What is a Pre-emphasis

Theory has proved, frequency of the output noise power spectrum by the square of the frequency law of increase. However, many actual message signals, such as language, music, and their power spectrum decreases with increasing frequency, most of its energy concentrated in low frequency range. This has resulted in high-frequency end of the message signal to noise ratio may be reduced to the extent not allowed. However, due to the higher frequency components of the message signal energy is small, rarely enough to produce the largest range of frequency offset, resulting in the maximum deviation of the signal amplitude mostly caused by the low frequency signal. On average, a minor deviation produced much less high frequency components. So FM signal and not fully occupy the bandwidth given to it. Because the FM bandwidth of the system need to send the message by the signal (modulation signal) of the most significant decisions of the frequency and maximum frequency deviation. However, the receiver input noise spectrum occupy the entire frequency bandwidth. This means that, in the frequency discriminator output noise power spectrum at higher frequencies have been increased.

To offset this undesirable phenomenon, in the FM system is widely used a technique called pre-emphasis and de-emphasis measures, the central idea is to use the signal and noise characteristics of the signal differences to effectively deal with. Before the introduction of noise with the appropriate network (pre-emphasis network), artificially increase (enhance) the transmitter input modulation signal frequency components. Then at the receiver output of frequency discriminator, and then vice versa, ie to increase the use of high frequency components to increase the network to restore the original signal power distribution. In the de-emphasis process, but also reduces high frequency noise, but the pre-emphasis on noise and not affect the output thus effectively improving the signal to noise ratio.