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19 Pre-emphasis_and_De-emphasis.pdf

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Mohamedshabana38

1. FM signals use pre-emphasis and de-emphasis circuits to suppress noise interference. At the transmitter, a pre-emphasis circuit amplifies high frequencies of the modulating signal. This makes the high frequency content stronger relative to high frequency noise. At the receiver, a de-emphasis circuit returns the frequency response to normal using a low-pass filter. 2. Pre-emphasis increases the signal-to-noise ratio by amplifying the high frequencies of the modulating signal that could otherwise be obscured by high frequency noise spikes. De-emphasis at the receiver then restores the original frequency response. This technique helps overcome frequency distortion caused by noise.

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1 1 PRE-EMPHASIS & DE-EMPHASIS Prof. Dr. G.Aarthi, Associate Professor, SENSE
2 Noise-Suppression Effects of FM Noise is interference generated by lightning, motors, automotive ignition systems, and power line switching that produces transient signals. Noise is typically narrow spikes of voltage with high frequencies. Noise (voltage spikes) add to a signal and interfere with it. Some noise completely obliterates signal information.
3 Noise-Suppression Effects of FM FM signals have a constant modulated carrier amplitude. FM receivers contain limiter circuits that deliberately restrict the amplitude of the received signal. Any amplitude variations occurring on the FM signal are effectively clipped by limiter circuits. This amplitude clipping does not affect the information content of the FM signal, since it is contained solely within the frequency variations of the carrier.
4 Noise-Suppression Effects of FM An FM signal with noise.
5 INTERFERENCE (capture effect) A major benefit of FM is that interfering signals on the same frequency will be effectively rejected. If the signal of one is more than twice the amplitude of the other, the stronger signal will "capture" the channel and will totally eliminate the weaker, interfering signal. This is known as the capture effect in FM. In FM, the capture effect allows the stronger signal to dominate while the weaker signal is eliminated. However, when the strengths of the two FM signals begin to be nearly the same, the capture effect may cause the signals to alternate in their domination of the frequency.
6 Despite the fact that FM has superior noise rejection qualities, noise still interferes with an FM signal. This is particularly true for the high-frequency components in the modulating signal. Since noise is primarily sharp spikes of energy, it contains a considerable number of harmonics and other high-frequency components. These high frequencies can at times be larger in amplitude than the high- frequency content of the modulating signal. This causes a form of frequency distortion that can make the signal unintelligible. To overcome this problem a technique known as Pre-emphasis and De- emphasis is used.
7 Pre-emphasis and De-emphasis. At the transmitter the modulating signal is passing through a simple network which amplifies the high frequency component more the low-frequency component. The simplest form of such circuit is a simple high pass filter. The pre-emphasis circuit increases the energy of the higher content of the higher-frequency signals so that will tend to become stronger than the high- frequency noise component. This improves the signal-to-noise ratio. To return the frequency response to its normal level, a de-emphasis circuit is used at the receiver. This is a simple low-pass filter The de-emphasis circuit provides a normal frequency response.
8 The combined effect of pre-emphasis and de-emphasis is to increase the high- frequency components during the transmission so that they will be stronger and not masked by noise Pre-emphasis and De-emphasis.

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19 Pre-emphasis_and_De-emphasis.pdf

  • 1. 1 1 PRE-EMPHASIS & DE-EMPHASIS Prof. Dr. G.Aarthi, Associate Professor, SENSE
  • 2. 2 Noise-Suppression Effects of FM Noise is interference generated by lightning, motors, automotive ignition systems, and power line switching that produces transient signals. Noise is typically narrow spikes of voltage with high frequencies. Noise (voltage spikes) add to a signal and interfere with it. Some noise completely obliterates signal information.
  • 3. 3 Noise-Suppression Effects of FM FM signals have a constant modulated carrier amplitude. FM receivers contain limiter circuits that deliberately restrict the amplitude of the received signal. Any amplitude variations occurring on the FM signal are effectively clipped by limiter circuits. This amplitude clipping does not affect the information content of the FM signal, since it is contained solely within the frequency variations of the carrier.
  • 4. 4 Noise-Suppression Effects of FM An FM signal with noise.
  • 5. 5 INTERFERENCE (capture effect) A major benefit of FM is that interfering signals on the same frequency will be effectively rejected. If the signal of one is more than twice the amplitude of the other, the stronger signal will "capture" the channel and will totally eliminate the weaker, interfering signal. This is known as the capture effect in FM. In FM, the capture effect allows the stronger signal to dominate while the weaker signal is eliminated. However, when the strengths of the two FM signals begin to be nearly the same, the capture effect may cause the signals to alternate in their domination of the frequency.
  • 6. 6 Despite the fact that FM has superior noise rejection qualities, noise still interferes with an FM signal. This is particularly true for the high-frequency components in the modulating signal. Since noise is primarily sharp spikes of energy, it contains a considerable number of harmonics and other high-frequency components. These high frequencies can at times be larger in amplitude than the high- frequency content of the modulating signal. This causes a form of frequency distortion that can make the signal unintelligible. To overcome this problem a technique known as Pre-emphasis and De- emphasis is used.
  • 7. 7 Pre-emphasis and De-emphasis. At the transmitter the modulating signal is passing through a simple network which amplifies the high frequency component more the low-frequency component. The simplest form of such circuit is a simple high pass filter. The pre-emphasis circuit increases the energy of the higher content of the higher-frequency signals so that will tend to become stronger than the high- frequency noise component. This improves the signal-to-noise ratio. To return the frequency response to its normal level, a de-emphasis circuit is used at the receiver. This is a simple low-pass filter The de-emphasis circuit provides a normal frequency response.
  • 8. 8 The combined effect of pre-emphasis and de-emphasis is to increase the high- frequency components during the transmission so that they will be stronger and not masked by noise Pre-emphasis and De-emphasis.