Pre-emphasis vs De-emphasis
As we know noise usually will have higher amplitude and higher frequency components. This high frequency noise will lead to frequency distortion when its amplitude is higher than the components present in the modulating signal. To overcome this situation most of the FM circuits use techniques known as Pre-emphasis at the transmitter and De-emphasis at the receiver.
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 cutoff 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 cutoff frequency from where signal enhancement will be flatten.
Upper cutoff frequency, Fu = R1+(R2/(2*pi*R1*R2*C))
Due to this characteristics, it is simple High Pass Filter with amplification.
The De-emphasis does exactly reverse of the Pre-emphasis counterpart. It is used at the receiver part. It helps bring pre-amplified signal back to the normal amplitude level. It is a simple Low Pass Filter with time constant of about 75 µs .
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 can not be masked or obliterate by the noise.
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