In telecommunications, preemphasis refers to a system process designed to increase, 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. That is the mirror of the Deemphasis. The whole system is called Emphasis.
The frequency curve is decided by special time constants. The Cutoff frequency can be calculated from that value.
Preemphasis is commonly used in digital audio recording, record cutting and in FM broadcasting transmissions.
See also Federal Standard 1037C and MIL-STD-188.