CH 1 PPTSnew

8/6/2019 CH 1 PPTSnew

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UNIT 1: FUNDAMENTALS OF

TELEVISION


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By R.R. Gulati


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Element r TV tr nsmission nd reception

Aspect tio

Need of sc nning

ect ng l r nd Interl ced sc nning

Persistence of vision

Flicker

Vertic l nd Horizont l resol tion

ndwidth

Composite video sign l: levels, Need of s nchroniz tion

S nchronizing p lses

Equalizing pulses


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1. In earl ears of the 20 th centur man scientistsexperimented with the idea of using seleniumphotosensitive cells for converting light from picturesinto electrical signals and transmitting them throughwires.

2. In 1927, first demo of actual TV was given b

J.L baird in UK

C.K. Jenkins in USA

ased upon Technique of mechanical scanningemplo ing rotating discs.

3. Invention of C T and success of V.K. Zwor kin of theUSA in perfecting the first camera tube (the iconoscope) based on storage principle.


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4. In 1930, Electromagnetic scanning of both cameraand

picture tubes and circuits for beam deflection , video

amplification were developed.

5. TV broadcast started in 1935.

6. Monochrome TV s stems :- ( /W )

i. 525 line American

ii. 625 line European

iii. 819 line French s stems.


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7. In UK initiall 415 line Monochrome s stem was in use

,that

has been changed to the 625 line s stem with some

modification in the channel bandwidth.

8. In India, transmission started in 959 , the 625-B

monochrome s stem was adopted.

9. 3 different s stems of colour TV compatible with the 3Monochrome s stems.


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1. USA --- NTSC S stem (American)

(National Television S stems Committee)

2. erman--- PAL ( Phase alteration b line s stems)

3. French --- SECAM ( Sequential Couleures a MemoireS stem )


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1. PAL (Colour TV)

2. CCI - ( /W or Monochrome TV)

3. Difference between P B and P G is thechannel bandwidth .

4. PAL- : Channel bandwidth of 7 MHz---

Does not provide an interchannel gap.

5. PAL : Channel bandwidth of 8 MHz ---Provides a band gap of 1 MHz in betweensuccessive channels.


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1. egular Color transmission started in the USA in 1954.

2. In 1960 Japan and Canadaadopted NTSC s stem .

3. PAL colour s stem was developed at Telefunken Laboratoriesin Federal epublic of erman (F ).

4. PAL s stem reduces colour display errors that occur in NTSCsystem during transmission.

5. PAL system was adopted by

F and UK in 1967Australia

Spain

Iran

West and South Asiaand several other countries.


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1. SECAM was developed and adopted in F ANCE in1967.

2. SECAM iv and SECAM v developed at ussian

National Institute of esearch (NI )

3. And is referred to as NI SECAM Systems

Adopted by :---

USSerman Democratic epublic

Hungary

East European countries

Algeria


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1. Public Entertainment

2. Social Education

3. Mass communication

4. Newscasts5. Weather reports

6. Political Organization and campaigns

7.

Announcements8. uidance at public places like Airport

terminals

9. Sales promotion


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1. Camera signals are made available over cable circuitsonly to specified destinations

2. Application:- where viewers need to see an area towhich they may not go for reasons of safety or

convenience.

3. Eg.

4. roup demonstrations of surgical operations

5. Scientific experiments6. Inspection of dangerous industrial or scientific processes

like Nuclear fuel processing

7. Underwater operations

8. Surveillance of areas for security purposes


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Wired community TV

1. Small communities that fall in the

shadow of tall eographical features like

hills, can jointly put up an antennaat a

suitable altitude and distribute the

programmes to subscribers premises

through cable circuits.


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1. In CCI 625 line Monochrome systemadopted by India the picture signal isamplitude modulated and the sound signalis frequency modulated .

2. The carrier frequencies are suitably spacedand the modulated outputs are radiatedthrough a common antenna.

3. Thus each broadcasting station can haveits own carrier frequencyand the receivercan then be tuned to select any desiredstation.


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1. The picture information is optical in character and may be

thought of as an assemblage of a large no. of bright and

dark areas representing picture details.

2. These elementaryareas into which the picture details may be

broken up are known as Picture elements , which whenviewed together , represent the visual information of thescene.

3. Picture transmission : At any instant there are almost infiniteno. of pieces of information , existing simultaneously , each

representing the level of brightness of the scene to thereproduced.

4. Ideally it would need an infinite no. of channels to transmitoptical information corresponding to all the picture elementssimultaneously.


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1. Practical difficulties of transmitting all theinformation simultaneously and decoding it at thereceiving end is done with the help of the scanningtechnique.

2. In scanning method , the conversion of opticalinformation to electrical form and its transmissionare carried out by element by element , one at atime and in sequential manner to cover the entirescene which is to be televised .

3. Scanning of the elements is done at a very fastrate and this process is repeated a large no. oftimes per second to create an illusion ofsimultaneous pick-up and transmission of picturedetails.


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1. The heart of TV camera is camera tube

which is used to convert the optical

information into a corresponding

electrical signal , the amplitude ofwhich varies in accordance with the

variations of brightness.


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1. An optical image of the scene to be transmitted isfocused bya lens assembly on the rectangularglass face plate of the camera tube .

2.

The inner side of the glass face plate has atransparent conductive coating on which is laid avery thin layer of a photoconductive material.

3. The photo layer has a very high resistance when no

light falls on it,

but decreases depending on the intensity of lightfalling on it.


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Optical

image of the

scene

Rectangular glass

face plate

Proportional tooptical informationof the picture


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4. Thus depending on the light intensity variations in the focused

optical image, the conductivity of each element of the photolayer changes accordingly.

5. An electron beam is used to pick yup the picture information nowavailable on the target plate in terms of varying resistance ateach point.

6. The beam is formed byan electron gun in the TV camera tube.

7. On its way to the inner side of the glass face plate it is deflectedbya pair of deflecting coils mounted on the glass envelope andkept mutually perpendicular to each other to achieve scanning ofthe entire target area.


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1. It is done in the same wayas one reads a written page tocover all the words in one line and all the lines on thepage.

2. To achieve this the deflecting coils are fed separatelyfrom two sweep oscillators which continuously generatesaw-tooth waveforms, each operating at a differentdesired frequency .

3. The magnetic deflection caused by the current in one coilgives horizontal motion to the beam from left to right at auniform rate and then brings it quickly to the left side tocommence the trace of next line.

4. The other coil is used to deflect the beam from top tobottom at auniform rate and for its quick retrace back tothe top of the plate to start this process all over again.


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1. 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.

2. As the beam moves from element to element , it encounters adifferent resistance across the target plate , depending on theresistance of the photoconductive coating .

3. The result is a flow of current which varies in magnitude as theelements are scanned.

4. This current passes through an load resistance l , connected to theconductive coating on one side and to a dc supply source on theother.

5. Depending on the magnitude of the current a varying voltage appearsacross the resistance l and this corresponds to the opticalinformation of the picture.


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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 theimage, the resultant electrical signal

contains the true information existing in the

picture during the time of the scan.


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The desired information is now in the form of asignal varying with time and scanning may thusbe identified as a particular process whichpermits the conversion of information existingin space andtime coordinates into timevariation only.

The electrical information obtained from the TVcamera tube is generally referred to as a videosignal .

This signal is amplified and then amplitude

modulatedwith the channel picture carrierfrequency.

The modulated output is fed to the transmitterantenna for radiation along with the soundsignal.


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The microphone : converts the sound associated

with picture being televised into proportionate

electrical signal, which is normallya voltage.

This electrical output is a single valued function

of time and needs a single channel for its

transmission.

The audio signal from the microphone after

amplification is frequency modulated , using

carrier frequency.


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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.

Fig. 1.1a, output of the sound FM transmitter

is finally combined with the AM picture

transmitter output , through a combiningnetwork, and fed to a common antenna for

radiation of energy in the form of

electromagnetic waves.


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Fig. 1.1b

The receiving antenna intercepts a radiated

picture and sound carrier signals and feeds

them to the F tuner.

The receiver is of the super heterodyne type

and uses two or three stages of Intermediate

frequency (IF) amplification.

The output from the last IF stage is

demodulated to recover the video signal.


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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 samedegree of black and white.


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It is similar to the cathode ray tube used to an

oscilloscope.

The glass envelope contains an electron gun structure that

produces a beam of electrons aimed at the fluorescentscreen.

When the electron beam strikes the screen, light is

emitted.

The beam is deflected bya 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.


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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 currents is

increased, making the spot of light on the screen

brighter.


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More negative grid voltage reduces the

brightness, if the grid voltages is negative

enough to cut off the electron beam current,

at the picture tube there will be no light.

This state corresponds to black.

Thus the video signal illuminates thefluorescent screen from white to black

through various shades of grey depending on

its amplitude at any instant.


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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 thestorage capability of the human eye.


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Video signal to grid or cathode


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In video detector stage the sound signal is

separated.

The frequency modulated audio signal isdemodulated after at least one stage of

amplification.

The audio output form the FM detector isgiven amplification before feeding it to the

loudspeaker.


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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 aretransmitted during the retrace, i.e. fly back

intervals of horizontal and vertical motions of

the camera scanning beam.


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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 thereceiver picture tube beam is in step with

the transmitter camera tube beam.


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Channel selector switch: used for selecting the

desired channel.

Fine tuning control: for obtaining best picturedetails in the selected channel.

Hold control : to get a steady picture in case it

rolls up or down.

rightness control: varies the beam intensity of

the picture tube and is set for optimum average

brightness of the picture.


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Contrast control: gain control of the video

amplifier. To obtain the desired contrast

between the white and black contents of the

reproduced picture.

Volume and Tone controls : They form part of

the audio amplifier in the sound section ,

and are used for setting the volume andtonal quality of the sound output form the

loudspeaker.


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Colour television is based on 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.


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Pure WHITE COLOU

ED= 30 %

LUE= 11%

EEN = 59%

Y SI NAL ( LUMINANCE SI NAL)= 0.3 + 0.11 + 0.59 .

It carries brightness information.


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Each colour TV system is compatible with the

corresponding monochrome system.

Compatibility means that colour broadcasts canbe received as black and white on monochrome

receivers.

Conversely colour receivers are able to receiveblack and white TV broadcasts.

Fig. 1.5, where the transmission paths from the

colour and monochrome cameras are shown to

both colour and monochrome receivers.


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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.


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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.


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NTSC colour television receivers have two

additional controls, known as Colour and Hue

controls.

These are provided at the front panel along

with other controls.

The colour or saturation control varies theintensity or amount of colour in the

reproduced picture.


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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 primarilyused to set the correct skin

colour, since when flesh tones are correct,

all other colours are correctly reproduced.


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PAL colour receivers do not need any tint

control while in SECAM colour receivers, both

tint and saturation controls are not

necessary.

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