The extent to which a system or component fails to reproduce accurately at its output the characteristics of the input. The modification of a waveform by a transmission system or network involves the introduction of features not present in the original input or the suppression or modification of features that are present. Distortion is a significant problem in telecommunication systems. There are several different types of distortion.

• • Amplitude distortion occurs when the ratio of the root-mean-square value of the output to the r.m.s. value of the input varies with the amplitude of the input, both waveforms being sinusoidal. If harmonics are present in the output waveform only the fundamental frequency is considered.

• • Crossover distortion occurs in push-pull operation when the transistors are not operating in the correct phase with each other.

• • Delay distortion is a change in the waveform because of the variation of the delay with frequency.

• • Deviation distortion occurs in frequency-modulated receivers that have an inadequate bandwidth or nonlinear discriminator.

• • Harmonic distortion is due to harmonics not present in the original waveform.

• • Intermodulation distortion results from spurious combination-frequency components in the output of a nonlinear transmission system when two or more sinusoidal voltages, applied simultaneously, form the input. Intermodulation distortion of a complex waveform arises from intermodulation within the waveform.

• • Nonlinear distortion is produced in a system when the instantaneous transmission properties depend on the magnitude of the input. Amplitude, harmonic, and intermodulation distortion are all results of nonlinear distortion.

• • Phase distortion occurs when the phase change introduced is not a linear function of frequency.

Optical distortion of an image is seen in electronic systems due to errors in the electron-lens focusing systems.

Optical distortion

• • Aperture distortion of an image occurs in a scanning system when the scanning spot has finite dimensions rather than infinitely small dimensions.

• • Barrel and pincushion distortion are seen when the lateral magnification is not constant but depends on image size (see diagram). Barrel distortion occurs when the magnification decreases with object size, pincushion distortion when it increases with object size.

• • Coma is a plumelike distortion of the spot occurring when the focusing elements of the electron gun are misaligned.

• • Keystone distortion is due to the length of the horizontal scan line varying with the vertical displacement of the line. It is most pronounced when the electron beam is at an acute angle to the screen and results in a trapezoidal image instead of a rectangular one. It can be removed using suitable transmitter circuits.

• • Trapezium distortion is a trapezoidal pattern on the screen of a cathode-ray tube instead of a square one and occurs when the deflecting voltage applied to the plates is unbalanced with respect to the anode.