Distortion
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224076720
2008-07-07T05:22:47Z
Binksternet
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Reverted edits by [[Special:Contributions/90.5.84.19|90.5.84.19]] ([[User talk:90.5.84.19|talk]]) to last version by ClueBot
{{otheruses4|technology, especially electrical engineering}}
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. In some fields, distortion is desirable, such as [[electric guitar]] (where distortion is often induced purposely with the [[instrument amplifier|amplifier]] or an electronic effect to achieve the electric guitar's desired, electrifying, aggressive sound). The slight distortion of analog [[magnetic tape|tapes]] and [[vacuum tube]]s is considered pleasing in certain situations. The addition of [[Electronic noise|noise]] or other extraneous signals ([[mains hum|hum]], [[interference (communication)|interference]]) is not considered to be distortion, though the effects of distortion are sometimes considered noise.
== Electronic signals ==
[[Image:Distorted waveforms square sine.png|353px|thumb|right|Graph of a waveform and the distorted versions of the same waveform]]
In [[telecommunication]] 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]] constant ''A'' and perfect [[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, 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.
# 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) will distort 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) will round 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 is gently compressing as may be typical of a tube audio amplifier, will compress the peaks of the sine wave. This will cause small amounts of low order harmonics to be generated.
# A hard-[[Clipping (audio)|clipping]] transfer function (red) will generate 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. [[Nonlinear]]ities 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 [[inhomogeneity|inhomogeneities]], [[Reflection (electrical)|reflections]], and so on in the [[wave propagation|propagation]] path.
=== Amplitude distortion ===
{{main|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.
=== Frequency distortion ===
This form of distortion occurs when different frequencies are amplified by different amounts, mainly caused by combination of active device and [[active component|components]]. For example, the non-uniform frequency response curve of RC-coupled [[cascade amplifier]] is an example of frequency distortion.
=== Phase distortion ===
{{main|Phase distortion}}
This form of distortion mostly occurs due to the reactive component, such as [[capacitive reactance]] or [[inductor]] [[capacitance]]. Here, all the components of the input signal are not amplified with the same phase shift, hence causing some parts of the output signal to be out of phase with the rest of the output.
=== Group delay distortion ===
Can be found only in [[dispersion (optics)|dispersive media]].
In a [[waveguide]], [[wave velocity|propagation 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 [[repeaters]].
=== 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 will be corrected.
An example of such correction is where LP/[[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 results in a loss of information, which is uncorrectable. Such a situation can occur when an amplifier is overdriven, resulting in [[clipping]] or [[slew rate]] distortion, when for a moment the output is determined by the characteristics of the amplifier alone, and not by the input signal.
== Teletypewriter or modem signaling ==
In binary [[Signalling (telecommunication)|signaling]] such as [[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 older problem, distorting the ratio between "mark" and "space" intervals. [http://www.freepatentsonline.com/3725787.html]
==Audio distortion==<!-- This section is linked from [[My Bloody Valentine]] -->
[[Image:Distortion waveform.png|200px|thumb|right|A graph of a waveform and the distorted version of the same waveform]]
In this context, distortion refers to any kind of deformation of a waveform, compared to an input. [[Clipping (music)|Clipping]], [[audio level compression|compression]], non-linear behavior of electronic components, [[modulation]], [[aliasing]], and [[mixing]] phenomena or power supply inefficiencies can cause distortion.
{{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]]}}
In most fields, distortion is characterized as unwanted change to a signal.
==Optics==<!-- This section is linked from [[Angle of view]] -->
In [[optics]], [[image 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 [[Greenland]] because of its high [[latitude]], in the sense that its shape and size are not the same as those on a [[globe]].
== See also ==
* [[Aliasing]]
* [[Amplitude distortion]]
* [[Attenuation distortion]]
* [[Bias distortion]]
* [[Crossover distortion]]
* [[Degree of isochronous distortion]]
* [[Delay distortion]]
* [[Distortion-limited operation]]
* [[Distortion (guitar)]]
* [[Distortion power factor]]
* [[Image warping]]
* [[Intermodulation distortion]]
* [[Lossy compression]]
* [[Overdrive (music)]]
* [[Quantization distortion]]
* [[Signal-to-noise-and-distortion]] (SINAD)
* [[Total harmonic distortion]] — a measurement of the amount of distortion in a sinusoidal waveform
* [[Valve sound]]
== References ==
{{FS1037C MS188}}
== External links ==
* [http://www.geofex.com/effxfaq/distn101.htm A Musical-Distortion Primer]
* [http://www.blackstoneappliances.com/dist101.html Guitar Distortion 101]
* [http://www.mcrow.net/Drive.htm Multiband Distortion ensemble for Reaktor]
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[[Category:Electronics terms]]
[[Category:Optics]]
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