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The noise power density at the demodulator output is increase with frequency. This means this the effect of noise increases with increase in frequency. This is unfortunately because, noise is strongest in the frequency range in which the signal is weakest. The signal to noise ratio therefore becomes poor at high frequency and the quality of FM reception degrades. The high frequency components of the message are badly affected by the noise. This problem can be solved by using circuits called pre – emphasis and de- emphasis.
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 modulation index \(m_a\) for higher modulating frequencies \(f_m\). This can be done by increasing the deviation ∆f and ∆f can be increased by increasing the amplitude of modulating signal at higher modulating 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 known as pre – emphasis. The pre – emphasis circuit and its output characteristics are given in Figure 28(a) and (b). Where x(t) and S(t) are the modulating signal and modulated signal after pre – emphasis.
Fig. 28(a) pre –emphasis circuit and (b) pre – emphasis characteristics
De – emphasis:
The artificial boosting gives to the higher modulating in the of pre – emphasis is nullified or compensated at the receiver by a process known as De –emphasis. The artificially boosted higher frequency signals are brought to their original amplitude using the de - emphasis circuit. The demodulated FM is applied to the De – emphasis circuit with increase in \(f_m\) the resonance of C goes on decreasing and the output of de – emphasis circuit will also reduce. The de – emphasis circuit and its output characteristics are shown in Fig 29 (a) and (b). Here x(t) and S(t) are the modulating signal and modulated signal before de - emphasis.
Fig. 29(a) De –emphasis circuit and (b) De – emphasis characteristics
FDM (Frequency division Multiplexing)
Frequency Division Multiplexing (FDM) is a technique by which sharing wire or wireless analog communication media.FDM voice channels are transmitted over terrestrial cable systems or radio systems. In long distance telephony employing FDM, a basic group is formed with 12 carriers, 4 KHz apart, each modulated with voice signal using sideband (SSB) an FDM forming a 12 – channel basic group. Each voice channel, which has actual band occupancy of 1 – 5 KHz, is shown in the Fig. 32 with a 4 KHz band. The first channel is modulated with a 64 KHz carrier producing and an upper sideband (64 – 68 KHz).
A band pass filter (BPF) selects the lower sideband occupying 60 – 64 KHz, producing the SSB signal. The second channel is similarly placed in the 64 – 68 KHz band by modulating it with a 68 KHz carrier and selecting the lower sideband from the modulating signal. The 12 voice signals, which are SSB modulated and multiplexed, occupy a 48 kHz band (60 – 108 KHz) as shown in Fig. 33.
FDM is carried out at several levels of which the first level is the basic group describe in Fig.32 five basic groups are multiplexed at the second level, forming a super group of 60 voice channels. A master group of 300 channels is formed at the third level by multiplexing 5 super groups.
Fig. 32 Frequency Division Multiplexing circuit for basic group formation
Fig. 33 Frequency Division Multiplexing circuit for basic super group formation
Advantage of FDM:
Drawbacks of FDM:
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