Pre-emphasis and De-emphasis Explained

As we know, noise usually has higher amplitude and higher frequency components. This high-frequency noise can lead to frequency distortion when its amplitude is higher than the components present in the modulating signal. To overcome this, most FM circuits use techniques known as Pre-emphasis at the transmitter and De-emphasis at the receiver.

Pre-emphasis

Pre-emphasis helps amplify high-frequency signal components so they have a magnitude higher than noise components. This leads to an improvement in the Signal to Noise Ratio (SNR).

Fig.1 Pre-emphasis

Fig. 1 depicts the pre-emphasis circuit. As shown in the figure, it consists of R and C components such that:

$t = R_1 * C$

Where $t$ should be about 75 µs. This circuit will have a lower frequency cutoff at approximately 2123 Hz. As mentioned in the curve, all frequencies higher than 2123 Hz are amplified at a rate of 6dB/octave.

The pre-emphasis circuit also has an upper cutoff frequency from where signal enhancement will flatten. The upper cutoff frequency, $F_u$, can be calculated as:

$F_u = \frac{R_1 + R_2}{2 * \pi * R_1 * R_2 * C}$

Due to these characteristics, it’s essentially a simple High Pass Filter with amplification.

De-emphasis

De-emphasis does exactly the reverse of its pre-emphasis counterpart. It’s used at the receiver. It helps bring the pre-amplified signal back to the normal amplitude level. It is a simple Low Pass Filter with a time constant of about 75 µs.

Fig.2 De-emphasis

The De-emphasis circuit will have a cutoff frequency of about 2123 Hz. To bring the signal back to its normal level, all frequency components above 2123 Hz are attenuated at 6dB/octave.

As explained, the pre-emphasis operation performed at the transmitter is compensated for by the de-emphasis operation at the receiver. Both of these modules help increase the amplitude of the high-frequency signal during transmission, ensuring it isn’t masked or obliterated by noise.