6.4.8 Pre-Emphasis And De-Emphasis

In frequency-modulated systems, channel noise introduces both amplitude and phase perturbations to the received carrier. Although amplitude noise is largely rejected by the FM demodulator, phase noise is converted into instantaneous frequency fluctuations, producing a baseband noise component whose power spectral density increases rapidly with frequency. In fact, the post-detection noise spectral density is approximately proportional to the square of frequency, so higher-frequency components are much more severely affected than lower-frequency components. This behavior is further compounded by the fact that higher-frequency components of typical message signals usually have smaller amplitudes, magnifying the perceptual impact of the non-linear noise introduced by the demodulation process.

To compensate for this inherent frequency-dependent degradation, pre-emphasis is applied at the transmitter. A pre-emphasis filter artificially boosts the higher-frequency components of the message signal prior to modulation, with a gain that increases with frequency (approximately a first-order high-pass characteristic). After demodulation, a complementary de-emphasis filter is applied at the receiver to restore the original frequency response of the message signal. This combination effectively suppresses high-frequency noise, resulting in a recovered baseband noise spectrum that is approximately flat across the message bandwidth.

Pre-emphasis and de-emphasis can therefore be viewed as techniques for improving the effective baseband signal-to-noise ratio for a given channel carrier-to-noise ratio. By selectively increasing the robustness of high-frequency components—without increasing the peak frequency deviation or the occupied transmission bandwidth—these techniques provide a significant performance advantage. In practical analog FM systems, pre-emphasis typically yields an improvement of approximately 4–8 dB for telephony and around 3 dB for television, representing a substantial enhancement in received signal quality without additional spectral cost.