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Definition: signal-to-noise ratio

The 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.