Pre-emphasis:

• To avoid amplitude and phase distortion we have to artificially increase the amplitude of FM signal on transmitter side. This concept is known as pre-emphasis.

• In this Modulating signal is passed through the RC filter which is basically high pass filter.

• Pre-emphasis circuit consist differentiator which will increase the amplitude of FM signal by slope +20dB/decade.

• Pre –emphasis process takes place for certain frequency range i.e. 20 to 20 KHz.

Output

Fig: Pre-emphasis characteristics.

3Fig: Typical pre-emphasis circuit

De-emphasis:

• To avoid amplitude and phase distortion we have to artificially decrease the amplitude of FM signal on receiver side. This concept is known as de-emphasis.

• De-emphasis circuit consists of integrator which will reduce amplitude of FM signal by slope -20dB/decade.

• Pre-emphasis and de-emphasis always reduce noise probability in FM

Fig: De-emphasis characteristics.

Fig: Typical de- emphasis circuit