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A distortion is the alteration of the original shape (or other characteristic) of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted, and often many methods are employed to minimize it in practice. In some fields, however, distortion is actually desirable; such is the case with electric guitar (where distortion is often induced purposely with the amplifier or an electronic effect to achieve an aggressive sound where desired), or censoring words. The slight distortion of analog tapes and vacuum tubes is considered pleasing in certain music listening situations.

The addition of noise or other extraneous signals (hum, interference) is not considered to be distortion, though the effects of quantization distortion are sometimes considered noise. A quality measure that explicitly reflects both the noise and the distortion is the Signal-to-noise-and-distortion (SINAD) ratio.


Electronic signals

In telecommunication and signal processing, a noise-free "system" can be characterised by a transfer function, such that the output y(t) can be written as a function of the input x as

When the transfer function comprises only a perfect gain constant A and perfect delay T

the output is undistorted. Distortion occurs when the transfer function F is more complicated than this. If F is a linear function, for instance a filter whose gain and/or delay varies with frequency, then the signal will experience linear distortion. Linear distortion will not change the shape of a single sinuosoid, but will usually change the shape of a multi-tone signal.

This diagram shows the behaviour of a signal (made up of a square wave followed by a sine wave) as it is passed through various distorting functions.

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