television (gnv64)

8/9/2019 Television (Gnv64)

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510HOW IT WORKS

BY NAVKALA ROY

DESIGNED AND ILLU STRATED BY

SUBIR ROY


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TH E MAG C OF TELEVISION

Bringing the world into the home ---


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Imagine. Sunil Gavaskar broken down intothousands of small bits of electricity as hetakes an outswinger and pushes it to sillymid-on. Rushed through the air at thephenomenal speed of light. Down yourantenna. Through the wires. And into yourreceiving set. To be seen exactly as he is onthat cricket field thousands and thousandsofkilometres away in the West Indies!

Imagine. But you don't have to. For, themagic of television does it fo r you everyday. It breaks up every picture intothousands of small charges of electricityand sends them through the air aselectromagnetic carrier waves at nearly

300,000 kms. per second, to be picked up bythe antenna on your roof-top andconducted into your receiving set wherethey are amplified and converted back intoa picture.

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Television takes you places indeed. Youcan go down to the bottom of the sea orright up into space as you sit munchingwafers before your TV set.

Time was when grandmother gatheredher grandchildren around her to tell themstories about the man in the moon. Now,both grandmother and grandchild sit gluedto the TV set as they watch and listen to theman on the moon.

-". . , . . /..

The first 'waves'

It began with sound broadcasting, in the

early 1920's, after Alexander Graham.Bellhad invented the telephone and GuglielmoMarconi had made wireless transmissionsacross the Atlantic:/tlf we can use waves totransmit speech," said some /tcan we notalso use them to transmit movingpictures?"

This set John Logie Baird, a Scotsman,thinking. One day, while in his bedroom, hepulled out some odd pieces of equipmentwhich included tw o cycle lamp lenses, atorch, an old electric motor, p a r t ~of an oldradio, string, wire, glue and sealing wax.Then he set about his task. The results,though not immediate, were dramatic. On

January 27, 1926 he finally proved to t ~ eworld that television was indeed a reality.

It was, of course, a combination of severaldiscoveries that helped Baird todemonstrate ho w moving pictures could be


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Dur'ng th e war

People did not take to televisionovernight. TV sets were expensive and not

many could afford to buy them, Besides,people had to be convinced that televisionwas worth their money.

And convinced they were in 1936, whenthe British Broadcasting Corporation(B.B.C.) set up the first regular televisionservice in the world. In the next couple of

years television didgrow

in popularity andwould have caught on in a big way, had thesecond world war not broken out.

This is, of course, no t to suggest thatpeople abandoned television thereafter. Itwas, in fact, during this time that some

scientists considered using television fo rmany purposes, fo r instance, to develop abomb. The idea was to fit a small TV camera

and transmitter to the flying bomb so thatits course could be followed by the planewhich had launched it. In this way the bombwould be guided nearer and nearer to itstarget. However, this was an idea that neverreally took shape.

The Germans, meanwhile, introduced apicture telegraph system fo r securityreasons. This system used the sameprinciples as television. Words in messageswere projected as images to the other endso that they could be read and understoodeasily,

While Europe was at war, engineers inAmerica threw themselves into establishinga regular television service, Numerousstations were opened at many of the largecities and a national network of cable andradio links, or what is more familiarlyknown as a 'national hook-up', was set up.


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Mickey's gala premiereAfter the war television spread. And

perhaps it was Mickey Mouse who made itpopular! For, the friendly little mouseendeared himself as much to people then ashe does today. The last transmission madeby the B.B.C. on September 1, 1939 justbefore the war, was a cartoon called,'Mickey's Gala Premiere', and on June 7,1946, to everyone's delight, the firstprogramme televised after the war was thesame cartoon!

Initially pictures could be transmitted

only over a limited distance. Onetransmitter could not serve people livingbeyond a radius of about 160 kms. To senda programme across the Atlantic was out ofthe question.

With the launching of space satellites, thiswas made possible. On July 11, 1962 thefirst

transatlantic transmission took placefrom Andover, U.S.A., via the satelliteTelstar 1, to Pleumeur, France. Baird'sdream had at last come true. Television hadby now become an accomplished fact inmany countries.

0


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Doordarshan

In India television was introduced in 1959.Known as J D o o r d a r s h a n ~the programmesinitially were entirely educational. The firstgeneral service on a regular basis wasstarted from Delhi in August 1965.

Now, besides having gone colour,television has reached practically everycorner of the country. This offers ustremendous prospects for development. TVhas great potential in the field of education,particularly basic education, and in othervital spheres such as transforming the

social environment, bringing us culturallynearer and generating a scienceconsciousness. Television has placed us in

the unique position of being able toJexperience simultaneously the sameenvironment.'

In 1963, people wh o had their TV setson, in the United States, saw Jack Ruby kill

PresidentJohn

Kennedy's presumedassassin, Lee Harvey Oswald, in thebasement of Dallas Police headquarters asit happened. Within hours the rest oftheworld also saw it - thanks to the SatelliteTelstar.


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F P

Today, television is shrinking the world.In

fact, it has gone far beyond the stageof

simply being one ofthe communicationmedia. TV has extended itself to manycomplex and intricate areas - fromunderground pipes to spacecraft, fromsupermarkets to operation theatres, andfrom police stations to your front door. Itcan go places where man perhaps cannot.

It was a television camera that showedthe world the first pictures ofthe moon longbefore Neil Armstrong set foot on it.

With possibilities of 3-D television takingshape, it may not be long before you cansee your favourite star reaching outtowards you as it were!

In a closed circuit TV system the signalsfrom the camera are not broadcast to all,but are transmitted through cables toselected receivers. It helps students watchthe surgeon perform his delicate task

without crowding into the operationtheatre. Similarly it helps the police toregulate traffic and spot thieves insupermarkets. And if you place one of thesecameras outside your front door, you caneven check who is calling!


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The cameraThe camera comes first in our bag of

television equipment. In this case, we take ablack and white one. Remember this is a TVcamera and not an ordinary one that youma y use to take photographs.

Several cameras are normally used fo r anoutside telecast, as in a studio transmission.You have a team of people operating the

equipment.One camera is so arranged that it takes in

the entire picture. Another camera showsclose-ups of the scene, and others arepositioned elsewhere to give differentviews of the same scene.

The producer positions the camerassuitably before the shooting is done. Theoverall effect is observed on a monitorwhich is placed before the producer. He canswitch from one camera to another during

the broadcast, in order to get the bestpossible coverage.The inside of a black and white TV camera .1. Object 2. Light from the object 3. Screen of photoelectriccells 4. Electron gun 5. Stream of electrons


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Th image-makerThe TV camera does no t have a film as in

the ordinary camera. The subject on whichit focuses - a monkey, let us say, throws animage on a plate inside the camera. This isknown as the signal plate. It is made up ofthousands of tiny dots of a special material.These dots are actually photo-cells. Whenlight falls on them, an electric charge isgenerated on each cell. The stronger the

light, the stronger the electrical charge.There are actually two images on the signalplate. One is visible, being formed by thescene in f ront o f the camera - in this caseth e monkey. The other is an invisibleelectrical 'image'. This image has strong

Television microscopes are used fo rbiological research. With ultra-violet lightthe microscope magnifies the object beingstudied more than a thousand times.

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1. Object 2. Lens 3. Photoelectric screen 4. A close-up ofscreen, showing ho w picture is broken into dots of varyingintensity

electrical charges where the scene beforethe camera is bright, and weaker electricalcharges where th e scene is less bright. Inplaces where the scene is dark, there is nocharge.


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Th e control r o o m - a close-up1. Monitors 2 C on tr ol p an el

Shooting in a TV studio1 Microphone 2 Cameras 3. Cameraman4. Sound engineer 5. Floor manager6. Director 7. Control gallery


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The scene reproduced onyour TV screen,. Antenna (on roof-top) 2. Cathode-raytube 3. Pi ct ur e a pp ea rs i n a s er ies of lines

4. The l in es m ak e u p t he p ic tu reI I

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Dot by dot

What exactly happens to this invisibleimage? Just as sound is recorded, imagesare recorded on magnetic tapes. In order tofacilitate this, the image has first to beturned into a stream of electrical signals.This process is known as scanning.

Scanning is done by an electron beam. Inthe TV camera there is a device called anelectron gun which shoots ou t a high-speed

beamof

electrons. This beam movesrapidly across the signal plate in a fixedpattern. By this process each image is sliced

In 1984,2,500 million people the worldover- the greatest number of viewers fo r

a televised event - watched the 23rdOlympic Games as they were being playedin Los Angeles, U.S.A.

into 625 lines. Each line is broken up intodots. Each do t on every line has a varyingcharge depending on whether it is black,

white or grey. That seems easy, doesn't it?Well, here is the difficult part.Each do t is positively charged. The

electron beam, however, has a negativecharge. When the negative beam strikes apositive dot, it cancels out the charges andwhoosh, the beam wipes ou t the electrical

image!As the charge is removed from each dot

of the signal plate, it flows into a wireconnected to the plate. The current comingfrom the plate varies in strength frommoment to moment, according to thecharges on the dots which vary according tothe visible image. These varying chargesare then recorded on a magnetic tape. Ahigh current indicates 'white', a lo w currentstands fo r 'grey', and no current for a 'black'spot.


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The ound effectWhile the TV camera picks up the picture

of the monkey, there must be some way inwhich the sounds that the monkey makesand the commentator's remarks can bepicked up and converted into electricalcharges.

First, let us understand sound. All soundsare the result of vibrations that travel inwaves through the air. Ifthere was no air we

would not hear any sound. You can prove. this by placing an alarm clock under a jar.You will be able to hear the alarm when itgoes off. Now, if you drawthe air ou t of thej ar b y a suction pump, you will find that youcannot hear the alarm ring. Astronauts,when in space, have to speak to each otherby radio. This is because in space there isno air.

Sound-waves can travel only a certaindistance. A loud noise causing powerfulwaves will travel farther than a soft noise.

Ai r helps sound to travel

Even a very powerful sound will no t travelfar enough fo r us to hear a noise if we are along wa y away. The normal human ear canhear vibrations from about 16 per second toabout 16,000 per second.

Just as water moves in ripples when youthrow a stone in a pond, sound-waves tootravel like ripples.

Electromagnetic waves, however, do notneed air to carry them. In fact, they travelbetter without.


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The electric ear

We have seen that sound travels byvibrations. If your ear is in the path of the

sound-wave, the latter sets your ear-drumvibrating, and you can hear the sound.The sound effects in a television

programme are picked up by a microphone.This is a kind of electric ear which has a thin

metal plate, called a diaphragm, in it. Thisvibrates whenever any sound reaches it andchanges the mechanical vibration into an

electric one.Different kinds of microphones are used

today. The principle behind all ofthem isthe same, that is, they pick up sound-wavesand turn them into electrical charges.

Just as a picture has different shades,sounds too vary in strength and frequency.The microphone alternates its voltageaccording to the frequency and strength ofthe original sound.

So, we have the picture of the monkey, itschatter and the remarks made by thecommentator recorded in the form ofelectrical variations on magnetic tapes.

These variations can be re-converted intoelectrical impulses which are thenbroadcast as sound and vision to all thehomes waiting to receive them. How doesthis happen?


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Th c rri

The transmission of pictures and soundalmost instantly and over great distanceswas made possible with the discovery ofelectromagnetic radio waves.

At the television studio the recordings onmagnetic tapes are fed into the transmitterby electrical impulses. In the transmitterthese impulses are changed intoelectromagnetic energy which moves along

the transmitting aerial in the form of waves.These waves radiate from the aerial out

into space at the speed of light and it is onthese waves that the vision and soundsignals are carried to your homes.

Brilliant, isn't it? And to think that theywere ordinary human beings like you and

me wh o made these fantastic discoveries!Or perhaps we should call themsuperhuman beings!

Electromagnetic waves produced bytransmitting aerials spread in all directions.

As these waves die over fairly shortdistances, they have to be kept going bysome means. It is fo r this reason that relaystations are set up. Here the waves arere-generated in order to cover longerdistances.

1 Main transmitter 2. Relay statIons


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Mod lationElectromagnetic carrier waves, as we

have seen, have a continuous f lo w o f wavesof the same strength or amplitude.However, the sound and vision that theycarry vary in strength. A loud sound at themicrophone, fo r instance, will cause a bigvariation and a soft sound a small variation

in the strength of the carrier wave. Similarlythe electron gun in the TV camera will causevariations in the current emitted by the lightdots.

When these varying currents mix with thecarrier wave, i t is known as modulation. Thevariations in strength according to thesound and vision being broadcast is knownas amplitude modulation.

sophisticated TV cameras. The focuses ofthese cameras are remotely controlled andwater-tight lamps are fitted to armsextending from the frame so as toilluminate th e objects on the ocean bed. o . ~ / oThe camera can also indicate the size of /

o the object and its distance away. :-;;::-;;.:

"

o

It is no longer necessary fo r humandivers to risk their lives in the depths ofthesea. As early as 1951, the submarine'Affray', which sank to the bottom of theEnglish Channel, was recovered by special

o

TV cameras designed fo r salvage work.Similarly in 1985, the wreckage of the Ai r

India Jumbo 'Kanishka' was recoveredfrom the Atlantic Ocean by extremely


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As electromagnetic waves radiate fromthe aerial of the TV station, they traveloutwards. In order to catch these, receivingaerials have to be placed in their path.

The receiving antenna is connected toyour TV set. In order to catch the wavesradiated from the transmitting tower, yourTV set has to be tuned to the samefrequency. Then a voltage similarlymodulated will be created in the antenna

and fed down a special cable to your TVreceiver.

The jo b of the sound equipment in a TVset is to extract from the high frequencycarrier wave the original sound variations.

In your TV set is a one-way rectifier. Thisrectifier chops the electromagnetic wave inhalf and passes on the half which is areplica of the original sound produced bythe microphone. This signal is then passedthrough smoothing circuits to an amplifying

1. Transmitting aerial2. Electromagnetic waves

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A moving coil loudspeaker1. Cone 2. Coil 3. Input

circuit wh ich produces an electrical outputat the original frequency. This is then fed toa loudspeaker. The loudspeaker has a conewhich vibrates according to the variationsofthe electrical current fed to it. Thevibrations ofthe cone send outsound-waves which in turn strike ourear-drums. The sound we hear is the soundthat went into the microphone manykilometres away.

How the picture comes alive

Like the sound, the waves carrying thepictures also pass through a rectifier. The

rectifier once again chops the wave in halfand passes on the half which is carrying theoriginal picture as it is produced by the TVcamera, to an amplifier. The amplifier helpsto give the pulses greater strength and feedthem into a tube in the TV set, known as thecathode-ray tube.

The cathode-ray tube has an electron gunsimilar to the one in the television camera.At the other end of this tube is a fluorescentscreen - the screen on which we see thepicture. It is known as fluorescent becausethe inside is coated with a special materialwhich gives of f light when the electron gunshoots electrons on to it. This material is

made up of tiny particles, each of whichgives of f a speck of light.The brightness of the light depends on

the strength of electrons that reach it. The


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1. Cathode 2. Deflecting coils 3. Scanning beam 4. Beamf or ms d ot of l ig ht f ro m t he picture 5 Fluorescent screen

electron gun scans the picture in the sameway as the camera's gun had scanned it.

Strong charges in current producestronger electrons which give a brighterlight speck. Weak charges in currentproduce weaker electrons which give aduller light speck on the screen. As theelectrons strike the tiny specks of materialthey glow fo r an instant. This processexactly reverses what the camera does.

In the camera the dots of light from thepicture cause varying charges of current,

whereas in the TV set the varying charges ofcurrent form dots of light to create thepicture on the screen. So, dot for dot, themonkey is faithfully reproduced on the TVscreen.

Would you believe it?The picture that you see on your

television set is not really there! No, this is

not a joke. It is absolutely true. For,if

youwere to s low everything down sufficiently,you would find that at any given secondthere is only one tiny speck on the screen.That would be the speck of fluorescentmaterial being hit by the electron from thecathode-ray tube at that instant. It is onlybecause the electron beam scans the screenat such a fantastic speed, together withsomething known as the 'persistence ofvision' that we see what appears to be acomplete picture.

Persistence of vision is a normal featureof human eyesight. This means - at leastas far as television is concerned - that

although the specks of light are going onand of f continuously, ou r eyes are not ableto detect the split second intervals, and thatis how we see the picture as a whole andnot broken up in dots.


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Colour TVAs if black and white television was not

fascinating enough, we have colour

television in many countries.It was Sir Isaac Newton who, in 1666,

showed that when light is passed through aprism, it is split into several colours. Thethree principal colours of the spectrum, as itis called, are red, green and blue. If thesecolours are mixed, they will give white light

again. Colour television was made possiblewith these discoveries.Like in a black and white TV camera, light

from the picture is focused on the colour TVcamera by a lens. When the light reachesthe camera, itis split into three beams by aset of special mirrors. Each beam of light isthen passed through a filter to producethree separate beams of red, green and bluelight. These beams are then directed tothree separate camera tubes which producesignals for each of these colours. These are

The inside o f a c ol ou r TV camera1. Lens 2. Special mirrors 3. Filters 4. Camera tubes

further processed to produce signalsdefining th e brightness, line and saturationof the scene. These three signals areeventually broadcast on one carrier wave.

At the receiving set the three signals areseparated and processed to reproduce thesignals fo r the red, green and blue. Theseare then sent through three guns in one


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cathode-ray tube. The screen in this tube ismade up of thousands of phosphoric dots,one-third of which emit red light, one-thirdblue light and the remaining third greenlight.

The dots are arranged in groups of three- one of each colour. Between the screenand the dots is a perforated metal mask.The perforations are exactly aligned withthe centres of each triangular group of threedots. This ensures that the red gun fires atthe dots emitting red light, the blue gun atthe dots emitting blue light, and the greengun at the dots emitting green light. Withthis, not only do the red, blue and greenparts come ou t clearly, but where the redoverlaps the green you get yellow and soon. The mixing rules fo r lights are quite

different from mixing paints. So, don't tryyour hand at that. What you could try, whenyou are older, is inventing even superiortelevision techniques. , P ' t . S Ii I ml't g a erial pi kp. 1= by

. anlerna and f ed I r t u T V ecelVE 2 R ec ei ve r bo x 3 Electrcgt.ns 4 neflectn coils


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Just th e first stepFor, when the first television broadcast

was made we thought we had reached theultimate in technology. Today it seemspretty certain that that was just the firststep. With satellites beaming programmesdirectly from foreign countries to theantennae on your roof-top, television hasmore up its tube than you can imagine.

Viewdata, fo r instance, can get youpractically any information you want withinseconds. This is a system that links a

household television set with a centralcomputer via the telephone line. All you dois use a key-pad containing numbers from 0to 9 and call up any information you requireprovided it is listed in the computer.

Perhaps a day will come when you willnot have to step out shopping, go to thebank, to your travel agent or to work!Television will be able to do all this fo r youwhile you sit at home and let your thoughtssoar. For, in the decades to follow the onlyl imit to television seems to be yourimagination.

A m in TV t that cane IIVbeheld between the fingers


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