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Pre-emphasis and deemphasis circuits.
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Pre-emphasis

• From the noise triangle it is observed that noise has a greater effect on higher modulating frequencies.
• Thus if we boost the amplitude of higher frequency signals artificially than it is possible to improve the noise immunity at higher modulating frequencies. Artificial boosting of higher modulating frequencies is called as pre-emphasis.
• Circuit is basically high pass filter. Modulating AF signal is passed through this circuit. As $f_m$ increases, reactance of c decreases and modulating voltage applied to FM modulators goes on increasing.

De-emphasis

Artificial boosting given to higher modulating frequencies in the process of pre-emphasis is nullified or compensated at the receiver by a process called De-emphasis. Artificially boosted high frequency signals are brought back to their original amplitudes using De-emphasis circuit. $75 \mu sec$ De-emphasis is used. It is a low pass filter.

Demodulated FM is applied to De-emphasis circuit in which with increase in $f_m$, the reactance of c goes on decreasing and output of De-emphasis will also reduce. $75 \mu sec$ De-emphasis corresponds to a frequency response curve that is 3 dB down at a frequency whose RC time constant is $75 \mu sec$.

$f = \frac {1}{2 \pi \ RC} = \frac {1}{2 \pi * 75 * 10^{-6}} = 2122Hz$

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Pre-emphasis

• The Pre-emphasis helps amplify high frequency signal components such that they will have magnitude higher than noise components.
• This lead to improvement in the Signal to Noise Ratio i.e. SNR.
• Fig-1 depicts the pre-emphasis circuit. As shown in the figure, it consists of R and C components such that t = R1*C where in t should be about 75 µs . This circuit will have lower frequency cut-off at 2123 Hz. As mentioned in the curve, all the frequencies higher than 2123 is amplified at the rate of 6dB/octave.
• Pre-emphasis circuit also has upper cut-off frequency from where signal enhancement will be flatten. Upper cut-off frequency, Fu = R1+(R2/(2piR1R2C))
• Due to these characteristics, it is simple High Pass Filter with amplification.

De-emphasis

• De-emphasis circuit will have cutoff frequency of about 2123 Hz. To bring the signal back to normal level, all the frequency components above 2123 Hz is attenuated at 6dB/octave.
• As explained, pre-emphasis operation performed at the transmitter is compensated by de-emphasis operation at the receiver.
• Both these modules help increase amplitude of high frequency signal while in transmission. Hence the same cannot be masked or obliterate by the noise.