unit -1

synchronization need?

It is essential that the same coordinates be scanned at any instant both at 'the camera tube target plate and at the raster of picture tube, otherwise, the picture details would split and get distorted. To ensure perfect synchronization between the scene being televised and the picture produced

on the raster, synchronizing pulses are transmitted during the retrace, i.e., flyback intervals of horizontal and vertical motions of the camera scanning beam. Thus; in addition to carrying

picture details, the radiated signal at the transmitter also contains synchronizing pulses. These pulses which are distinct for horizontal and vertical motion control, are processed at the receiver and fed to the picture tube sweep circuitry thus ensuring that the receiver picture tube beam is

in step with the transmitter camera tube beam.

The Frequency synchronization system is designed to eliminate the interference 'for two or more TV transmitters operating on same frequency, bysynchronizin g the equipment through a precision source. Systems and methods are described for frequency synchronization for improved AM and TV broadcast reception. A method includes synchronizing a carrier

frequency of a broadcast signal with a remote reference frequency. An apparatus includes a reference signal receiver,a phase comparator coupled to the reference signal receiver, a

voltage controlled oscillator coupled to the phase comparator and a radio frequency outputcoupled to the voltage controlled oscillator.

4:3 aspect ratio?

For obtaining best viewing comfoit, panoramic effect and artistic appreciation, the picture raster should be in a rectangular format with an aspect ratio i.e., Width-to-height ratio of 4:3. This is because the pair of eyes have a binocular vision in the horizontal plane when compared to that in the vertical plane. One more reason is that the region of maxirnurn resolution at that centre of the retina i.e., fovea has greater area along the width than along the height. More efficient use of the area of fovea can be obtained when the Width of the raster is made large. Therefore an aspect ratio of 4:3 is commonly used for the television transmission and reception as it is more pleasing and less fatiguing to the eye.

kell factor?

Consider a beatn. spot scanning closely spaced horizontal black and white lines whose thickness is nearly equal to the thickness of the line.

The output will exactly follo W the black or white levels when the beam is perfectly aligned. But, the output will follow a gray level between the black and white levels if the beam is shifted slightly and is positioned on the junction of black and white lines. The output would be a continuous gray with no vertical resolution at all. The line in which the alignment of the beam is more when compared to the adjacent line will experience a more gray level than- the other line. Thus the finite beam size results in the degradation of vertical resolution. Various tests have indicated that the average number of effective lines is 0.7 times the total active scan lines present. This factor indicating the-reduction in effective number of lines is known as kell factor, which lies between 0.64 and 0.85. Thus, this kell factor indicates that not all lines or parts of lines are completely effective at all times and that vertical resolution cannot be equal to the number of active lives. Vertical resolution which is the limitup to which the scanning system can resolve picture details along the vertical direction can be given as,

Vr = Vertical resolution

Na = Number of active lines

m = Resolution or kell factor.

For a 625 line system, 40 lines are lost per frame as blanked lines during the retrace interval. Therefore, the number of active lines Na is,

Na = 625 -40 = 585

Assuming m = 0.69,

We have,

Therefore, vertical resolution increases with the increase in number of scanning lines as the factor k remains the same.

Illusion of continuity?

Image Continuity

In order to achieve illusion of continuity and any motion in the scene appear on the picture tube screen as a smooth and continuous change, the advantage of persistence of vision or storage characteristics of human eye is taken. The sensation produced by the nerves of retina by incident light, does not cease immediately after light is removed but persists for about l/16th of a second. Thus if the scanning rate per second is made greater than sixteen or the number of pictures

shown per second is more than sixteen the eye is able to integrate the changing levels of brightness in the scene, so when picture elements are scanned rapidly enough, they

appear to the eye as a complete picture unit, with none of the individual elements visible separately.

No. of scanning lines?

Scanning lines are defined as the sequence of lines strips of an image formed in a raster scan of a picture tube. Scanning lines play a very significant role in an image where resolution is a matter of concern. The more the number of scanning lines and pixels present in a system, the higher is

the resolution. In other words. inorder to increase the quality of the picture, the number of scanning lines should be large enough so that they can accomodate more number of picture

details. However there are some factors which tend to reduce the number of scanning lines. They are,

(i) Need for large channel bandwidth.

(ii) The minimum possible thickness of scanning beam.

(iii) Limited resolving capability ofhuman eye.

In order to avoid these factors, the total number of scanning lines have been selected as 625 in the CCIR television system and 525 in 525 line American system, since these involve higher order scanning rates for one complete picture.

why 25 frames required not 24?

Image Continuity

In order to achieve illusion of continuity and any motion in the scene appears on the picture tube screen as a smooth and continuous change, the advantage of persistenceeof vision or storage characteristics of human eye is taken. The sensation produced by thenerves of retina by incident light, does not cease immediately after light is removed but persists for about 1/ l6th of a second. Thus, if the scanning rate per second is made greater than sixteen or the number of pictures

shown per second is more than sixteen the eye is able to integrate the changing levels of brightness in the scene, so when picture elements are scanned rapidly enough, they appear to the eye as a complete picture unit,with none of the individual elements visible separately.

Choice of Frame Repetition Rate of 25 :

The frame repetition rate of 25 is chosen rather than 24 as in motion pictures to make the field frequency equal to the power line frequency of 50 Hz. This helps in reducing undesired effect of hum due to any pickup from the mains, because suck effects in the picture stay still, instead of drifting up or down on the screen.

horizontal resolution?vertical resolution?

Picture Resolution:

The ability of image reproducing system to represent fine structure of `an object is known as its resolving power or resolution. It is necessary to consider this aspect separately in the vertical and horizontal planes of the picture.

Vertical Resolution:

The extent to which scanning system is capable of resolving picture details in the vertical direction is referred as its vertical resolution. As already explained, vertical resolution is a function of scanning lines into which the picture is divided in the vertical plane. Thus, vertical resolution can be expressed as, Vr = Na x k where Vr is the vertical resolution expressed as

number of lines, Na is the active number of lines in the system and k the resolution factor also known as KELL factor. The use of resolution factor arises from the fact that in practice, picture elements are not arranged as equally spaced segments but have random distribution of black, grey and white pixels depending on the nature of picture details - in the scene under

consideration. Statistical analysis and subjective tests carried out to determine average number of effective lines suggest that about 70 percent of the total lines for segments get separately scanned in the vertical direction and the remaining 30 percent get merged with other elements due to the beam falling equally on two consecutive (one after the other) lines. Thus the effective

number of lines distinctly resolved gets reduced by the resolution (k) factor. Assuming k = 0.69 we get vertical resolution for the 625 line system as, Vr = 585 X 0.69 =400 lines

Horizontal Resolution:

The capability of the system to resolve maximum number of picture elements along the scanning line determines horizontal resolution. While aiming at equal vertical and horizontal resolutions and assuming the same KELL factors (k) the effective number of alternate black and white segments (N) that gets scanned in one horizontal line are,

N = Na x aspect ratio x k= 585 x4/3 x 0.69 = 533

For the extreme situation where alternate picture segments are black and white, like that in a chess board, the scanning spot must develop 533/2 = 266 complete cyclic changes while switching alternately to voltage levels corresponding to black and peak white light values. Since 26 cyclic voltage changes get generated in a time period of 52 us that the beam takes to scan

one line in the horizontal plane the time period (th) of each cycle comes to, t suffix h= Active period of each horizontal line/ Number of cycles

Assuming the cyclic changes to be nearly sinusoidal thefrequency of this periodic wave is equal to

The significance of determining this frequency is to show the maximum video frequency than can get generated with the assumptions made is 5 MHz and the system must be designed to handle a bandwidth that extends up to this frequency. In addition, the TV channel bandwidth must be large enough to transmit a modulating frequency up to 5 MHZ. This in turn

explains the need of a 7 MHz channel bandwidth for the 625 line system when sound signal transmission and other related factors are taken into account.

prove f=5mhz using vertical resolution equals horizontal resolution?

The horizontal and vertical resolutions are said to be equal if the resolution factor, which is used to find the effective number of distinct picture elements in horizontal line is same. The resolution factor is denoted by K. Therefore, for equal horizontal and vertical resolutions the effective number of black and White lines in a horizontal picture is,

The value of K is 0.69 and NM = 585, aspect ratio is 4 : 3.

The vertical and horizontal resolutions are according to similar blackaand white bars in a horizontal and vertical plane of the picture frame. So, We can consider a chessboard

pattern to explain the process. This is shown in following figure.

In the above figure, the bars are replaced by black and white square elements in order to find the capability of scanning system to regenerate' the object that is televised.

The size of the square element is similar to the thickness of scarming beam. Over the screen of 5l cm the square element has a size of 0.5 mm2. As the elements are very small, the shape of variation over the scanning line is not disinguished by them. Only alternate black and white lines are properly distinguished. Due to this, the eye will not be able to differentiate the square wave of brightness variation and sine wave of brightness variation in the reproduced picture. Thus, to obtain clarity in TV signals, the amplifier must be able to generate a sine wave

frequency equal to the repetition frequency of rectangular wave. For example, to have a square wave of 5 MHz, it is essential to reproduce a periodic sine wave of 5 MHz up to its

llth harmonic with the help of related electronic circuitry.Therefore, the overall frequency will be 5x 1 l = 55 MHZ, this is highly impossible in practical terms. Hence, the bandwidth must be limited up to 5 MHz. This bandwidth has proved to be 'adequate to generate sufficient details of scene that is televised.

From above details, the highest modulation frequency fn of a 625-B monochrome television which should be able to handle transmitting and receiving picture details is,

Therefore, the highest modulating frequency fi for a 625-B monochrome television is approximately 5 MHZ

4)Interlaced scanning?how it eliminates flicker and reduce band width?

5)Additive color mix?

subtractive color mix?

hue?saturation?

luminance?chrominance?

color bar pattern?chromacity diagram?

gamma correction?

frequency interleaving technique?

compatibility?

6)PAL encoder

7)color difference signals?y signal? g-y interms of b-y and r-y ?formation of chrominance signal?

It is essential that the same coordinates be scanned at

any instant both at 'the camera tube target plate and at the

raster of picture tube, otherwise, the picture details would

split and get distorted. To ensure perfect synchronization

between the scene being televised and the picture produced

on the raster, synchronizing pulses are transmitted during the

retrace, i.e., flyback intervals of horizontal and vertical motions

of the camera scanning beam. Thus; in addition to carrying

picture details, the radiated signal at the transmitter also

contains synchronizing pulses. These pulses which are

distinct for horizontal and vertical motion control, are

processed at the receiver and fed to the picture tube sweep

circuitry thus ensuring that the receiver picture tube beam is

in step with the transmitter camera tube beam.

The Frequency synchronization system is designed

to eliminate the interference 'for two or more TV transmitters

operating on same frequency, bysynchronizin g the equipment

through a precision source.

Systems and methods are described for frequency

synchronization for improved AM and TV broadcast

reception. A method includes synchronizing a carrier

frequency of a broadcast signal with a remote reference

frequency. An apparatus includes a reference signal receiver,

a phase comparator coupled to the reference signal receiver, a

voltage controlled oscillator coupled to the phase comparator

and a radio frequency outputcoupled to the voltage

controlled oscillator.

Interlaced Scanning T

In television pictures an effective rate of 50 vertical

scans per second is utilized to reduce flicker. This is

accomplished by increasing the downward rate of travel of

the scanning electron beam, so that every alternate line gets

scanned instead of every successive line. Then, when the

beam reaches the bottom of picture frame, it quickly retums to

the top to scan those lines that were missed in the previous

scanning. Thus the total number of lines are divided into two

groups called tields. Each field is scanned alternately. This

method of scanning is known as interlaced scanning and is

illustrated in figure. It reduces flicker to an acceptable level

since the area of screen is covered at twice the rate. This is

like reading altemate lines of a page from top to bottom once

and then going back to read the remaining lines down to the

bottom..

Note that the vertical retrace time has been assumed

r- zero. The retrace lines are also omitted for clarity. 5

In the 625 line TV,_system,' for successful interlaced

| u ng, the 625 lines of each frame or picture are divided

Q sets of 312.5 lines and each set is scanned altemately to

er the entire picture area. To achieve this, the horizontal

p oscillator is madegto work at a frequency of 15625 Hz

5 >< 50 = 15625) to scan the same number of lines per frame

a 25/25 == 625 lines), but the vertical sweep circuit is run ata

ency of 50 instead of 25 Hz. Note that since the beam is

deflected fromlto;p, to bottom in half the time and the

f ontal oscillator is still operating at 15625 Hz, only half the

l1nes,i.e., 312.5 (625/2 = 3 l2.5) get scanned during each

cal sweep. Since the first field ends in a half line and the

nd fieldvcommences at middle of the line on top of the

plate or screen (see figure (l)), the beam is able to scan

iemaining 312.5 alternate lines during its downward

if

I;

.A

_i

In all then, the beam scans 625 lines (312.5 >< 2 = 625)

at the same rate of 15625 lines (312.5 >< 50 = 15625)

Therefore, with interlaced scanning the flicker

is eliminated without increasing the speed of scanning,

in turn does not need any increase in the channel

Flicker is the effect of discontinuity between two picture

In motion pictures, frzunes are moved at the rate of 24

per second and in T.V 25 picture frames ure scanned per

This rate is sufficient to create an illusion or continuity

eye. But, these rate are not rapid enough to blend the

S.

of one picture frame smoothly into next picture

causes flickering of iight on the screen. This effect

flicker. Flicker is avoidedjin motion pictures by

each frame twice. ' _

In the 625 lines monochrome T.V system, for successful

interlaced scanning, the 625 lines of each frame or picture are

divided into sets of 312.5 lines and each set is scanned

alternately to` cover the entire picture area. To accomplish

this, the horizontal sweep oscillator is made to function at a

frequency of 15625 Hz (312.5 x 50 = 15625 Hz) to scan the

same number of lines per frame_(l 5625/25 = 625 lines), but the

vertical sweep oscillator is made to work at a frequency of 50

Hzinstead of 25 Hz. Since, the electron beam is now deflected

from top to bottom iii half the time and the horizontal oscillator

is still operating at 15625 Hz, only half the total lines i.e., 312.5

(625/2 = 3125) get scanned dunng each vertical sweep. Since,

the first field -ends in a half line and the second field

commences at middle of the line on the top of target plate or

screen, the beam is able to scan the remaining 312.5 alternate

lines during its downward journey. In all the beam scans 625

lines per frame at the same rate of 15625 lines per second.

Therefore. with interlaced scanning. the flicker effect is

eliminated without increasing the speed of scanning, which

in turn does not need any increase in the channel bandw

Perception of Brightness and Colors ,

The lens system ofthe eye is used for sharply focusing

the object viewed by us onto the retina. The retina is located

at the back of the eye and the light-sensitive organs of the

retina are connected to the optical centre of brainthrough the

optic newe. The two typesof light sensitive organs, which

measure visual sensation are,

1. Rods

2. Cones.

Rods, perceive the objects in black and white as th

are sensitive to brightness. By the additive mix of red, gr

and blue in suitable proportion remaining colours

produced. Each of the three sets of cones at the retina

,sensitive to one of these colours. The actual colour of

object is perceived by enabling the brain with the integratio

of three colour impressions. The basis for ,colour television `

formed by this additive mixing of colours when the red an

green groups of cones are excited at the same time in a suitabl

intensity ratio by the falling colour light, an yellow colour is

seen by the eye. Similarly, two or all the three cones are excited

by the colours other than primary colours to generate

cumulative sensation of that colour.

The set of cones are excited by the respective primary

colour and the remaining sets will not contribute any sensation

while exciting all the three sets of cones, white colour is

produced, which is a mix of red, green and blue. Thus, rods

and cones perceive the brightness and colour of any object

seen by the eye by enabling the optic centre of brain. The

approximate relative response of the eye to different colours

is shown in the figure.

(i) Complementary Colors

The colors which give white light when added together

are called complementary colors.

(ii) Primary Colors

Red, Green and Blue are the primary colors, it is not

possible to produce the primary colors by mixing them.

(iii) Color Circle

The triangular chrfomaticity figure in color televisions is changed into a color circle. The primary colors R, G and B are

radial vectors phased about 120 apart measurement of the degree of saturation is done along the vector from the centre to the

circumference.

(iv) Color BurstIt is a /signal used to keep the chrominance subcarrier synchronized in a color television signal. By synchronizing an

oscillator with the color burst at the beginning of each scan line, a television receiver is able to restore the suppressed carrier of

the chrominance signals, and in turn decode the color information.

-_u v-

(b) Color Bar Pattern - _ V ` e T l

Color bar pattem is the pattern consisting of different colors in vertical bars. The pattern is generally used to check the

performance of a color television receiver. It gives a close approximation to the NTSC standardcolor bar. The color bars in a

color bar pattern are referred as color control bars, Color control strips or proofing bars. These are generally rows of different

colored patches printed inthe small area of the press sheet. They are generally used by proof readers and press operators to

control the trapping, ink density, dot gain and print contrast of the printed sheet. . . s

The different colors present in a color bar are, top colors namely gray, yellow, cyan, green, magenta, red, blue and black.

The bottom colors are very dark such as green, white, blue and black.

Color bars are available in different types and are selected depending upon the type of printing being used. Print

suppliers purchase original film or rights to digital files of color bars andother test images. The film and file can then serve as

constant reference points throughout the proofing, platemaking and print process.

Color bars are used as an essential trouble shooting tool. They are easy to analyze visually with instruments used for

measuring the relative density of any part of image.

A two-dimensional co-ordinate systems representing all the spectral colours and their mixtures is reffered as chromaticity

diagram. These mixtures of spectral colours are based on the tristimulus values of primary colours. Figure (1) shows the

chromaticity diagram with hue and saturation as two coordinates (X and Yrespectively). `

Y

A .

520 ' A. g

-9- (nm) 530

8_ Yellowish green `

.71 A