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Of course, provision for live broadcast also has to be there for VIP interviews, sports events and the like. For remote pick-ups the signal is relayed by cable or RF link to the studio for broadcasting in the assigned channel. This usually varies between 75 and km depending on the topography and radiated power. Area of TV broadcast coverage can be extended by means of relay stations that rebroadcast signals received via microwave links or coaxial cables. A matrix of such relay stations can be used to provide complete national coverage.
With the rapid strides made in the technology of space and satellite communication it has now become possible to have global coverage by linking national TV systems through satellites.
Besides their use for international TV networks, large countries can use satellites for distributing national programmes over the whole area. One method for such national coverage is to set up a network of sensitive ground stations for receiving signals relayed by a satellite and retelecasting them to the surrounding area.
Another method is to employ somewhat higher transmitter power on the satellite and receive the down transmissions directly through larger dish antenna on conventional television receivers fitted with an extra front-end converter. Recent Trends In the last decade, transistors and integrated circuits have greatly improved the quality of performance of TV broadcasting and reception.
Modern camera tubes like vidicon and plumbicon have made TV broadcast of even dimly lit scenes possible. Special camera tubes are now used for different specific applications. The most sensitive camera tubes available today can produce usable signals even from the scenes where the human eye sees total darkness. With rapid advances in solid state technology, rugged solid state image scanners may conceivably replace the fragile camera tubes in the not-too-distant future. Experimental solid state cameras are already in use for some special applications.
Solid state picture-plates for use in receivers are under active development. Before long the highly vulnerable high vacuum glass envelope of the picture tube may be a thing of the past. Since solid state charge coupled devices are scanned by digital addressing, the camera scanner and picture plate can work in exact synchronism with no non-linear distortions of the reconstructed picture.
An important recent technological advance is the use of pseudo-random scan. The signal so generated requires much less bandwidth than the one for conventional method of scanning.
Wiley Eastern Ltd. Television and Video Engineering R. R, Wiley Eastern Limited publication. Television and video engineering- Dhake A. M, Tata McGraw Hill publication. Shilpa Gulati. Gulati, Modern Television. Gupta, Audio Video Engineering. Dhake, Television and video Engin. Bernard Grob, Charles Herdone,. Monochrome and Colour TV — R. Gulati, New Age International Publication, Television and Video Engineering - A.
Dhake, 2nd Edition. PDF K. Monochrome and Colour Television - R. Amplitude Modulation The Picture Tube Basic Principle Basic Television Broadcasting Monochrome Picture Tube Television Camera Tubes Reception of Vestigial Sideband Signals Television Studio Television Receiver Types of Television Receivers Choice of Intermediate Frequencies Television Signal Propagation and Antennas Radio Wave Propagation Television Applications Television Broadcasting Video Detector Video Picture Signal Detection Video Section Fundamentals Picture Reproduction Video AmplifiersDesign Principles Vacuum Tube Amplifier Video Amplifier Circuits Direct Coupled Video Amplifier Advantages of AGC Sync Separation Circuits Sync SeparatorBasic Principle Improved Noise Gate Sync Separator Sync Waveform Separation Deflection Oscillators Deflection Current Waveforms Vertical Deflection Circuits Requirements of the Vertical Deflection Stage Horizontal Deflection Circuits Horizontal Output Stage Sound System Sound Signal Separation RF Tuner Tuner Operation Video IF Amplifiers Video IF Section Receiver Power Supplies Low Voltage Power Supplies Essentials of Colour Television Colour Picture Tube Colour Signal Transmission and Reception Colour Signal Transmission Remote Control and Special Circuits Remote Control Electronic Control Systems Alignment and Servicing Equipment Receiver Circuits and Alignment Monochrome TV Receiver Circuit Receiver Servicing Conversion Factors and Prefixes Transient Response and Wave Shaping Television Broadcast Channels Satellite Television The desire in man to do so has been there for ages.
In the early years of the twentieth century many scientists experimented with the idea of using selenium photosensitive cells for converting light from pictures into electrical signals and transmitting them through wires.
The first demonstration of actual television was given by J. Baird in UK and C. Jenkins in USA around by using the technique of mechanical scanning employing rotating discs. However, the real breakthrough occurred with the invention of the cathode ray tube and the success of V.
Zworykin of the USA in perfecting the first camera tube the iconoscope based on the storage principle. By electromagnetic scanning of both camera and picture tubes and other ancillary circuits such as for beam deflection, video amplification, etc.
Though television broadcast started in , world political developments and the second world war slowed down the progress of television. With the end of the war, television rapidly grew into a popular medium for dispersion of news and mass entertainment. Television Systems At the outset, in the absence of any international standards, three monochrome i. These are the line American, the line European and the line French systems.
This naturally prevents direct exchange of programme between countries using different television standards. Later, efforts by the all world committee on radio and television CCIR for changing to a common line system by all concerned proved ineffective and thus all the three systems have apparently come to stay.
The inability to change over to a common system is mainly due to the high cost of replacing both the transmitting equipment and the millions of receivers already in use. However the UK, where initially a line monochrome system was in use, has changed to the line system with some modification in the channel bandwidth.
In India, where television transmission started in , the B monochrome system has been adopted. The three different standards of black and white television have resulted in the development of three different systems of colour television, respectively compatible with the three monochrome systems. Regular colour transmission started in the USA in This system incorporates certain features that tend to reduce colour display errors that occur in the NTSC system during transmission.
Since this system is compatible with the B monochrome system, India also decided to adopt the PAL system. This was initially developed and adopted in France in When both the quality of reproduction and the cost of equipment are taken into account, it is difficult to definitely establish the superiority of any one of these systems over the other two. All three systems have found acceptance in their respective countries.
The deciding factor for adoption was compatibility with the already existing monochrome system. Applications of Television Impact of television is far and wide, and has opened new avenues in diverse fields like public entertainment, social education, mass communication, newscasts, weather reports, political organization and campaigns, announcements and guidance at public places like airport terminals, sales promotion and many others.
Though the capital cost and operational expenses in the production and broadcasting of TV programmes are high compared to other media, its importance for mass communication and propagation of social objectives like education are well recognized and TV broadcasts are widely used for such purposes.
Closed Circuit Television CCTV is a special application in which the camera signals are made available over cable circuits only to specified destinations. This has important applications where viewers need to see an area to which they may not go for reasons of safety or convenience. Group demonstrations of surgical operations or scientific experiments, inspection of noxious or dangerous industrial or scientific processes e. Small communities that fall in the shadow of tall geographical features like hills can jointly put up an antenna at a suitable altitude and distribute the programme to the subscribers premises through cable circuits.
Another potential use of CCTV that can become popular and is already technically feasible is a video-telephone or visiphone. Equipment Television broadcasting requires a collection of sophisticated equipment, instruments and components that require well trained personnel. Television studios employ extensive lighting facilities, cameras, microphones, and control equipment. Transmitting equipment for. A wide variety of support equipment essential in broadcast studios, control rooms and outside includes video tape recorders, telecine machines, special effects equipment plus all the apparatus for high quality sound broadcast.
Coverage Most programmes are produced live in the studio but recorded on video tape at a convenient time to be broadcast later. Of course, provision for live broadcast also has to be there for VIP interviews, sports events and the like.
For remote pick-ups the signal is relayed by cable or RF link to the studio for broadcasting in the assigned channel. This usually varies between 75 and km depending on the topography and radiated power.
Area of TV broadcast coverage can be extended by means of relay stations that rebroadcast signals received via microwave links or coaxial cables.
A matrix of such relay stations can be used to provide complete national coverage. With the rapid strides made in the technology of space and satellite communication it has now become possible to have global coverage by linking national TV systems through satellites. Besides their use for international TV networks, large countries can use satellites for distributing national programmes over the whole area.
One method for such national coverage is to set up a network of sensitive ground stations for receiving signals relayed by a satellite and retelecasting them to the surrounding area. Another method is to employ somewhat higher transmitter power on the satellite and receive the down transmissions directly through larger dish antenna on conventional television receivers fitted with an extra front-end converter.
Recent Trends In the last decade, transistors and integrated circuits have greatly improved the quality of performance of TV broadcasting and reception.
Modern camera tubes like vidicon and plumbicon have made TV broadcast of even dimly lit scenes possible. Special camera tubes are now used for different specific applications. The most sensitive camera tubes available today can produce usable signals even from the scenes where the human eye sees total darkness. With rapid advances in solid state technology, rugged solid state image scanners may conceivably replace the fragile camera tubes in the not-too-distant future.
Experimental solid state cameras are already in use for some special applications. Solid state picture-plates for use in receivers are under active development. Before long the highly vulnerable high vacuum glass envelope of the picture tube may be a thing of the past. Since solid state charge coupled devices are scanned by digital addressing, the camera scanner and picture plate can work in exact synchronism with no non-linear distortions of the reconstructed picture.
An important recent technological advance is the use of pseudo-random scan. The signal so generated requires much less bandwidth than the one for conventional method of scanning. Besides all this, wider use of composite devices, made by integrated solid state technology, for television studio and transmitter equipment as well as for receivers will result in higher quality of reproduction, lower costs and power consumptions with increased reliability and compactness.
Special mention may be made of the surface acoustic wave filter to replace the clumsy and expensive IF transformer. Further, large screen TV reception systems based on projection techniques now under development will make it possible to show TV programmes to large audience as in a theatre. With the rapid development of large scale integrated LSI electronics in the last decade, digital communication by pulse code modulation PCM has made immense progress.
The advantage gained is, that virtual freedom from all noise and interference is obtained by using a somewhat larger bandwidth and a specially coded signal. Even if the final transmission in TV is retained in its present form, so that all previous receivers remain usable, the processing of pictures from the camera to the transmitter input is likely to change over to PCM techniques.
Unlike the case of monochrome TV standards, the International Telecommunication Union ITU , a UN special agency, has already adopted a single set of standards accepted by all member countries for the production and processing of picture signals by digital methods. Digital TV has become all the more attractive since solid state cameras compatible with digital signal processing and deflection circuitry have also been developed and are at present in the field testing stage.
Essentially then, a TV system is an extension of the science of radio communication with the additional complexity that besides sound the picture details are also to be transmitted. In most television systems, as also in the C. The carrier frequencies are suitably spaced and the modulated outputs radiated through a common antenna. Thus each broadcasting station can have its own carrier frequency and the receiver can then be tuned to select any desired station.
Figure 1. The picture information is optical in character and may be thought of as an assemblage of a large number of bright and dark areas representing picture details. These elementary areas into which the picture details may be broken up are known as picture elements, which when viewed together, represent the visual information of the scene.
Thus the problem of picture transimission is fundamentally much more complex, because, at any instant there are almost an infinite number of pieces of information, existing simultaneously, each representing the level of brightness of the scene to the reproduced. In other words the information is a function of two variables, time and space. Ideally then, it would need an infinite number of channels to transmit optical information corresponding to all the picture elements simultaneously.
Presently the practical difficulties of transmitting all the information simultaneously and decoding it at the receiving end seem insurmountable and so a method known as scanning is used instead. Here the conversion of optical information to electrical form and its transmission are carried out element by element, one at a time and in a sequential manner to cover the entire scene which is to be televised.
Scanning of the elements is done at a very fast rate and this process is repeated a large number of times per second to create an illusion of simultaneous pick-up and transmission of picture details. A TV camera, the heart of which is a camera tube, is used to convert the optical information into a corresponding electrical signal, the amplitude of which varies in accordance with the variations of brightness.
An optical image of the scene to be transmitted is focused by a lens assembly on the rectangular glass face-plate of the camera tube. The inner 8. Loudspeaker Sound IF amplifier. The photolayer has a very high resistance when no light falls on it, but decreases depending on the intensity of light falling on it. Thus depending on the light intensity variations in the focused optical image, the conductivity of each element of the photolayer changes accordingly.
An electron beam is used to pick-up the picture information now available on the target plate in terms of varying resistance at each point.
The beam is formed by an electron gun in the TV camera tube. On its way to the inner side of the glass faceplate it is deflected by a pair of deflecting coils mounted on the glass envelope and kept mutually perpendicular to each other to achieve scanning of the entire target area. Scanning is done in the same way as one reads a written page to cover all the words in one line and all the lines on the page see Fig.
To achieve this the deflecting coils are fed separately from two sweep oscillators which continuously generate saw-tooth waveforms, each operating at a different desired frequency. The magnetic deflection caused by the current in one coil gives horizontal motion to the beam from left to right at a uniform rate and then brings it quickly to. The other coil is used to deflect the beam from top to bottom at a uniform rate and for its quick retrace back to the top of the plate to start this process all over again.
Two simultaneous motions are thus given to the beam, one from left to right across the target plate and the other from top to bottom thereby covering the entire area on which the electrical image of the picture is available. As the beam moves from element to element, it encounters a different resistance across the target-plate, depending on the resistance of the photoconductive coating. The result is a flow of current which varies in magnitude as the elements are scanned.
This current passes through a load resistance RL, connected to the conductive coating on one side and to a dc supply source on the other. Depending on the magnitude of the current a varying voltage appears across the resistance RL and this corresponds to the optical information of the picture.
Glass plate. If the scanning beam moves at such a rate that any portion of the scene content does not have time to move perceptibly in the time required for one complete scan of the image, the resultant electrical signal contains the true information existing in the picture during the time of the scan. The desired information is now in the form of a signal varying with time and scanning may thus be identified as a particular process which permits the conversion of information existing in space and time coordinates into time variations only.
The electrical information obtained from the TV camera tube is generally referred to as video signal video is Latin for see. This signal is amplified and then amplitude modulated with the channel picture carrier frequency. The modulated output is fed to the transmitter antenna for radiation along with the sound signal. The microphone converts the sound associated with the picture being televised into proportionate electrical signal, which is normally a voltage.
This electrical output, regardless of the complexity of its waveform, is a single valued function of time and so needs a single channel for its transmission. The audio signal from the microphone after amplification is frequency modulated, employing the assigned carrier frequency. In FM, the amplitude of the carrier signal is held constant, whereas its frequency is varied in accordance with amplitude variations of the modulating signal.
As shown in Fig. The receiving antenna intercepts the radiated picture and sound carrier signals and feeds them to the RF tuner see Fig. The receiver is of the heterodyne type and employs two or three stages of intermediate frequency IF amplification. The output from the last IF stage Control grid. This signal that carries the picture information is amplified and coupled to the picture tube which converts the electrical signal back into picture elements of the same degree of black and white.
The picture tube shown in Fig. The glass envelope contains an electrongun structure that produces a beam of electrons aimed at the fluorescent screen.
When the electron beam strikes the screen, light is emitted. The beam is deflected by a pair of deflecting coils mounted on the neck of the picture tube in the same way and rate as the beam scans the target in the camera tube. The amplitudes of the currents in the horizontal and vertical deflecting coils are so adjusted that the entire screen, called raster, gets illuminated because of the fast rate of scanning.
The video signal is fed to the grid or cathode of the picture tube. When the varying signal voltage makes the control grid less negative, the beam current is increased, making the spot of light on the screen brighter. More negative grid voltage reduces the brightness. This state corresponds to black.
Thus the video signal illuminates the fluorescent screen from white to black through various shades of grey depending on its amplitude at any instant. This corresponds to the brightness changes encountered by the electron beam of the camera tube while scanning the picture details element by element. The rate at which the spot of light moves is so fast that the eye is unable to follow it and so a complete picture is seen because of the storage capability of the human eye.
The path of the sound signal is common with the picture signal from antenna to the video detector section of the receiver. Here the two signals are separated and fed to their respective channels. The frequency modulated audio signal is demodulated after at least one stage of amplification.
The audio output from the FM detector is given due amplification before feeding it to the loudspeaker.
It is essential that the same coordinates be scanned at any instant both at the camera tube target plate and at the raster of the 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.
Thus, in addition to carrying picture detail, 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 front view of a typical monochrome TV receiver, having various controls is shown in Fig. The channel selector switch is used for selecting the desired channel. The fine tuning control is provided for obtaining best picture details in the selected channel. The hold control is used to get a steady picture in case it rolls up or down. The brightness control varies the beam intensity of the picture tube and is set for optimum average brightness of the picture. The contrast control is actually the gain control of the video amplifier.
This can be varied to obtain the desired contrast between the white and black contents of the reproduced picture. The volume and tone controls form part of the audio amplifier in the sound section, and are used for setting the volume and tonal quality of the sound output from the loudspeaker.
Colour television is based on the theory of additive colour mixing, where all colours including white can be created by mixing red, green, and blue lights.
The colour camera provides video signals for the red, green, and blue information. These are combined and transmitted along with the brightness monochrome signal. Compatibility means that colour broadcasts can be received as black and white on monochrome receivers. Conversely colour receivers are able to receive black and white TV broadcasts.
This is illustrated in Fig. At the receiver, the three colour signals are separated and fed to the three electron guns of colour picture tube.
The screen of the picture tube has red, green, and blue phosphors arranged in alternate dots. Each gun produces an electron beam to illuminate the three colour phosphors separately on the fluorescent screen. The eye then integrates the red, green and blue colour information and their luminance to perceive the actual colour and brightness of the picture being televised.
These are provided at the front panel along with other controls. The colour or saturation control varies the intensity or amount of colour in the reproduced picture. For example, this control determines whether the leaves of a tree in the picture are dark green or light green, and whether the sky in the picture is dark blue or light blue.
The tint or hue control selects the correct colour to be displayed. This is primarily used to set the correct skin colour, since when flesh tones are correct, all other colours are correctly reproduced. It may be noted that PAL colour receivers do not need any tint control while in SECAM colour receivers, both tint and saturation controls are not necessary. The reasons for such differences are explained in chapters exclusively devoted to colour television.
Optical filters R Object. Signal transmission paths illustrating compatibility between colour and monochrome TV systems. R, G and B represent three camera tubes which develop video signals corresponding to the red, green and blue contents of the scene being televised. Review Questions 1. Why is scanning necessary in TV transmission? Why is it carried out at a fast rate?
What is the basic principle of operation of a television camera tube? What is a raster and how is it produced on the picture tube screen? Why are synchronizing pulses transmitted along with the picture signal? Why is FM preferred to AM for sound signal transmission? Describe briefly the functions of various controls provided on the front panel of a TV receiver. Describe the basic principle of colour television transmission and reception.
Geometric form and aspect ratio of the picture. Scanning and its sequence. Resolution of picture details. Interlaced scanning. Vertical and horizontal resolution. Picture brightness transfer characteristics of the system. There are many reasons for this choice. In human affairs most of the motion occurs in the horizontal plane and so a larger width is desirable.
The eyes can view with more ease and comfort when the width of a picture is more than its height. This enables direct television transmission of film programmes without wastage of any film area.
It is not necessary that the size of the picture produced on the receiver screen be same as that being televised but it is essential that the aspect ratio of the two be same, otherwise the scene details would look too thin or too wide.
This is achieved by setting the magnitudes of the current in the deflection coils to correct values, both at the TV camera and receiving picture tube. Another important requirement is that the same coordinates should be scanned at any instant both by the camera tube beam and the picture tube beam in the receiver. Synchronizing pulses are transmitted along with the picture information to achieve exact congruence between transmitter and receiver scanning systems.
While televising picture elements of the frame by means of the scanning process, it is necessary to present the picture to the eye in such a way that an illusion of continuity is created and any To achieve this, advantage is taken of persistence of vision or storage characteristics of the human eye.
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 the picture elements are scanned rapidly enough, they appear to the eye as a complete picture unit, with none of the individual elements visible separately. In present day motion pictures twenty-four still pictures of the scene are taken per second and later projected on the screen at the same rate.
Each picture or frame is projected individually as a still picture, but they are shown one after the other in rapid succession to produce the illusion of continuous motion of the scene being shown. A shutter in the projector rotates in front of the light source and allows the film to be projected on the screen when the film frame is still, but blanks out any light from the screen during the time when the next film frame is being moved into position.
As a result, a rapid succession of still-film frames is seen on the screen. With all light removed during the change from one frame to the next, the eye sees a rapid sequence of still pictures that provides the illusion of continuous motion.
A similar process is carried out in the television system. The scene is scanned rapidly both in the horizontal and vertical directions simultaneously to provide sufficient number of complete pictures or frames per second to give the illusion of continuous motion.
Instead of the 24 as in commercial motion picture practice, the frame repetition rate is 25 per second in most television systems. Horizontal scanning. The linear rise of current in the horizontal deflection coils Fig.
At the peak of the rise, the sawtooth wave reverses direction and decreases rapidly to its initial value. This fast reversal produces the retrace or flyback. The start of the horizontal trace is at the left W Start of a line.
The finish is at the right edge, where the flyback produces retrace back to the left edge. Note, that up on the sawtooth wave corresponds to horizontal deflection to the right.
The heavy lines in Fig. Vertical scanning. The sawtooth current in the vertical deflection coils see Fig. Thus the beem produces complete horizontal lines one below the other while moving from top to bottom. Then the rapid vertical retrace returns the beam to the top. Note that the maximum amplitude of the vertical sweep current brings the beam to the bottom of the raster.
Because of motion in the scene being televised, the information or brightness at the top of the target plate or picture tube screen normally changes by the time the beam returns to the top to recommence the whole process. This information is picked up during the next scanning cycle and the whole process is repeated 25 times to cause an illusion of continuity.
The actual scanning sequence is however a little more complex than that just described and is explained in a later section of this chapter. It must however be noted, that both during horizontal retrace and vertical retrace intervals the scanning beams at the camera tube and picture tube are blanked and no picture information is either picked up or reproduced.
Instead, on a time division basis, these short retrace intervals are utilized for transmitting distinct narrow pulses to keep the sweep oscillators of the picture tube deflection circuits of the receiver in synchronism with those of the camera at the transmitter. This ensures exact correspondence in scanning at the two ends and results in distortionless reproduction of the picture details. Most scenes have brightness gradations in the vertical direction.
The ability of the scanning beam to allow reproduction of electrical signals according to these variations and the capability of the human eye to resolve these distinctly, while viewing the reproduced picture, depends on the total number of lines employed for scanning. It is possible to arrive at some estimates of the number of lines necessary by considering the bar pattern shown in Fig.
If the thickness of the scanning beam is equal to the width of each white and black bar, and the number of scanning lines is chosen equal to the number of bars, the electrical information corresponding to the brightness of each bar will be correctly reproduced during the scanning process. Obviously the greater the number of lines into which the picture is divided in the vertical plane, the better will be the resolution. However, the total number of lines that need be employed is limited by the resolving capability of the human eye at the minimum viewing distance.
Angle subtended at the eye by picture elements at critical viewing distance a. Adjacent black and white lines of resolution. For the eye this resolution is determined by the structure of the retina, and the brightness level of the picture.
Substituting these values of and we get. This perhaps explains the use of lines in the original French TV system. In practice however, the picture elements are not arranged as equally spaced segments but have random distribution of black, grey and white depending on the nature of the picture details or the scene under consideration. Statistical analysis and subjective tests carried out to determine the average number of effective lines suggest that about 70 per cent of the total lines or segments get separately scanned in the vertical direction and the remaining 30 per cent get merged with other elements due to the beam spot falling equally on two consecutive lines.
Thus the effective number of lines distinctly resolved, i. However, there are other factors which also influence the choice of total number of lines in a TV system. Tests conducted with many observers have shown that though the eye can detect the effective sharpness provided by about scanning lines, but the improvement is not very significant with line numbers greater than while viewing pictures having motion.
Also the channel bandwidth increases with increase in number of lines and this not only adds to the cost of the system but also reduces the number of television channels that can be provided in a given VHF or UHF transmission band.
Thus as a compromise between quality and cost, the total number of lines inclusive of those lost during vertical retrace has been chosen to be in the B monochrome TV system. In the line American system, the total number of lines has been fixed at because of a somewhat higher scanning rate employed in this system.
Although the rate of 24 pictures per second in motion pictures and that of scanning 25 frames per second in television pictures is enough to cause an illusion of continuity, they are not rapid enough to allow the birghtness of one picture or frame to blend smoothly into the next through the time when the screen is blanked between successive frames.
This results in a definite flicker of light that is very annoying to the observer when the screen is made alternately bright and dark.
This problem is solved in motion pictures by showing each picture twice, so that 48 views of the scene are shown per second although there are still the same 24 picture frames per second. As a result of the increased blanking rate, flicker is eliminated. 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 the picture frame, it quickly returns 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 fields.
Each field is scanned alternately. This method of scanning is known as interlaced scanning and is illustrated in Fig. It reduces flicker to an acceptable level since the area of the screen is covered at twice the rate.
This is like reading alternate lines of a page from top to bottom once and then going back to read the remaining lines down to the bottom.