Q. What is meant by pre-emphasis and de-emphasis?

In telecommunication, emphasis is the intentional attenuation of amplitude vs frequency characteristics of a signal to reduce the adverse effects of noise in a communication system. 

            The noise that may occur include both signal frequency interference and thermal noise. Noise is inherently greater in amplitude at higher modulating signal frequency than at lower frequencies because the signals with higher frequency have low SNR.

Pre-emphasis:

            Signals with higher modulation frequency have low SNR. In order to compensate this, the high frequency signals are emphasized or boosted at the transmitter section of a communication system prior to the modulation process.

 That is, pre-emphasis network allows the higher frequency modulating signal to modulate the carrier at higher level, this causes more frequency deviation.

Example: High Pass Filter

De-emphasis:

            It is the reverse process of pre-emphasis, used to attenuate the high frequency signal that is boosted at the transmitter section. The de-emphasis network at the receiver section restores the original amplitude vs frequency characteristics of information signal after the demodulation process.

Example: Low Pass Filter 

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