Distortion
In telecommunications and signal processing, a noise-free system can be characterised by a transfer function, such that the output
Linear distortion does not introduce new frequency components to a signal but does alter the balance of existing ones.
The transfer function of an ideal amplifier, with perfect gain and delay, is only an approximation.
Nonlinearities in the transfer function of an active device (such as vacuum tubes, transistors, and operational amplifiers) are a common source of non-linear distortion; in passive components (such as a coaxial cable or optical fiber), linear distortion can be caused by inhomogeneities, reflections, and so on in the propagation path.
Harmonic distortion adds overtones that are whole number multiples of a sound wave's frequencies.
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.
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.
The magnitude of the distortion is expressed in percent of an ideal unit pulse length.
Other forms of audio distortion are non-flat frequency response, compression, modulation, aliasing, quantization noise, wow and flutter from analog media such as vinyl records and magnetic tape.
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" 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.
The Mercator projection, for example, distorts by exaggerating the size of regions at high latitude.
