Preemphasis

In telecommunication, a preemphasis is 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.

Note: Preemphasis has applications, for example, in audio recording and FM transmission.

Source: from Federal Standard 1037C and from MIL-STD-188