Distortion

Insignal processing,distortionis the alteration of the original shape (or other characteristic) of asignal.Incommunicationsandelectronicsit means the alteration of thewaveformof an information-bearingsignal,such as anaudio signalrepresenting sound or avideo signalrepresenting images, in an electronic device orcommunication channel.

Distortion is usually unwanted, and so engineers strive to eliminate or minimize it. In some situations, however, distortion may be desirable. For example, innoise reductionsystems like theDolby system,an audio signal is deliberately distorted in ways that emphasize aspects of the signal that are subject toelectrical noise,then it is symmetrically "undistorted" after passing through a noisy communication channel, reducing the noise in the received signal. Distortion is also used as amusical effect,particularly withelectric guitars.

The addition ofnoiseor other outside signals (hum,interference) is not considered distortion, though the effects ofquantization distortionare sometimes included in noise. Quality measures that reflect both noise and distortion include thesignal-to-noise and distortion(SINAD) ratio andtotal harmonic distortion plus noise(THD+N).

Electronic signals

Intelecommunicationsandsignal processing,a noise-freesystemcan be characterised by atransfer function,such that the output $y(t)$ can be written as a function of the input $x$ as

y(t)=F(x(t))

When the transfer function comprises only a perfectgainconstantAand perfectdelayT

y(t)=A\cdot x(t-T)

the output is undistorted. Distortion occurs when the transfer functionFis more complicated than this. IfFis alinear function,for instance a filter whose gain and/or delay varies with frequency, the signal suffers linear distortion. Linear distortion does not introduce new frequency components to a signal but does alter the balance of existing ones.

Graph of a waveform and some distorted versions of the same waveform

This diagram shows the behaviour of a signal (made up of asquare wavefollowed by asine wave) as it is passed through various distorting functions.

The first trace (in black) shows the input. It also shows the output from a non-distorting transfer function (straight line).
Ahigh-pass filter(green trace) distorts the shape of a square wave by reducing its low frequency components. This is the cause of the "droop" seen on the top of the pulses. This "pulse distortion" can be very significant when a train of pulses must pass through an AC-coupled (high-pass filtered) amplifier. As the sine wave contains only one frequency, its shape is unaltered.
Alow-pass filter(blue trace) rounds the pulses by removing the high frequency components. All systems are low pass to some extent. Note that thephaseof the sine wave is different for the lowpass and the highpass cases, due to the phase distortion of the filters.
A slightlynon-lineartransfer function (purple), this one gently compresses the peaks of the sine wave, as may be typical of atube audio amplifier.This generates small amounts of low order harmonics.
A hard-clippingtransfer function (red) generates high order harmonics. Parts of the transfer function are flat, which indicates that all information about the input signal has been lost in this region.

The transfer function of an ideal amplifier, with perfect gain and delay, is only an approximation. The true behavior of the system is usually different.Nonlinearitiesin the transfer function of anactive device(such asvacuum tubes,transistors,andoperational amplifiers) are a common source of non-linear distortion; in passivecomponents(such as acoaxial cableoroptical fiber), linear distortion can be caused by inhomogeneities,reflections,and so on in thepropagationpath.

Amplitude distortion

Amplitude distortion is distortion occurring in a system, subsystem, or device when the output amplitude is not a linear function of the input amplitude under specified conditions.

Harmonic distortion

Harmonic distortion addsovertonesthat arewhole numbermultiples of a sound wave's frequencies.^[1]Nonlinearities that give rise to amplitude distortion in audio systems are most often measured in terms of theharmonics(overtones) added to a puresinewavefed to the system. Harmonic distortion may be expressed in terms of the relative strength of individual components, indecibels,or theroot mean squareof all harmonic components:Total harmonic distortion(THD), as a percentage. The level at which harmonic distortion becomes audible depends on the exact nature of the distortion. Different types of distortion (likecrossover distortion) are more audible than others (likesoft clipping) even if the THD measurements are identical. Harmonic distortion inradio frequencyapplications is rarely expressed as THD.

Frequency response distortion

Non-flat frequency response is a form of distortion that occurs when different frequencies are amplified by different amounts in afilter.For example, the non-uniform frequency response curve of AC-coupledcascade amplifieris an example of frequency distortion. In the audio case, this is mainly caused by room acoustics, poor loudspeakers and microphones, long loudspeaker cables in combination with frequency dependent loudspeakerimpedance,etc.

Phase distortion

This form of distortion mostly occurs due toelectrical reactance.Here, all the components of the input signal are not amplified with the same phase shift, hence making some parts of the output signal out of phase with the rest of the output.

Group delay distortion

Can be found only indispersive media.In awaveguide,phase velocityvaries with frequency. In a filter, group delay tends to peak near thecut-off frequency,resulting in pulse distortion. When analog long distance trunks were commonplace, for example in12 channel carrier,group delay distortion had to be corrected inrepeaters.

Correction of distortion

As the system output is given by y(t) = F(x(t)), then if the inverse function F⁻¹can be found, and used intentionally to distort either the input or the output of the system, then the distortion is corrected.

An example of a similar correction is where LP/vinylrecordings orFM audiotransmissions are deliberately pre-emphasised by alinear filter,the reproducing system applies an inverse filter to make the overall system undistorted.

Correction is not possible if the inverse does not exist—for instance if thetransfer functionhas flat spots (the inverse would map multiple input points to a single output point). This produces an uncorrectable loss of information. Such a situation can occur when an amplifier is overdriven—causingclippingorslew ratedistortion when, for a moment, the amplifier characteristics alone and not the input signal determine the output.

Cancellation of even-order harmonic distortion

Many symmetricalelectronic circuitsreduce the magnitude of even harmonics generated by the non-linearities of the amplifier's components, by combining two signals from opposite halves of the circuit where distortion components that are roughly the same magnitude but out of phase. Examples includepush-pull amplifiersandlong-tailed pairs.

Teletypewriter or modem signaling

In binarysignalingsuch asFSK,distortion is the shifting of the significant instants of the signal pulses from their proper positions relative to the beginning of the startpulse.The magnitude of the distortion is expressed in percent of an ideal unitpulselength. This is sometimes calledbias distortion.

Telegraphic distortion is a similar and older problem, distorting the ratio betweenmarkandspaceintervals.^[2]

Audio distortion

A graph of a waveform and the distorted version of the same waveform

Distorted waveforms

An audio example of a short sample followed by different distorted versions of it.

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With respect to audio, distortion refers to any kind of deformation of an output waveform compared to its input, usuallyclipping,harmonic distortion,orintermodulation distortion(mi xingphenomena) caused bynon-linearbehavior of electronic components and power supply limitations.^[3]Terms for specific types of nonlinear audio distortion include:crossover distortionandslew-induced distortion(SID).

Other forms of audio distortion are non-flatfrequency response,compression,modulation,aliasing,quantization noise,wowandflutterfrom analog media such asvinyl recordsandmagnetic tape.The human ear cannot hearphase distortion,except that it may affect thestereo imaging.

In most fields, distortion is characterized as unwanted change to a signal.Distortion in musicis oftenintentionally used as an effectwhen applied to anelectric guitarsignal in styles ofrock musicsuch asheavy metalandpunk rock.

Distortion in art

In thevisual artsa distortion is any change made by an artist to the size, shape or visual character of a form in order to express an idea, convey a feeling, or enhance visual impact. Such distortions or "abstractions" primarily refer to purposeful deviations fromphotorealisticperspective or from realistic proportionality. Examples include "The Weeping Woman"by Picasso and"The Adoration of the Shepherds"by El Greco, whose human subject matters are irregularly and (as is often with physical distortions) asymmetrically proportioned in a way that is not possible in standardperspective.

Optics

Inoptics,image/optical distortion is a divergence fromrectilinear projectioncaused by a change inmagnificationwith increasing distance from theoptical axisof an optical system.

Map projections

Incartography,a distortion is the misrepresentation of the area or shape of a feature. TheMercator projection,for example, distorts by exaggerating the size of regions at highlatitude.

References

^Moscal, Tony (1994).Sound Check: The Basics of Sound and Sound Systems.Hal Leonard. p. 55.ISBN 9780793535590.
^"Telegraphic Type Services Standard Interface Specifications"(PDF).The Mindway.July 1970.Archived(PDF)from the original on 2022-10-09.
^Audio Electronics by John Linsley Hood; page 162

This article incorporatespublic domain materialfromFederal Standard 1037C.General Services Administration.Archived fromthe originalon 2022-01-22.(in support ofMIL-STD-188).

External links

Media related toDistortionat Wikimedia Commons

[1] Moscal, Tony (1994).Sound Check: The Basics of Sound and Sound Systems.Hal Leonard. p. 55.ISBN 9780793535590.

[2] "Telegraphic Type Services Standard Interface Specifications"(PDF).The Mindway.July 1970.Archived(PDF)from the original on 2022-10-09.

[3] Audio Electronics by John Linsley Hood; page 162

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