chowning's FM synthesis research, just at a rate 200 times slower and with a modulation index around 5.
Implementation
In a plugin, tremolo is trivial: multiply the output buffer by an LFO that oscillates between (1 - depth) and 1.0. No phase modulation is involved. A standard sine LFO and a gain stage are all that are required.
Vibrato requires modulating the playback position or delay time rather than the amplitude. The standard implementation is a short delay line (2 to 10 milliseconds) with a time-varying read pointer. The LFO modulates how far behind the write pointer the read pointer sits, which changes the effective sample rate at the output and therefore the pitch. Depth is controlled by how much the read pointer varies; rate by the LFO frequency.
The Leslie rotary cabinet used with Hammond organs produces both simultaneously. The rotating horn changes the distance to the listener as it spins, creating Doppler-based pitch variation (vibrato) while the rotation pattern also creates amplitude variation as the horn alternately faces and turns away from the listener (tremolo). The combination is why the Leslie sounds distinctly different from electronic vibrato or tremolo in isolation.
Practical Distinction
When programming or mixing modulation effects: tremolo preserves the pitch center exactly and rhythmically accentuates amplitude, making it more perceptually distinct and rhythmic at the same rate. Vibrato spreads spectral energy and creates a rounder, fuller modulation that blends more naturally into sustained tones. For pads and strings, vibrato tends to sound more organic. For rhythmic effects and tremolo picking emulation, amplitude modulation is more direct. The Fender naming mix-up aside, the physical distinction is consistent and precise.