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Signal-to-noise ratio (abbreviated SNR or ''S/N'') is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power, often expressed in decibels. A ratio higher than 1:1 (greater than 0 dB) indicates more signal than noise. While SNR is commonly quoted for electrical signals, it can be applied to any form of signal (such as isotope levels in an ice core or biochemical signaling between cells). The signal-to-noise ratio, the bandwidth, and the channel capacity of a communication channel are connected by the Shannon–Hartley theorem. Signal-to-noise ratio is sometimes used informally to refer to the ratio of useful information to false or irrelevant data in a conversation or exchange. For example, in online discussion forums and other online communities, off-topic posts and spam are regarded as "noise" that interferes with the "signal" of appropriate discussion. ==Definition== Signal-to-noise ratio is defined as the ratio of the power of a signal (meaningful information) and the power of background noise (unwanted signal): : where ''P'' is average power. Both signal and noise power must be measured at the same or equivalent points in a system, and within the same system bandwidth. If the variance of the signal and noise are known, and the signal is zero-mean:〔(【引用サイトリンク】title=Signal-to-noise ratio )〕 : If the signal and the noise are measured across the same impedance, then the SNR can be obtained by calculating the square of the amplitude ratio: : where ''A'' is root mean square (RMS) amplitude (for example, RMS voltage). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Signal-to-noise ratio」の詳細全文を読む スポンサード リンク
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