noise
In common use the word noise means unwanted sound or noise pollution. In electronics noise can refer to the electronic signal corresponding to acoustic noise (in an audio system) or the electronic signal corresponding to the (visual) noise commonly seen as ’snow’ on a degraded television or video image. In signal processing or computing it can be considered data without meaning; that is, data that is not being used to transmit a signal, but is simply produced as an unwanted by-product of other activities. In Information Theory, however, noise is still considered to be information. In a broader sense the film grain or even advertisements in web pages can be considered noise. Radio noise is interference with radio transmissions caused either by thermal noise from receiver input circuits or by radiated electromagnetic noise picked up by the receiver’s antenna. If no noise was picked up with radio signals, even weak transmissions could be received at virtually any distance by making a radio receiver that was sensitive enough. In practice this doesn’t work, and a point is reached where the only way to extend the range of a transmission is to increase the transmitter power. Thermal noise can be made lower by cooling the circuits, but this is only usually worthwhile on radio telescopes. In other applications the limiting noise source depends on the frequency range in use. At low freqencies (longwave or mediumwave) and at high frequencies (shortwave), interference caused by lightning or by nearby electrical impulses in electrical switches, motors, vehicle ignition circuits, computers, and other man-made sources tends to swamp transmissions with thermal noise. These noises are often referred to as static. At very high frequency and ultra high frequency these sources can still be important, but at a much lower level, such that thermal noise is usually the limiting factor. In the microwave region, cosmic background noise may be relevant. Electromagnetic noise can interfere with electronic equipment in general, causing malfunction, and in recent years standards have been laid down for the levels of electromagnetic radiation that electronic equipment is permitted to radiate. These standards are aimed at ensuring what is referred to as electromagnetic compatibility, or EMC.
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