Television Concepts Viva Questions
Television Concepts Viva Questions, Fundamental Concepts of Television Viva Questions, Short Answer Type Questions on Fundamental Concepts of Television, Engineering Viva Questions, Television Engineering Questions
Short Answer Questions
Q.1. Why flicker is not removed by progressive scanning?
Ans. Flicker can be removed by doubling the speed of progressive scanning, but doubling the speed will double the bandwidth, which is not desirable.
Q.2. How does a picture appear one whole while it is actually presented truncated in two halves alternately in odd and even fields?
Ans. As the scanning lines of the even fields do not appear with the lines of the odd fields, the picture does not glow as one whole in the two fields. Parts that do not glow in odd fold glow in even fields and vice versa. However, the two fields appear so quickly one after the other that the two parts of the picture are integrated by the eyes and hence appear as one whole picture due to the persistence of vision.
Q.3. What would happen if the circuit bandwidth is much higher than required by the sidebands?
Ans. Noise will increase and the number of channels available for transmission will decrease.
Q.4. Why is the number of scanning lines per frame always odd?
Ans. To assist interlacing. Each field should have n + 1/2 lines, where n is a positive integer, and therefore two fields or a frame should have 2n+1 lines, which is always an odd number.
Q.5. Why is vertical scanning very slow in comparison to horizontal scanning?
Ans. To cover a larger number of pixels in scanning.
Q.6. Why are sync pulses needed in TV systems?
Ans. Sync pulses are needed to ensure that scanning in the receiver is identical to the scanning in the transmitter.
Q.7. Why is retrace blanked?
Ans. If the retrace is visible on the screen, it will cause distortion in the picture.
Q.8. What type of polarity of a video signal is needed at the picture tube and how is it achieved?
Ans. The picture tube requires a positive polarity signal so that the stronger signal produces a brighter glow. After demodulation of the negative modulation signal, the final polarity of the video signal may be positive or negative depending on the number of amplification stages. If it is positive, it is applied to the control grid of the picture, keeping the cathode grounded. If it is negative, it is applied to the cathode and the control grid is kept grounded. Thus in both cases grid remains positive with respect to the cathode for the increasing video signal.
Q.9. What is the importance of the front porch?
Ans. The front porch provides a fixed pedestal from which the sync pulse builds up. Thus sync starts correctly in each cycle.
Q.10. What is the importance of a back porch?
Ans. The back porch absorbs ringing oscillations due to fly back. In the absence of the back porch, these oscillations would have distorted the video signal.
Q.11. Why are pre-equalizing pulses used in vertical blanking?
Ans. To assist in correct interlacing of lines in scanning.
Q.12. Why are post-equalizing pulses used in vertical blanking?
Ans. Post equalizing pulses are needed to maintain half-line rhythm in the back portion of the V-blanking pulse.
Q.13. Why is AMVSB preferred to SSB and FM for video signals?
Ans. AMVSB is preferred for video signals because on the one hand, it saves bandwidth, and on the other, its detection is simple, keeping the receiver’s cost low. SSB, although it provides more saving in bandwidth, but requires a costly receiver because of the complex detector circuits. Receivers should be of low cost as they are items of mass consumption. FM is not used because it can detect random changes in phase which will cause noise in the picture. Further, FM requires very high bandwidth for modulated waves.
Q.14. Why is AMSC chosen for color transmission?
Ans. AMSC is chosen for color so that the baseband colors may not be detected in the video detector. They will distort the luminance signal.
Q.15. Mention important factors which are considered for designing the transmission and reception systems to get a good quality picture. Draw relevant current waveforms.
Ans. A good quality picture should be free of geometrical distortions, should he of adequate brightness and comfortable contrast. The following factors contribute to the above qualities of the picture.
The most important factor in the formation of a correct raster. i.e. accurate scanning and perfect synchronization. Sawtooth current signals for horizontal and vertical deflection are shown below.
Another important factor is the gain of RF and IF stages. Their tuned circuits should be correctly tuned and aligned. Gain of all other stages contributes to the quality of the picture.
The trap circuit eliminates cross-interference between sound and picture.
Q.16. Draw CVS for three scanning lines?
Ans. A diagram (not to scale) illustrating the video signal for three scanning lines is given below.