Deemphasis
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In telecommunication, deemphasis is a system process designed to decrease, within a band of frequencies, the magnitude of some (usually higher) frequencies with respect to the magnitude of other (usually lower) frequencies in order to improve the overall signal-to-noise ratio by minimizing the adverse effects of such phenomena as attenuation differences or saturation of recording media in subsequent parts of the system. It is the mirror of preemphasis, and the whole system is called emphasis. The frequency curve (response) is decided by special time constants, from which one can calculate the cutoff frequency.
Preemphasis is commonly used in audio digital recording, record cutting and FM radio transmission.
See also Federal Standard 1037C and MIL-STD-188.de:Deemphase