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Study Guide

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• Clarify problems in tutorials

(A1) Why is interlaced video scanning adopted in Television system?

Hint:Consider bandwidth and human perception

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(A1.1) The resolution of a video signal is 320(H)X240(V). The number of pictures per second is 25. Determine the difference in bandwidth of the video signal with (i) interlaced scanning, and(ii) interlaced scanning.

(A2) What are the purposes of separating RGB video signals to Luminance and Chrominance in TV transmission?

Hint:Consider bandwidth, human perception and compatibility problems

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(A2.1) Consider a line of RGB signal with 256 pixels

R = [0,1,2,3,...........,255]G = [0,1,2,3,...........,255]B = [0,1,2,3,...........,255]

Determine the entropy between of the video signal in the form of(i) RGB , and(ii) YUV representation.

Note: Entropy reflects the amount of redundancy in a signalWhat can you tell about the redundancies of the two representations from their entropies?

(A3) What is meant by vestigial sideband? Why is it better than double sideband modulation in TV system

Hint:Based your discussion on bandwidth, filter response, and importance of low frequency components in video signal

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(A4) Describe how luminance and chrominance signals are interleaved in the frequency domain

Hint:Determine the repetitive frequency of Y, U and V. Applied Fourier Transform to illustrate the interleaving mechanism

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(A4.1) Given the frequency of the color subcarrier is N times the line frequency. Calculate, with respect to the luminance signal, the locations in the frequency spectrum at which the chrominance components exist with N = 4.

(A4.2) Repeat the above calculations with N = 2 and 1.

(A5) How can luminance be extracted in the receiver?

Hint:Remember: human visual response is less sensitive to high frequency video contents

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(A6) Describe how chrominance components are extracted in the receiver?

Hint:How does ‘Comb Filter’ works?

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(A6.1) Sketch the structure of a comb filter and express its transfer function in the ‘z’ domain.

(A6.2) Determine the transfer function of a comb filter in the Fourier domain.

Try this: construct the time expression for the input and output signals of a comb filter. Next, transform the time expression.

(A7) Describe how are the baseband of U and V extracted in the receiver?

Hint:Derive the steps involved in quadrature modulation and demodulation?

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Note: It is the baseband component to be extracted, i.e. free from the color sub-carriers.

(A8) How is phase error in quadrature demodulation be compensated in the PAL system.

Hint:•Sketch the phasor diagram for U and V.•How does ‘U+V’ changes with error?

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(A9) What is the difference between PAL-S and PAL-I systems. Which is better?

Hint:How is the compensation mechanism in PAL implemented in practice?

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(A10) Why is it necessary to convert RF to IF in TV receiver?

Hint:Try to think of it in the frequency domain. Make a few sketches is often helpful.

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(A11) Sketch the IF response of a TV receiver? How can it be implemented in practice?

Hint:The IF response is shaped to adapt to the spectral characteristics of the composite video signal.

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(A12) What are the function of the color burst?

Hint:•What is needed in quadrature demodulation?•How can correct chrominance level be maintained in the receiver?

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(A13) Why is DC restoration necessary?

Hint:• Sketch a few arbitrary video waveforms

with different levels.• Align these waveform with their

estimated DC levels. What do you observed?

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(A14) What is meant by ‘swinging burst’?

Hint:Sketch the phasor diagram for the color burst.

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(A15) Describe the principles of operation of a PAL decoder?

Hint:•How are the 2 LOs generated?•How to demodulate U and V? •How to compensate the phase error

using alternate line information

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(A16) Why is it necessary to include the ‘color killer’ in a TV?

Hint:Study the color video signal spectrum and the PAL decoder, imagine what would happen if the signal is black and white.

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