What is added to an FM speech channel to boost the higher audio frequencies?

In FM transmission, higher frequency audio components often naturally have lower amplitudes and can be easily masked by high-frequency background noise (hiss) in the receiver. To improve the overall signal-to-noise ratio, a "pre-emphasis network" is added to the transmitter's audio stages. This circuit artificially boosts the higher audio frequencies before the signal modulates the carrier. At the receiving end, a corresponding "de-emphasis network" is used to reduce these high frequencies back to their normal, natural levels. As it lowers the artificially boosted highs, it simultaneously cuts down the high-frequency hiss picked up during transmission, resulting in much clearer audio.