In this lecture, we are going to learn about the pre emphasis and de emphasis circuits and their behavior in a very detailed manner. So let’s discuss them one by one.
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Pre Emphasis
- It has been proved that in Fm, the noise has a greater effect on the higher modulating frequencies.
- This effect can be reduced by increasing the value of modulating indexm_f for higher modulating frequencies f_m. This can be done by increasing the deviation of \Delta and \Delta can be increased by increasing the amplitude of the modulating signal at higher frequencies.
- Thus, if we boost the amplitude of higher frequency modulating signals artificially then it will be possible to improve the noise immunity at higher modulating frequencies. The artificial boosting of higher modulating frequencies is called Pre Emphasis.
Pre Emphasis Circuit
- Boosting of the higher frequency modulating signal is achieved by using the pre emphasis circuit shown in the figure below.
- The modulating AF signal is passed through a high-pass RC filter, before applying it to the FM modulator.
- As f_m increases, the reactance of C decreases, and modulating voltage applied to the FM modulator goes on increasing. The frequency response characteristics of the RC high pass network are shown in the figure also.
- The boosting is done according to this pre-arranged curve. The amount of pre emphasis in US FM transmission and sound transmission in TV has been standardized at 75usec.
- The pre emphasis circuit is basically a high-pass filter. The pre emphasis is carried out at the transmitter.
- The pre emphasis circuit is used at the transmitter as shown in the figure below.
De Emphasis
- The artificial boosting given to the higher modulating frequencies in the process of pre emphasis is nullified or compensated at the receiver by a process called De Emphasis.
- The artificially boosted high-frequency signal is brought to its original amplitude using the de emphasis circuit.
- The 75usec de emphasis circuit is standard and it is shown in the below figure.
- It shows that it is a low-pass filter.
- 75usec de emphasis corresponds to a frequency response curve that is 3dB down at a frequency whose RC time constant is 75usec.
f=\frac{1}{2\pi RC}= \frac{1}{2\pi \times 75 \times 10^{-6}}=2,122\;Hz
- The demodulated FM is applied to the de emphasis circuit with an increase in f_m the reactance of C goes on decreasing and the output of de emphasis circuit will also be reduced as shown in the below figure.
De Emphasis Circuit
Frequently Asked Questions on Pre Emphasis and De Emphasis
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What is meant by pre-emphasis?
Answer: the intentional alteration of the relative strengths of signals at different frequencies (as in radio and disc recording) to reduce adverse effects (such as noise) in the following system parts.
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Why pre-emphasis is used?
Answer: Pre-emphasis should be used when the signal loss in the transmission channel between Transmitter and Receiver is heavy and the signal observed at the end of the Receiver is less than the receiving sensitivity required for the Receiver.
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What is the difference between pre-emphasis and de-emphasis?
Answer: Pre-emphasis works by boosting the high-frequency portion of the signal. This compensates for the high-frequency loss in the cable. De-emphasis works by cutting the low-frequency portion of the signal. This may be coupled with an increased transmit voltage.
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What is the use of de-emphasis?
Answer: The De-Emphasis filter is pre-configured, with set parameters, and is used to attenuate the high-frequency components boosted during recording with pre-emphasis. When the De-Emphasis block is used on audio recorded with pre-emphasis, its signal-to-noise ratio is greatly improved.
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What are pre-emphasis and de-emphasis why it is required?
Answer: De-emphasis means attenuating those frequencies by the amount they are boosted. However, pre-emphasis is done at the transmitter and de-emphasis is done at the receiver. The purpose is to improve the signal-to-noise ratio for FM reception.
