Important Short Questions and Answers: Fundamentals of Television

FUNDAMENTALS OF TELEVISION

Mention some important characteristics of human eye?

Visual acuity,

persistence of vision,

brightness and color sensation.

Why is scanning necessary in television system?

Scanning is the important process carried out in a television system in order to obtain continuous frames and provides motion of picture. The scene is scanned both in the horizontal and vertical directions simultaneously in a rabid rate.

As a result sufficient number of complete picture of frames per second is obtained to give the illusion of continuous motion.

What is the main function of the blanking pulses?

The composite video signal consist of blanking pulses to make the retrace lines invisible by increasing the signal amplitude little above the black level of 75 percent during the time scanning the circuits develop retrace.

How is interlaced scanning is different from sequential scanning?

Sequential scanning is the process in which both horizontal and vertical directions are scanned simultaneously to provide complete pictures. Here horizontal lines are scanned one by one. So, complete picture will be scanned through this type of scanning. But the drawback is that the time taken to complete the cycle is high. Also flicker will be produced, because of the low scanning rate.

In interlaced scanning, all the odd-numbered lines in the entire frame are scanned first, and then the even numbered lines. This process produces two distinct images per frame, representing two distinct samples of the image sequence at different points in time. The set of odd-numbered lines constitute the odd field, and the evennumbered lines make up the even field.

Interlace cleverly preserves the high-detail visual information and, at the same time, avoids visible large-area flicker at the display due to temporal post filtering by the human eye.

What is known as flicker?

The result of 24 pictures per second in motion pictures and that of scanning 25 frames per second in television pictures is enough to make an illusion of continuity.

But, they are not rapid enough to permit the brightness of one picture or frame to blend smoothly in the next through the time when the screen is blanked between successive frames.

This develops in a definite flicker of light that is very irritating to the observer when the screen is made alternately bright and dark.

Why is the number of scanning lines in a frame always odd?

For half line separation between the two fields only the topmost and the extreme bottom lines are then half lines whereas the remaining lines are all full lines.

If there are x number of full lines per field, where x may be even or odd, the total number of full lines per frame is then 2x, an even number.

To this, when the two half lines get added the total number of lines per frame becomes odd. Thus for interlaced scanning the total number of lines in any TV system must be odd.

What is the significance of choosing the number of lines as 625 and not 623 or 627 and the frame reception rate as 25 and not 24 as in motion pictures?

In motion illusion the minimum refresh rate is 24 frames / sec.

To avoid flicker and brightness blends smoothly by showing each frame twice.

For half line separation between the two fields only the topmost and the extreme bottom lines are then half lines whereas the remaining lines are all full lines.

If there are x number of full lines per field, where x may be even or odd, the total number of full lines per frame is then 2x, an even number.

To this, when the two half lines get added the total number of lines per frame becomes odd. Thus for interlaced scanning the total number of lines in any TV system must be odd.

What is photoconductivity?

Since the conductivity of the material varies in accordance with the light falling on it, the magnitude of the current represents the brightness variations of the scene. This varying current completes its path under the influence of an applied dc voltage through a load resistance connected in series with path of the current.

Why back porch is longer than front porch?

It is longer enough to complete the flyback(retrace) period. It also permits the time for reverse the current to start next time scanning.

List out the frequency bands of TV broadcast channels.

•        Lower UHF range I        41-68 MHZ

•        Upper VHF range          III      174-230 MHZ

•        UHF range  IV      470-582 MHZ

•        UHF range  V       606-790 MHZ

Why the neck of the picture tube is made narrow?

For the compactness it is designed with a narrow neck and a large deflection angle of the beam.

Horizontal deflection coil and Vertical deflection coil for deflecting the Electron beam is positioned at the Neck of the picture tube. For this reason the tubes are made with a narrow neck to put the deflection yoke closer to the electron beam.

List out the CCIR-B standards of a V-blanking pulses.

The duration of the vertical blanking period is equal to 20 line periods

VB = 20 H = 1280 µ s

Pre-equalizing(2.5 H) + vertical sync(15 H) + Post equalizing(2.5 H) = 20 H = 1280 µ s

Calculate the width and height of the TV screen for 60 cm size TV when the aspect ratio is 4:3.

Total length in diagonal is 60 cm // hyp of the triangle which has the width to height ratio of 4:3.

Now the actual length of diagonal is calculated by Pythagoras theorem. i.e. 12 cm

And actual width is 4 * 12 = 48 cm / height is 3* 12 = 36 cm

Define picture resolution.

The ability of the system to resolve maximum number of picture elements along the scanning lines over the frame determines picture resolution.

Justify the choice of negative modulation for TV transmission.  What are the advantages of negative modulation?

The negative polarity of modulation permits a large increase in peak power output and for a given setup in the final transmitter stage the output increases by about 40%.

In negative system of modulation, amplitude level is the peak of sync pulses which remains fixed at 100 per cent of signal amplitude and is not affected by picture details. This level may be selected simply by passing the composite video signal through a peak detector. In the positive system of modulation the corresponding stable level is zero amplitude at the carrier and obviously zero is no reference, and it has no relation to the signal strength.

In negative modulation the noise pulses would tend to produce black spots which are less noticeable against a grey background. This merit of lesser noise interference on picture information with negative modulation has led to its use in most TV systems.

Give the list of semiconductor materials coated on the target plates of Image orthicon and vidicon.

Image orthicon – Cesium silver

Vidicon – Antimony, Sb2 S3 / Selenium

What is potential well?

The application of small positive potentials to the gate electrodes results in the development of depletion regions just below them. These are called potential wells.

The depth of each well (depletion region) varies with the magnitude of the applied potential. The gate electrodes operate in groups of three, with every third electrode connected to a common conductor. The spots under them serve as light sensitive elements. When any image is focused onto the silicon chip, electrons are generated within it, but very close to the surface.

The number of electrons depends on the intensity of incident light. Once produced they collect in the nearby potential wells. As a result the pattern of collected charges represents the optical image.

Define aspect ratio.

Aspect ratio can be defined as the ratio of width to height of the picture frame. For television, it is standardized as 4:3.

Why is an aluminized coating provided on the phosphor screen?

In older picture tubes a magnetic beam, bender commonly known as ‘ion-trap’ was employed to deflect the heavy ions away from the screen. In present day picture tubes having a thin metal coating on the screen, it is no longer necessary to provide an ion-trap. This is because the ions on account of their greater mass fail to penetrate the metal backing and do not reach the phosphor screen.

Thus an aluminized coating when provided on the phosphor screen, not only improves screen brightness and contrast but also makes the use of ‘ion-traps’ unnecessary.

Advantages of aluminum layer:

It increases the light output from the phosphor to the viewer.

It protects the phosphor coating from damage due to ions by absorbing their energy.

    Since it is connected to high voltage, it helps in collecting the secondary emission electron at the screen.

Define positive and negative modulation.

When the intensity of picture brightness causes increase in amplitude of the modulated envelope, it is called ‘positive’ modulation.

When the polarity of modulating video signal is so chosen that sync tips lie at the 100 per cent level of carrier amplitude and increasing brightness produces decrease in the modulation envelope, it is called ‘negative modulation’.

What is composite video signal?

A signal that contains all three of these components intensity information, horizontal-retrace signals, and vertical-retrace signals is called a composite video signal.

What are the IF of video and sound of TV?

•        Video IF     ----     38.9 MHz

•        Sound IF     ----     33.4 MHz

Compare between the number of scanning lines of PAL and NTSC systems. Compare between the number of scanning lines and frames of Indian and American TV systems.

PAL --       British (Indian) 625 lines system / 50 frames per sec.

NTSC --American 525 lines system / 60 frames per sec.

Distinguish between hue and saturation.

Hue or tint can be defined as the predominant spectral color of the received light. The color of any object is distinguished by its hue or tint.

Saturation refers to the spectral purity of the color light. It indicates the degree by which the color is diluted by white.