Write short notes on Pre-emphasis and de-emphasis.
2 Answers
  • Pre-emphasis: The noise suppression ability of FM decreases with the increase in the frequencies. Thus increasing the relative strength or amplitude of the high frequency components of the message signal before modulation is termed as Pre-emphasis. The Fig3 below shows the circuit of pre-emphasis.

enter image description here

Fig3. Pre-emphasis circuit

  • De-emphasis: In the de-emphasis circuit, by reducing the amplitude level of the received high frequency signal by the same amount as the increase in pre-emphasis is termed as De-emphasis. The Fig4. below shows the circuit of de-emphasis.

enter image description here

Fig4. De-emphasis circuit

  • The pre-emphasis process is done at the transmitter side, while the de-emphasis process is done at the receiver side.
  • Thus a high frequency modulating signal is emphasized or boosted in amplitude in transmitter before modulation. To compensate for this boost, the high frequencies are attenuated or de-emphasized in the receiver after the demodulation has been performed. Due to pre-emphasis and de-emphasis, the S/N ratio at the output of receiver is maintained constant.
  • The de-emphasis process ensures that the high frequencies are returned to their original relative level before amplification.
  • Pre-emphasis circuit is a high pass filter or differentiator which allows high frequencies to pass, whereas de-emphasis circuit is a low pass filter or integrator which allows only low frequencies to pass.

The artificial boosting of the amplitude of higher frequency modulating s/g in FM, emitter is called pre-emphasis.

The process of pre-emphasis is carried out prior to FM process.

enter image description here

The pre-emphasis circuit is basically HPF(RC) before applying it to FM modulator.

As FM increases the capacitive reactance decreases and modulating voltage applied to FM modulator goes on increasing.

The 3dB cut off frequency for pre-emphasis ckt can be computed as:

$f_1(cut \ off) = \frac{1}{2 \pi R C}$

Need of pre-emphasis ckt :

The noise voltage present at the i/p of frequency modulator may produce some, random variation of frequency even in the absence of a modulating s/g.

In FM, noise greater effect in higher side of modulating frequency range.

This effect can be reduced by increasing the value of frequency modulation index (mf) for higher modulating frequencies (fm).

De-emphasis :

The artificially boosted high frequency s/g in the process of pre-emphasis at FM, transmitter are bought to their original amplitude levels using de-emphasis ckt at FM Rx.

The time constant of de-emphasis ckt of FM Tx must be kept some as that in pre-emphasis ckt in FM transmitter.

enter image description here

Please log in to add an answer.