Pre-emphasis, and the corresponding pre-emphasis is de-emphasis. It is a technology about the principle of noise shaping technology in analog signal processing. The so-called pre-emphasis means that the high-frequency components of the analog signal are properly upgraded before the signal is sent. After receiving the signal, the signal is inversely processed, that is, de-emphasized, and the high-frequency components are appropriately attenuated. This pre-emphasis and de-emphasis technology can reduce the influence of high frequency loss in signal transmission and change the noise spectrum, which is the principle of analog noise reduction.

Theoretically, it is proved that the output noise power spectrum of frequency discriminator increases according to the square law of frequency. However, the power spectrum of many practical information signals, such as language and music, decreases with the increase of frequency, and most of its energy is concentrated in the low frequency range. This may cause the signal-to-noise ratio of the high-frequency end of the message signal to drop to an unacceptable level. However, because the energy of the higher frequency components in the message signal is very small, the amplitude enough to produce the maximum frequency offset is very small, so the signal amplitude that produces the maximum frequency offset is mostly caused by the low frequency components of the signal. On average, high-frequency components with smaller amplitude produce much smaller frequency offset. Therefore, the FM signal will not completely occupy the bandwidth allocated to it. Because the transmission bandwidth of FM system is determined by the highest effective frequency and the maximum frequency offset of the message signal (modulation signal) to be transmitted. However, the noise spectrum input by the receiver occupies the whole FM bandwidth. In other words, the noise power spectrum at the output of the frequency discriminator is enhanced at higher frequencies. In order to offset this bad phenomenon, people generally adopt a measure called pre-emphasis and de-emphasis in FM system, and its central idea is to effectively process the signal by using the difference between signal characteristics and noise characteristics. That is, before noise is introduced, an appropriate network (pre-emphasis network) is adopted to artificially emphasize (boost) the high-frequency component of the modulation signal input by the transmitter. Then, at the output end of the receiver frequency discriminator, reverse processing is carried out, that is, the high-frequency components are de-emphasized by using the de-emphasis network to restore the original signal power distribution. In the process of de-emphasis, the high-frequency components of noise are also reduced, while pre-emphasis has no effect on noise, so the output signal-to-noise ratio is effectively improved.