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signal-to-noise ratio[′sig·nəl tə ′nȯiz ‚rā·shō]
The quantity that measures the relationship between the strength of an information-carrying signal in an electrical communications system and the random fluctuations in amplitude, phase, and frequency superimposed on that signal and collectively referred to as noise. For analog signals, the ratio, denoted S/N, is usually stated in terms of the relative amounts of electrical power contained in the signal and noise. For digital signals the ratio is defined as the amount of energy in the signal per bit of information carried by the signal, relative to the amount of noise power per hertz of signal bandwidth (the noise power spectral density), and is denoted Eb/N0. Since both signal and noise fluctuate randomly with time, S/N and Eb/N0 are specified in terms of statistical or time averages of these quantities.
The magnitude of the signal-to-noise ratio in a communications systems is an important factor in how well a receiver can recover the information-carrying signal from its corrupted version and hence how reliably information can be communicated. Generally speaking, for a given value of S/N the performance depends on how the information quantities are encoded into the signal parameters and on the method of recovering them from the received signal. The more complex encoding methods such as phase-shift keying or quadrature amplitude-shift keying usually result in better performance than simpler schemes such as amplitude- or frequency-shift keying. As an example, a digital communication system operating at a bit error rate of 10-5 requires as much as 7 dB less for Eb/N0 when employing binary phase-shift keying as when using binary amplitude-shift keying. See Electrical communications, Information theory
signal-to-noise ratio (S/N ratio)
signal-to-noise ratioThe ratio of the power or volume (amplitude) of a signal to the amount of disturbance (the noise) mixed in with it. Measured in decibels, signal-to-noise ratio (SNR, S/N) measures the clarity of the signal in a circuit or a wired or wireless transmission channel. See interference-to-noise ratio.
The Larger the Number, the Better
The greater the ratio (the larger number), the less noise and more easily it can be filtered out. The lowest number is an SNR of 0, which means that noise and signal levels are the same. Although signals contain non-random intelligence and can be isolated and separated, with a 0 SNR, it is more difficult to isolate the signal in real time and might be better handled in a batch process offline.
As a Metaphor (Quality Vs. Junk)
The terms "signal" and "noise" are also commonly used to refer to meaningful content (signal) versus useless content (noise). For example, in a search engine result, signal refers to relevant results compared to the noise, which is information of little value. The terms can be applied to any quantity of material that is valuable versus that which is not.