Distortion

Distortion describes a difference between the input and output of an audio system that alters the shape of the waveform. While some distortion can be musically desirable, particularly in analogue circuits, it generally indicates a loss of fidelity or accuracy.

There are several types of distortion, each caused by different mechanisms. Harmonic distortion produces overtones that are integer multiples of the original signal frequency. It is common in valve amplifiers and magnetic tape, where even-order harmonics can add warmth or richness. Intermodulation distortion occurs when two or more frequencies interact non-linearly, producing additional tones not harmonically related to the originals. This type is generally unpleasant and indicates circuit overload or instability.

Clipping distortion results when an amplifier or converter is driven beyond its voltage limits, flattening the peaks of the waveform and introducing harsh-sounding artefacts. Crossover distortion appears in some push-pull amplifier stages when there is a discontinuity at the point where one device hands off to another. Phase and time-based distortion arise from misaligned filters, converters, or playback systems, altering the timing relationships within the signal.

Distortion is measured as a percentage of unwanted content relative to the original signal, often specified as Total Harmonic Distortion (THD) or THD+N (including noise). In high-fidelity systems, THD values below 0.01% are considered excellent, while musical applications may intentionally use higher amounts for colouration.

In professional audio, understanding and controlling distortion is crucial to maintaining signal integrity. Whether using analogue or digital equipment, engineers aim to preserve the original waveform unless a specific type of distortion is introduced deliberately for creative effect.