Emphasis

Emphasis: In FM transmission, the intentional alteration of the amplitude-vs.-frequency characteristics of the signal to reduce adverse effects of noise in a communication system.

Note: The high-frequency signal components are emphasized to produce a more equal modulation index[?] for the transmitted frequency spectrum, and therefore a better signal-to-noise ratio for the entire frequency range[?].

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