præemphasis

Præemphasis, also spelled pre-emphasis, is a signal processing technique in which high-frequency components of a signal are boosted before transmission or subsequent processing. The goal is to compensate for the greater attenuation and higher susceptibility to noise that high-frequency content experiences in many channels, so that the received signal can be reconstructed with a flatter overall frequency response.

In analog systems, pre-emphasis is commonly implemented with an RC network that increases gain as frequency rises. In practice the boost is defined by a time constant, typically around 50 microseconds in European systems and 75 microseconds in North American systems for FM broadcasting. The corresponding receiver applies a de-emphasis network with the same time constant to restore the original spectrum.

Præemphasis is used not only in FM broadcast but also in telephone and data transmission to improve signal-to-noise ratio on channels with poor high-frequency response. In digital systems it can be realized as a pre-emphasis filter in the baseband or in codecs, shaping the spectrum prior to quantization or equalization.

Historically, the technique became standardized in FM radio and has influenced other areas; the term describes a general class of high-frequency emphasis applied before processing, with de-emphasis serving as the complementary stage at the receiver.