Q&A 1 UNIT 2 - Prof. Prajeesha PDF

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Prof. Prajeesha...

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1. What is VSB? Vestigial Sideband Modulation or VSB Modulation is the process where a part of the signal called as vestige is modulated, along with one sideband. Along with the upper sideband, a part of the lower sideband is also being transmitted in this technique. 2. Why is VSB used in television? Back in the analog TV days, VSB (vestigial sideband) transmission was used as a way to reduce the already-high bandwidth required for video. ... (Early TV sets had problems with sync on weak signals, so the extra power helped — better to get a snow-filled image that still had sync). 3. What are the advantages of VSB am? The main advantage of VSB modulation is the reduction in bandwidth. It is almost as efficient as the SSB . Due to allowance of transmitting a part of lower sideband, the constraint on the filter have been relaxed. 4. What is VSB used for? VSB modulation is standard for the transmission of TV signals. Because the video signals require a large transmission BW using the techniques like DSB-FC otherwise DSF-SC. This is a type amplitude modulation that is mainly used for the TV broadcast worldwide. 5. How do you calculate transmission efficiency? Efficiency = Pout/Pin*100% = IVout/IVin*100% or simply Vout/Vin*100% since the current is the same everywhere in the transmission line. 6. What is the maximum transmission efficiency? The transmission efficiency (η) of AM wave is defined as the percentage of total power contributed by side bands of the AM signal. The maximum transmission efficiency of an AM signal is 33.33%, i.e., only one third of the total transmitted power is carried by the side bands in an AM wave. 7. What is TV bandwidth? By the time you add in sound, something called a vestigial sideband and a little buffer space, a TV signal requires 6 MHz of bandwidth. Therefore, the FCC allocated three bands of frequencies in the radio spectrum, chopped into 6-MHz slices, to accommodate TV channels: 54 to 88 MHz for channels 2 to 6.

8. Mention few demerits of VSB transmission. (a) A small portion of the transmitter power is wasted in the vestigial sideband filters which remove the remaining lower sideband. (b) The attenuation slope of the receiver to correct the boost at lower video frequencies places the carrier at 50 per cent output voltage which amounts to introducing a loss of about 6 db in the signal to noise voltage ratio relative to what be available if double sideband transmission is used. (c) Some phase and amplitude distortion of the picture signal occurs despite careful filter design at the transmitter. Also, it is very difficult to tune IF stages of the receiver to correspond exactly with the ideal desired response and this toointroduces some phase and amplitude distortion. (d) More critical tuning at the receiver becomes necessary because for a given amount of local oscillator mismatch or drift after initial tuning, the degeneration of picture quality is less with wider lower sideband than with narrow lower sideband. In this respect the British 625 line system is superior because it allows 1.25 MHz unattenuated lower sideband transmission as compared to 0.75 MHz in most other systems. 9. Explain the allocation of frequency bands for Television Signal Transmission For effective amplitude modulation and better selectivity at the RF and IF tuned amplifiers in the receiver, it is essential that the carrier frequency be chosen about ten times that of the highest modulating frequency. Since the highest modulating frequency for picture signal transmission is 5 MHz, the minimum carrier frequency that can be employed, cannot be much less than 40 MHz. As an illustration consider a carrier frequency fc = 10 MHz. With the highest video modulating frequency = 5 MHz, a deviation of 50 per cent from the centre frequency would be necessary in any tuned circuit to accommodate the lower and upper sideband frequencies. However, if the carrier frequency is fixed at, say 50 MHz, the percentage deviation required to pass the upper and lower sideband frequencies for the same modulating frequency would be only 10 per cent. It is obvious from these observations that selectivity is bound to be poor at the receiver tuned amplifiers with a carrier frequency of 10 MHz. The 3 db down points with a carrier frequency of 50 MHz are within 5 per cent deviation from the carrier frequency and

thus the selectivity is bound to be much better. Further, each television channel occupies about 7 MHz. In order to accommodate several TV channels, the carrier frequencies have to be in the region of the spectrum above about 40 MHz. This explains why television transmission has to be carried out at very high frequencies in the VHF and UHF bands. In radio broadcastwhere the highest modulating frequency is only 5 kHz, lower carrier frequencies can be used, and accordingly transmission is carried out in the medium wave band (550 kHz to 1600 kHz)and short wave bands extending up to about 30 MHz. Transmission at very high frequencies has its own problems and limitations for long distance transmission. 10. Explain the different types of wave propagation. (a) Ground Wave Propagation Vertically polarized electromagnetic waves radiated at zero or small angles with ground, are guided by the conducting surface of the ground, along which they are propagated. Such waves are called ground or surface waves. The attenuation of ground waves, as they travel along the surface of the earth is proportional to frequency, and is reasonably low below 1500 kHz. Therefore, all medium wave broadcasts and longwave telegraph and telephone communication is carried out by ground wave propagation. (b) Sky Wave Propagation Ground wave propagation, above about 1600 kHz does not serve any useful purpose as the signal gets very much attenuated within a short distance of its transmission. Therefore, most radio communication in short wave bands up to 30 MHz (11 metres) is carried out by sky waves. When such waves are transmitted high up in the sky, they travel in a straight line until the ionosphere is reached. This region which begins about ‘120 km above the surface of the earth, contains large concentrations of charged gaseous ions, free electrons and neutral molecules. The ions and free electrons tend to bend all passing electromagnetic waves. The angle by which the wave deviates from its straight path depends on (i) frequency of the radio wave ( ii) angle of incidence at which the wave enters the ionosphere (iii) density of the charged particles in the ionosphere at the particular moment and (iv) thickness of the ionosphere at the point....