Introduction to Sound Synthesis Effects
Distortion
Distortion changes the shape of a waveform, usually by flattening its peaks when it goes past a limit. The flattened peaks add harmonics, so the further the signal is driven past the limit, the more high harmonics it contains and the brighter it sounds.
Every circuit has a largest signal it can pass, and every digital system has a largest value it can store. Drive a signal past that limit and the peaks are flattened, or clipped, because the output cannot go any further. Guitarists in the 1950s found that valve amplifiers turned up past their clean range clipped in this way, and players began to use the sound deliberately. Dedicated fuzz boxes followed, such as the Maestro FZ-1 of 1962, built to clip on purpose. In synthesis, distortion is used on bass and drums to add upper harmonics, on leads for brightness, and in small amounts on whole mixes to round off the peaks. Like wavefolding, it is waveshaping: each input level is mapped to an output level by a curve.
The tool below drives a source into one of four curves. The plot shows the curve itself, one cycle of a sine wave after it, and the harmonics that the curve has added.
Distortion
A sine wave, a chord or drums are driven into a soft clipper, a hard clipper, an asymmetric clipper or a bit reducer. Drive sets how far past the limit the signal goes, and tone sets the cutoff of a lowpass filter after the curve.
Listen for On the sine, soft clipping adds harmonics gradually as the drive rises. Hard clipping adds strong high harmonics as soon as the peaks reach the limit, and the tone becomes buzzy. At high drive both turn the sine into something close to a square wave. The asymmetric curve adds even harmonics as well as odd ones, which shows in the harmonic bars. The bit reducer turns the sine into a staircase with a gritty, digital sound. On the chord, every type sounds rougher than on the single note, because distortion also produces sum and difference frequencies from each pair of notes.
Soft and hard
A hard clipper passes the signal unchanged up to the limit and cuts it off flat above it, so the waveform gains sharp corners, and a waveform with sharp corners contains strong high harmonics. A soft clipper begins to bend the waveform before it reaches the limit and approaches the limit gradually, so the corners are rounded and the high harmonics are weaker; it is often described as warmer. Both approach a square wave as the drive rises, because a sine driven far past the limit spends almost all of each cycle flattened at one limit or the other.
Even and odd
If the curve treats the positive and negative halves of the wave the same way, the output keeps the half-cycle symmetry of the input, and, as with a square wave, it contains only odd harmonics. Many real circuits, including single valve stages, clip one half earlier than the other. That breaks the symmetry and adds even harmonics, which sound fuller and less harsh to many listeners. The asymmetric curve in the tool does this.
Bit reduction and chords
Early digital samplers and game consoles stored audio with few bits, so each sample could take only a small number of levels. Reducing the number of levels turns a smooth wave into a staircase, a different kind of distortion with a gritty, digital character. Whatever the curve, distortion of a chord also produces sum and difference frequencies between every pair of notes, as ring modulation does. For an octave, those frequencies are harmonics of the lower note. For a fifth tuned to the ratio 3:2, the difference is an octave below the lower note and the sums are harmonics of that same frequency, so the result keeps one clear pitch; the equal-tempered fifth is within two cents of 3:2, so the same holds very nearly. With thirds and denser chords the sum and difference frequencies are more numerous and more closely spaced. In equal temperament a major third is about 14 cents wider than the ratio 5:4, so those frequencies land near each other without coinciding and beat against each other. This is one reason distorted guitar parts so often play fifths.
Common questions
What is the difference between overdrive, distortion and fuzz?
They are names for increasing amounts of the same thing: a waveform pushed past the level a circuit can pass cleanly, so its peaks are flattened. Overdrive usually means gentle, soft clipping; distortion means more; fuzz means clipping so hard that the waveform becomes close to a square wave.
What is the difference between soft and hard clipping?
Hard clipping cuts the waveform off flat at a fixed level, which makes sharp corners and adds strong high harmonics. Soft clipping bends the waveform gradually as it approaches the limit, which rounds the corners and adds weaker high harmonics. Soft clipping is often described as warmer for that reason.
Why does distortion add even or odd harmonics?
When the input is a wave whose negative half-cycle mirrors its positive one, such as a sine, clipping that treats the two halves alike keeps that symmetry, so it adds only odd harmonics. Clipping that treats the halves differently, as many valve and transistor circuits do, breaks the symmetry and adds even harmonics as well.
Why do distorted chords sound rough?
Distortion creates sum and difference frequencies between every pair of notes as well as harmonics of each. For octaves and fifths those extra frequencies are harmonics of a single lower note, or very nearly so for an equal-tempered fifth, so the result keeps one clear pitch. With thirds and denser chords there are many more of them, closely spaced, and in equal temperament they land near each other without coinciding, so they beat against each other. This is one reason distorted guitar often plays fifths, called power chords.
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