View source for Distortion

{{Short description|Alteration of the original shape of a signal}}
{{About|technology, especially electrical engineering}}
{{Redirect|Distort}}

In [[signal processing]], '''distortion''' is the alteration of the original shape (or other characteristic) of a [[signal]].  In [[communications]] and [[electronics]] it means the alteration of the [[waveform]] of an information-bearing [[signal]], such as an [[audio signal]] representing sound or a [[video signal]] representing images, in an electronic device or [[communication channel]].

Distortion is usually unwanted, and so engineers strive to eliminate or minimize it. In some situations, however, distortion may be desirable.  For example, in [[noise reduction]] systems like the [[Dolby noise-reduction system|Dolby system]], an audio signal is deliberately distorted in ways that emphasize aspects of the signal that are subject to [[electrical 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 a [[Distortion (music)|musical effect]], particularly with [[electric guitar]]s.

The addition of [[Electronic noise|noise]] or other outside signals ([[mains hum|hum]], [[interference (communication)|interference]]) is not considered distortion, though the effects of [[quantization distortion]] are sometimes included in noise. Quality measures that reflect both noise and distortion include the [[SINAD|signal-to-noise and distortion]] (SINAD) ratio and [[THD+N|total harmonic distortion plus noise]] (THD+N).

==Electronic signals==
In [[telecommunications]] and [[signal processing]], a noise-free [[system]] can be characterised by a [[transfer function]], such that the output <math>y(t)</math> can be written as a function of the input <math>x</math> as

: <math>y(t) = F(x(t))</math>

When the transfer function comprises only a perfect [[gain (electronics)|gain]] constant ''A'' and perfect [[propagation delay|delay]] ''T''

: <math>y(t) = A\cdot x(t-T)</math>

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, the signal suffers linear distortion. Linear distortion does not introduce new frequency components to a signal but does alter the balance of existing ones.

[[File:Distorted waveforms square sine.svg|353px|thumb|right|Graph of a waveform and some distorted versions of the same waveform]]

This diagram shows the behaviour of a signal (made up of a [[Square wave (waveform)|square wave]] followed by a [[sine 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).
# A [[high-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.
# A [[low-pass filter]] (blue trace) rounds the pulses by removing the high frequency components. All systems are low pass to some extent. Note that the [[Phase (waves)|phase]] of the sine wave is different for the lowpass and the highpass cases, due to the phase distortion of the filters.
# A slightly [[non-linear]] transfer function (purple), this one gently compresses the peaks of the sine wave, as may be typical of a [[tube audio amplifier]]. This generates small amounts of low order harmonics.
# A hard-[[Clipping (audio)|clipping]] transfer 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. [[Nonlinearity|Nonlinearities]] in the transfer function of an [[active device]] (such as [[vacuum tube]]s, [[transistor]]s, and [[operational amplifier]]s) are a common source of non-linear distortion; in passive [[electronic component|components]] (such as a [[coaxial cable]] or [[optical fiber]]), linear distortion can be caused by inhomogeneities, [[Reflection (electrical)|reflections]], and so on in the [[wave propagation|propagation]] path.

===Amplitude distortion===
{{Main|Amplitude distortion}}
{{See also|Clipping (signal processing)}}
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 adds [[overtone]]s that are [[Integer|whole number]] multiples of a sound wave's frequencies.<ref>{{Cite book
  | last = Moscal
  | first = Tony
  | title = Sound Check: The Basics of Sound and Sound Systems
  | publisher = Hal Leonard 
  | year = 1994
  | page = 55
  | url = https://books.google.com/books?id=_omgNjqf7GAC&q=harmonic+distortion+whole+integer&pg=PA55
  | isbn = 9780793535590}}</ref> Nonlinearities that give rise to amplitude distortion in audio systems are most often measured in terms of the [[harmonic]]s (overtones) added to a pure [[sinewave]] fed to the system. Harmonic distortion may be expressed in terms of the relative strength of individual components, in [[decibel]]s, or the [[root mean square]] of 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 (like [[crossover distortion]]) are more audible than others (like [[soft clipping]]) even if the THD measurements are identical.  Harmonic distortion in [[radio frequency]] applications is rarely expressed as THD.

===Frequency response distortion===
{{See also|Frequency response}}
Non-flat frequency response is a form of distortion that occurs when different frequencies are amplified by different amounts in a [[Filter (signal processing)|filter]]. For example, the non-uniform frequency response curve of AC-coupled [[cascade amplifier]] is 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 loudspeaker [[Electrical impedance|impedance]], etc.

===Phase distortion===
{{Main|Phase distortion}}
This form of distortion mostly occurs due to [[electrical 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 in [[dispersion (optics)|dispersive media]]. In a [[waveguide]], [[phase velocity]] varies with frequency. In a filter, group delay tends to peak near the [[cut-off frequency]], resulting in pulse distortion.  When analog long distance trunks were commonplace, for example in [[12 channel carrier]], group delay distortion had to be corrected in [[repeater]]s.

==Correction of distortion==
As the system output is given by y(t) = F(x(t)), then if the inverse function F<sup>−1</sup> can 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/[[gramophone record|vinyl]] recordings or [[FM broadcasting|FM audio]] transmissions are deliberately pre-emphasised by a [[linear 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 the [[transfer function]] has 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—causing [[clipping (audio)|clipping]] or [[slew rate]] distortion when, for a moment, the amplifier characteristics alone and not the input signal determine the output.

===Cancellation of even-order harmonic distortion===
Many symmetrical [[electronic circuits]] reduce 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 include [[Push–pull output|push-pull amplifiers]] and [[long-tailed pair]]s.

==Teletypewriter or modem signaling==
In binary [[signaling]] such as [[Frequency-shift keying|FSK]], distortion is the shifting of the significant instants of the signal pulses from their proper positions relative to the beginning of the start [[pulse]]. The magnitude of the distortion is expressed in percent of an ideal unit [[pulse]] length. This is sometimes called ''bias distortion''.

Telegraphic distortion is a similar and older problem, distorting the ratio between [[Mark and space|''mark'' and ''space'']] intervals.<ref>{{Cite web |date=July 1970 |title=Telegraphic Type Services Standard Interface Specifications |url=https://www.ridethemindway.com/phones/Disk_1/Bell_Sytstem_Practices_000-000-000_to_500-000-000/312-011-101_I1R.pdf |url-status=live |archive-url=https://web.archive.org/web/20160228063249/https://www.ridethemindway.com/phones/Disk_1/Bell_Sytstem_Practices_000-000-000_to_500-000-000/312-011-101_I1R.pdf |archive-date=2016-02-28 |website=The Mindway}}</ref>

==Audio distortion==<!-- This section is linked from [[My Bloody Valentine]], [[Loudness war]] and [[Audio distortion]] (redirect) -->
[[File:Distortion waveform.svg|thumb|left|A graph of a waveform and the distorted version of the same waveform]]
{{Listen
| filename     = Distortion_effect.ogg
| title        = Distorted waveforms
| description  = An audio example of a short sample followed by different distorted versions of it.
| format       = [[Ogg]]
}}

With respect to audio, distortion refers to any kind of deformation of an output waveform compared to its input, usually [[Clipping (music)|clipping]], [[harmonic distortion]], or [[intermodulation distortion]] ([[Mixing (physics)|mixing]] phenomena) caused by [[non-linear]] behavior of electronic components and power supply limitations.<ref>Audio Electronics by John Linsley Hood; page 162</ref> Terms for specific types of nonlinear audio distortion include: [[crossover distortion]] and [[slew-induced distortion]] (SID).

Other forms of audio distortion are non-flat [[frequency response]], [[audio level compression|compression]], [[modulation]], [[aliasing]], [[quantization noise]], [[wow (recording)|wow]] and [[flutter (electronics and communication)|flutter]] from analog  media such as [[vinyl records]] and [[magnetic tape]]. The human ear cannot hear [[phase distortion]], except that it may affect the [[stereo imaging]].

In most fields, distortion is characterized as unwanted change to a signal. [[Distortion (music)|Distortion in music]] is often [[Tube sound#Intentional distortion|intentionally used as an effect]] when applied to an [[electric guitar]] signal in styles of [[rock music]] such as [[heavy metal music|heavy metal]] and [[punk rock]].

{{clear}}

==Distortion in art==
In the [[visual arts]] a 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 from [[photorealistic]] perspective or from realistic proportionality.  Examples include "[[The Weeping Woman]]" by Picasso and "[[The Adoration of the Shepherds (El Greco)|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 standard [[Perspective (graphical)|perspective]].

==Optics==<!-- This section is linked from [[Angle of view]] -->
{{Main|Distortion (optics)}}

In [[optics]], image/optical distortion is a divergence from [[rectilinear projection]] caused by a change in [[magnification]] with increasing distance from the [[optical axis]] of an optical system.

==Map projections==
{{Main|Map projection}}

In [[cartography]], a distortion is the misrepresentation of the area or shape of a feature. The [[Mercator projection]], for example, distorts by exaggerating the size of regions at high [[latitude]].

==See also==
{{div col|colwidth=20em}}
* [[Aliasing]]
* [[Attenuation distortion]]
* [[Audio system measurements]]
* [[Bias distortion]]
* [[Distortion synthesis]]
* [[Fading]]
* [[Image warping]]
* [[Lossy compression]]
{{div col end}}

==References==
{{Reflist}}
{{FS1037C MS188}}

==External links==
* {{Commons category-inline}}

{{Noise}}
{{Analogue TV transmitter topics}}

[[Category:Audio amplifier specifications]]
[[Category:Audio effects]]
[[Category:Cartography]]
[[Category:Geometrical optics]]
[[Category:Effects units]]