signal noise
In science, and especially in physics and telecommunication, noise is fluctuations in and the addition of external factors to the stream of target information (signal) being received at a detector. In communications, it may be deliberate as for instance jamming of a radio or TV signal, but in most cases it is assumed to be merely undesired interference with intended operations. Natural and deliberate noise sources can provide both or either of random interference or patterned interference. Only the latter can be cancelled effectively in analog systems; however, digital systems are usually constructed in such a way that their quantized signals can be reconstructed perfectly, as long as the noise level remains below a defined maximum, which varies from application to application.
More specifically, in physics, the term noise has the following meanings:
 An undesired disturbance within the frequency band of interest; the summation of unwanted or disturbing energy introduced into a communications system from man-made and natural sources.
A disturbance that affects a signal and that may distort the information carried by the signal.
Random variations of one or more characteristics of any entity such as voltage, current, or data.
A random signal of known statistical properties of amplitude, distribution, and spectral density.
Loosely, any disturbance tending to interfere with the normal operation of a device or system.
Noise and what can be done about it has long been studied. Claude Shannon established information theory and in so doing clarified the essential nature of noise and the limits it places on the operation of electronic equipment.
In some cases a little noise may be considered advantageous, allowing a dithered representation of signals below the minimum strength, or between two quantization levels. This is especially true for signals intended for human appreciation, since the brain seems to expect signals to contain a degree of noise. See, for example  , or the phenomenon of stochastic resonance, where small amount of noise improves the detection of signals in non-linear sensors.
See also
Electronic noise
Colors of noise
Signal-to-noise ratio
Electromagnetic interference
Noise level
Noise power
Noise-equivalent power
Phase noise
List of noise topics
Audio system measurements
Stochastic resonance
External links
Noise and Operational Amplifier Circuits (PDF) — Analog Devices Application note
Calculation of thermal or Johnson noise, Nyquist noise - Noise voltage in microvolts, and noise level in dBu and dBV
PEAR Lab: consciousness - machine interaction research using diode Johnson noise
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