Historically, noise was considered to be defined as the interference that was heard on a pair of headphones placed at the output of a radio receiver when no signal was being transmitted, the noise being the product of random voltages appearing in the transmission system. Radio operators still regard this as the definition of noise, but noise is now given a broader definition to take into account any random fluctuations of voltage or current that are not intentionally injected into a system. The noise need not be at audio frequencies; it can be at any frequency, such as the television frequencies where interference noise results in white flashes of light on a television picture.
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