TELEVISION AND VIDEO ENGINEERING UNIT I

SNS COLLEGE OF TECHNOLOGY

Electronics and Communication Engineering

Seventh Semester

EC 2034-TELEVISION AND VIDEO ENGINEERING

IMPORTANT QUESTIONS AND ANSWER

2MARKS

PREPARED BY: VIGNESHWARAN.S AP/ECE

1. What is vertical and horizontal scanning?

The capability of resolving picture details in the vertical direction is called

vertical resolution & The ability of the system to resolve maximum number of

picture elements along the scanning lines determines horizontal resolution.

2. Differentiate monochrome and colour camera tube?

In black and white camera only one pickup tube is needed but three such

tubes or its equivalent is necessary in color cameras to develop separate signals for

red, green and blue information present in the scene.

3. Define Focal length.

The focal length of the lens is the distance between the centre of the lens and

the image formed by it of an object at infinity.

4. Define Field of view.

This describe the width and height of the scene viewed and is determined by

the focal length, lens to object distance and the width and height of the scanned

area

Field of view = scanned width x (lens to object distance/focal length)

5. What do you refer by persistence of eye?

The persistence of eye refers to the storage capability of the human eye. It

is 60ms

6. Define kell factor and number of scanning lines. The pixels are distributed

randomly in a picture. So the spot of scanning beam at some places may cover

Page 1 of 27

two pixels partly. This will reduce resolution subjective tests made on several

people showed that reduction in general is form 28% to 32% (average

value=30%).Hence while calculating practical resolution the theoretical

resolution should be multiplied by a factor of 0.7.

7. Define Aspect ratio and image continuity.

Aspect ratio can be defined as the ratio of width to height of the picture

frame. For television, it is standardized as 4:3. If each frame is displayed in

25frames/second due to our persistence of vision we will see it continuous

8. What kind of modulation is used for video signal?Why?

The type of modulation in which the modulated video signal amplitude

decreases with the increase in brightness of the picture is called as negative

modulation. Here the peak white signal corresponds to near zero level.The blanking

level is at 75% modulation level

Less disturbances due to noise pulses.

Less transmitting power is required to transmit because most of the

picture information is white.

Less transmitting power in transmitter.

Gain of the receiver circuit is automatically adjusted using AGC circuit

9. List the contents of composite video signal .

Composite video signal consists of a camera signal, blanking pulses and

synchronizing pulses.

10. Compare image orthicon and vidicon camera tubes.

S.No IMAGE ORTHICON VIDICON CAMERA TUBES

Works on photoconduction

principle

Works on photo emission principle

Consists of bismuth silver cesium

compound

Antimony trisulphide(a

semiconductor)

Resolution is poor Resolution is better than image

orthicon

Page 2 of 27

11. Define pedestal level and pedestal height.

In positive polarity, the pedestal level is represented by zero IRE ,This is a

fixed level and is treated as reference .IRE values of all other levels are

indicated with respect to reference level .In negative modulation ,it is

represented by75% of the carrier signal. The pedestal level is also called

blanking level.

Pedestal height is the distance between the pedestal level and the average

value axis of the video signal.

12. Why is vertical scanning very slow in comparison with horizontal scanning.

To cover large number of pixels in scanning.

13. Define Active lines.

All the scanning lines are not visible on the screen due to blanking during

the retrace period.The actual lines which reproduce pixels are called active

lines.The number of active lines (NA) is equal to the total number of scanning

lines per frame(NT) minus the line lost in vertical blanking(NL),as expressed

by NA= NT- NL

14. What is dark current?

In camera tubes the target used is semiconductor material it is sensitive to

light in absence of light it should hold no charge but due to room temperature

there may be charge on the target which pocess current on the output circuit

this is known as dark current.

15. Determine height and width of a TV screen of 30cm size?

SOLUTION

5x=30cm,therefore x=6cm

Hence,height=6*3=18cm and Width=6*4=24cm

16. What do you understand by TV standards? Why are these standards different

in different countries?

TV standards consist of a set of specification for transmission of TV

programmes.Broadly speaking these specifications pertain to the scanning

process.The reason for this being different is electric main frequency of the country .

17. Describe scanning and justify interlaced scanning.

Scanning is the process of converting optical image into electrical signal.

When the scanning 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. Hence the total number of lines are divided into two groups called

fields. Each field is scanned alternately. This way of scanning is called

interlaced scanning.

Page 3 of 27

PART B ANSWERS

18. Neatly draw and explain a vidicon camera tube cross section. (8)

CONSTRUCTION

It is a small size camera tube operates on the principle of photo conduction.

Here the conduction of target plate increases with the intensity of light

falling on it.

Vidicon camera tube has the following three sections

Photo conductive target plate

Scanning section and

Electron gun section

TARGET SECTION

The target consist of thin photo-conductive layer madeup of either antimony

or selenium compounds.

On the inner surface of the face plate a transparent, conductive film is coated

and on this the photo-conductive material is deposited.

The conductive coating is also called as signal electrode.

SCANNING SECTION

The scanning section is used to move the electron beam horizontally (left to

right) and vertically (T-B) to scan the entire target plate.

Horizontal and vertical deflection coils are used for this purpose.

Electron beam approaches the target plate with low velocity and due to low

velocity, secondary emission are avoided.

ELECTRON GUN SECTION

Heater, cathode, control grid, focussing coils and accelerating anode forms

the electron gun section.

Cathode: used to emit electron

Page 4 of 27

Control grid: used to control the electrons coming out of the cathode

Focussing coils: used to focus the electron beam to a sharp point on the target

plate

ELECTRIC CIRCUIT:

ADVANTAGES OF VIDICON

Resolution is high

Long life and small size

By varying the target voltage, it can be operated at different levels of

sensitivity.

Gamma cancellation circuit is not necessary.

DISADVANTAGES OF VIDICON

High dark current

Poor sensitivity

High image lag

Scanning

NOTE:This diagram is common to all camera tubes

Page 5 of 27

19. Explain in detail about image orthicon camera tube. (8)

Developed in 1945 by radio corporation of america.

The main transducer in image orthicon camera tube is photocathode.

It consists of coating of bismuth-silver-cesium compound on the inside of the

glass face plate of the tube.

When light from the scene is focused on the photo cathode, electrons are

emitted from it. The energy of the electrons emitted from different points of

the cathode depends on the intensity of light(energy of the photons)

Being a conductor, the photocathode cannot store charge.

Design of cathode is such that electron comes out in parallel beams.

The electrons are accelerated towards the target, strike a n-type silicon plate

placed parallel to the cathode. They cause emission of secondary electron

from the target plate.

The number of secondary electrons emitted from the target depends on the

energy of the striking electrons.

Loss of secondary electrons from the n-type semiconductor makes positively

charged.

The positively charge at any point on the semiconductor depends on the

number of electrons lost from that point.

Different points of the target possess different positive charges proportional

to the intensity of light of the corresponding points in the scene.

Hence a charge image of the picture is formed on the surface of the target.

On the other end electron gun scans the target plate and the return beam is

collected by dynode for amplification.

20.Draw the composite video signal and explain its components in detail. (12)

Page 6 of 27

CVS CONSISTS OF

Camera signal: corresponding to picture or scene to be transmitted.

Blanking pulses: to make the horizontal and vertical retrace invisible.

Synchronizing pulses: to synchronize the transmitter and receiver scanning

system

Information about colour signal and some sample of colour sub-carrier

frequency also be send if we are using colour TV transmission

VIDEO SIGNAL DIMENSIONS

Video signal amplitude is separated into 2 sections:

Camera signal: occupies 75%

Sync. Pulses: occupies 25%

The level of video signal obtained for maximum white light is called as peak

white level. This level exist between 10 to 12.5%

Black level: which is at 73% corresponds to dark picture.

The blanking pulses are added at 75% level and called as blanking level.

Black level and blanking level is closed to each other practically they merge

with each other.

The difference between black level and blanking level is called pedestal

height

Useful video signal information lies between 10% to 75% level.

Since the modulation used is –ve modulation, if the picture is dark then the

amplitude of video signal is high.

The region between 0% to 10% is called as whiter than white region.

The signals in those region create more bright spots on the screen and it will

create disturbance to eye of the viewer.

To reduce noise effect, usually the whiter than white regions are not used

The average background brightness of the scene is represented by the DC

level.

Since the DC level for different lines are at different level, background

brightness are different depending on the scene to be transmitted.

The distance between DC level and blanking level is called as pedestal

height.

It is used as a measure of average brightness of the scene.

It is not possible to follow the changes that takes place in background

brightness if the DC levels are not properly maintained

Page 7 of 27

DC COMPONENT

The average background brightness of the scene is represented by the DC

level.

Since the DC level for different lines are at different level, background

brightness are different depending on the scene to be transmitted.

The distance between DC level and blanking level is called as pedestal

height.

It is used as a measure of average brightness of the scene.

It is not possible to follow the changes that takes place in background

brightness if the DC levels are not properly maintained

DC INSERTION

The video control operator, who observe the scene to be telecast directly at

the studio knows the background brightness and it is his responsibility to

properly set (insert) the DC level with the video signal. This process is called

as DC insertion.

BLANKING PULSES

To make the retrace invisible, blanking pulses are added at 75% level.

Since this is above black level, no picture is seen on the screen.

Page 8 of 27

Synchronization pulses are also added in this blanking region to achieve the

synchronization

It is not possible to use blanking pulses as synchronization pulse coz some

time extreme black signal may rise above the blacking level. If this happens

then it may false trigger the synchronization oscillator

VIDEO SIGNAL TO SYNC. PULSE RATIO

The picture signal amplitude to sync. Pulse amplitude ratio is 4:3. This has

been found satisfactory due to the following reasons

Let picture signal amplitude is increased and sync. Pulse amplitude is

decreased: S/N ratio of Rx. Signal falls (ie., noise level increases) below a

limit, then amplitude of sync pulse is not sufficient to keep the

synchronization properly. However picture signal can able to produce

acceptable picture

Let sync. Pulse amplitude is increased and picture signal amplitude is

decreased: S/N ratio falls below the limit, then amplitude of sync. Pulse is

sufficient to synchronize and lock the picture frame.

But video signal amplitude is insufficient to produce a good acceptable

picture on the screen

21.Discuss the merits and demerits of positive and negative modulation and

Justify the choice of negative modulation in most TV systems. (4)

Transmitting power is less in negative modulation hence it is the best choice

for video modulation

22.Describe the construction and working of monochrome picture tube. (13)

The picture tube is very similar to the cathode-ray tube used in an

oscilloscope.

The glass envelope contains an electron Gun structure that produces a beam

of electrons aimed at the fluorescent screen.

When the electron beam strikes the screen, light is emitted.

Page 9 of 27

A monochrome picture tube has one electron gun and a continuous phosphor

coating that produces a picture in black and white. For colour picture tubes

the screen is formed of three different phosphors and there are three electron

beams, one for each colour phosphor. The three colours are red, green and

blue produced by three phosphors combined to produce different colours.

MONOCHROME PICTURE TUBE

6S,VIGNESHWARAN

ELECTROSTATIC FOCUSSING

The electric field due to the positive potential at the accelerating grid extends

through the opening the of the control grid right to the cathode surface.

The orientation of this field is such that besides accelerating electrons down

the tube. It also brings all the electrons in the stream into a tiny spot called

the cross over. This is known as the first electrostatic lens action.

The electrode voltages are so chosen or the electric field is so varied that the

second point where all the electrons get focused is the screen of the picture

tube. Electrostatic focusing is preferred over magnetic focusing because it is

not affected very much by changes in the line voltage.

BEAM VELOCITY

In order to give the electrons stream sufficient velocity to reach the screen

material with proper energy to cause it to fluoresce, a second anode is

included within the tube.

This is a conductive coating with colloidal graphite on the inside of the wide

bell of the tube.

Page 10 of 27

This coating called aquadag usually extends from almost half way into the

narrow neck to within 3 cm of the fluorescent screen.

DEFLECTION YOKE

It may be noted that a perpendicular displacement results because the

magnetic field due to each coil reacts with the magnetic field of the electron

beam to produce a force that deflects the electrons at right angles to both the

beam axis and the deflection field.

Page 11 of 27

BEAM DEFLECTION

As already stated the electron beam must attain a very high velocity to

deliver enough energy to the atoms of the phosphor coating. Because of this

the electrons of the beam remain under the influence of the deflecting field

for a very short time.

This necessitates application of high deflecting fields to achieve the desired

deflection. It is very difficult to generate such high voltages at the deflection

frequencies. On the other hand with magnetic deflection it is a large current

that would be necessary to achieve the same deflection.

Since it is more convenient to generate large currents than high voltages. All

picture tubes employ electromagnetic deflection. With electrostatic deflection

the beam electros gain energy. Thus, larger deflection angles tend to defocus

the beam.

The deflection plates need to be placed further apart as a deflection angle is

made larger. Thus requiring higher voltages to produce the same deflection

fields.

Magnetic deflection is free from both these shortcomings and much larger

deflection angles can be achieved without defocusing or nonlinearities with

these consequent saving in tube length and cabinet size.

PICTURE TUBE

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.

23.What is the purpose of thin aluminium coating over fluroscent screen

in a picture tube? (3)

To reflect light back towards the front and to stop heavy ions from going to

the screen,while allowing high velocity light electron to pass

24. Explain the following

(i)Solid state image scanner. (8)

These scanners operates on the principle of charge coupled devices

(CCD).

Page 12 of 27

ARRANGEMENTS OF PHOTO DIODE,GATES AND

SHIFT REGISTERS IN A CCD

Large number of CCD array are formed together to construct the image plate.

It can store and transfer analog signals .

Aspect ratio 4:3 is followed in the above arrangement.

In actual set up there may be 600 diodes in each rows and 450 diodes in each

column in 1/3 inch size sensor.

There are as many photodiodes as there are the number of pixels that is one

photo diode for one pixel.

Horizontal and vertical scanning are done by timing it is accurately

controlled by clock and multivibrator circuit.

TREATING PHOTO DIODE AS CAPACITOR

Page 13 of 27

ADVANTAGES

No need for electron gun and electron beam.

No need for high voltage.

No need for vacuum envelope.

Light weight, small size, portable.

High resolution.

24.(ii)Plumbicon camera tube. (8)

Page 14 of 27

PLUMBICON CIRCUIT

25 Draw the horizontal and vertical sync pulse and mark their time duration.

Explain their application in scanning process in detail (16)

HORIZONTAL SYN DETAILS

Out of 64 μs line period, the blanking period occupies 12 μs and useful

information occupies 52 μs.

Within the blanking period sync. Pulses are also added.

Page 15 of 27

Leading edge of these sync pulses are used to trigger the deflection oscillator

circuit

Horizontal blanking period consists of following 4 main regions:

Front porch

Line (Horizontal) sync pulse

Back porch and

Colour burst signal

FRONT PORCH

It is a brief cushioning period of 1.5 μs length.

It is inserted b/w the end of picture information and leading edge of the sync

pulse

Pulling on whites: the video signal for peak white information exists in the

whiter than white region and nearly touches the zero level.

LINE SYNC PULSES

At the end of front porch, sync pulses necessary for horizontal

synchronization are added.

It having a period of 4.7 μs (see fig)

These pulses are separated at the receiver and after differentiation used for

triggering the horizontal deflection oscillator

BACK PORCH

The back porch is having a period of 5.8 μs.

It gives necessary time for the retrace (fly back) to be completed.

During this period, the reversal of current direction of the horizontal time

base circuit takes place.

Also at the receiver side, this level is used for the production of AGC voltage

in the AGC circuit

COLOUR BURST SIGNAL

If we are using colour signal, then some samples of these colour subcarrier

frequency called colour burst signal is added for each line and send along this

back porch

S.

No.

period Time (μs)

1 Horizontal blanking pulse 12

2 Horizontal sync pulse 4.7

3 Front porch 1.5

Page 16 of 27

4 Back porch 5.8

5 Visible time period 52

6 Total time period 64

VERTICAL SYNC DETAILS

Vertical sync pulses (VSP) are added at the end of each field.

Construction of VSP is tedious than horizontal sync pulses.

The requirements to satisfy by VSP train are:

It should provide vertical blanking interval so that vertical retrace is not

visible on screen.

It should provide VSP for proper triggering of vertical oscillator

During the vertical blanking and sync pulse period, horizontal scanning and

triggering of horizontal oscillator should not affected.

Vertical retrace has 20 horizontal lines.

It has a period of 1280 μs or 1.28 ms (20 x 64 μs).

Page 17 of 27

To blank the vertical retrace, we have to add such a long pulse having the

duration of 1.28 ms. If we are using such a long pulse then large amount of

power is wasted. So we are using a pulse duration of 2.5 line period as VSP

SERRATED VERTICAL PULSE

From the figure of vertical sync pulse train and sync pulse generation circuit,

the following problems are to be considered for future modifications

Horizontal sync pulses are available during both vertical trace and retrace

period. But during the vertical sync period (2.5 line period) horizontal sync

pulse are not available. Due to this the horizontal oscillator may come out of

synchronization during vertical sync period

At the end of first field, the leading edge of vertical sync pulse came in the

midway of a line (line 313). This may false trigger the horizontal oscillator.

To achieve the horizontal sync during the vertical sync period, 5 narrow slots

of 4.7 μs duration is inserted (see fig). The leading edges of these pulses are

used to trigger the horizontal oscillator

The vertical slots of 4.7 μs duration that are inserted in the vertical sync

period to achieve horizontal sync are called as serrated pulses. This process is

called notching or serration of field sync pulse

26. Discuss the advantage of AM for video signal and explain VSB

detail. (16)

The AM is suitable for long transmission and conserve

bandwidth so VSBAM is used in TV transmission

VESTIGIAL SIDEBAND TRANSMISSION

The maximum video frequency obtained in the 625 line system is 5 Mhz.

If we are using double side band AM modulation, the video signal bandwidth

becomes 10 Mhz.

Due to practical difficulty of filter design, on both side of the channel we have

an attenuation slope of 0.5 Mhz.

To avoid interference with adjacent channel a guard band of 0.25 Mhz should

be allowed. So the channel bandwidth is 11.25 Mhz

If we are using such a long bandwidth channels, then within the given range

of frequencies, the number of channels used are reduced. So we have to

reduce the bandwidth

Since the upper and lower side bands have the same information, one of the

side band may be completely removed and we can use single sideband

system. Due to this upto 5 Mhz channel bandwidth is reduced.

The lower video frequency components are near the carrier frequency. Using

filter circuits they cannot be completely attenuated.

Also the fine details of picture are in low frequency range only.

Page 18 of 27

So if completely eliminating one side band, phase distortion may occur.

Coz of this problem, some part (vestige) of lower side band, upto 0.75 Mhz is

retained and others are attenuated. This method of transmitting a full upper

side band and vestige of lower side band is called as vestigial side band

transmission

Channel bandwidth = Attenuation slope + vestigial part (VLSB) + USB +

attenuation slope + guard band

= (0.5 + 0.75 + 5 +0.5 + 0.25)

= 7 Mhz

So, if we are using VSB modulation, then channel bandwidth is reduced

from 11.25 Mhz to 7 Mhz and bandwidth saving upto 4.25 Mhz is achieved

VSB TRANSMISSION

VESTIGIAL SIDEBAND RECEPTION

In VSB modulation, low frequencies upto 0.75 Mhz are available on both the

sideband.

But the signal in the frequency range of 0.75 Mhz to 5 Mhz are available in

one band only.

Due to this, the gain of voltage level at the receivers, RF and IF should be

properly controlled.

Page 19 of 27

The o/p voltage obtained for low frequency (0 to 0.75 Mhz) signal are twice,

than the high frequency (0.75 to 5 Mhz) signals. To correct this problem the

response curve of IF amplifier should be properly designed

ADVANTAGES

Channel bandwidth is reduced

Reduction in transmitted power

DISADVANTAGES

Some powers are wasted in VSB filters

S/N ratio is reduced upto 6 db, due to IF picture carrier placed at 50% gain

at the receiver

Chances for amplitude and phase distortion.

The tuning at the receiver is more critical

27 What is interlaced scanning?How does it reduce flicker and conserve

Bandwith?Explain (10)

INTERLACED SCANNING

In interlaced scanning each frame is scanned in two sequence and each

sequence is called field.this cause two fields /frame and 50fields/second and

this eliminate flicker in progressive scanning and also conserve bandwidth

Page 20 of 27

SCANNING PERIOD

SAWTOOTH CURRENT FOR HORIZONTAL DEFLECTION

SAWTOOTH CURRENT FOR VERTICAL DEFLECTION

Page 21 of 27

28.Discuss the importance of scanning and synchronization pulse? (6)

Scanning is the important process carried out in a television system in

order to obtain continuous frames and provides motion of picture. The scene

is scanned both in the horizontal and vertical directions simultaneously in a

rabid rate. As a result sufficient number of complete picture of frames per

second is obtained to give the illusion of continuous motion.

Synchronization pulse synchronizes the scanning in camera at receiver and

scanning in picture tube at receiver providing no delay.

29. Show the standard channel characteristics of CCIR B adopted in India.

(16)

30. Draw a block diagram of a monochrome TV receiver and explain the

function of each block? (16)

The block schematic diagram below of a typical monochrome TV receiver. As

shown there, the receiver has been divided into several main sections

depending on their functions and are discussed below.

Page 22 of 27

MONOCHROME TV RECEIVER

IMPORTANT SECTIONS IN THE RECEIVER ARE:

Antenna section

RF tuner section

Video signal and AGC section

Audio signal section

Sync separator section

ANTENNA SECTION

Mounted top of the building

TUNED ANTENNAS: to receive good signal strength

FOLDED DIPOLE TYPE ANTENNAS: good reception

A folded dipole with one reflector and more directors called as yagi-uda

antenna

RF TUNER SECTION

RF tuner: RF amp., mixer and local osc. Collectively called RF tuner

To amplify the weak signal RF amp. is used

Amplified signal is given as one i/p to mixer.

Other i/p of mixer is from L.O

o/p of mixer is IF of audio and video signal

Page 23 of 27

L.O produces freq. equal to channel picture freq. of +38.9 Mhz or channel

audio freq. of +33.4 Mhz

SO THE O/P OF MIXER FOR ANY CHANNEL SELECTION

Picture IF = 38.9 Mhz and

Sound IF = 33.4 Mhz

VIDEO SIGNAL AND AGC SECTION

Video section: (IF amplifiers, video detector, video amplifier and picture tube

The signal amplitude obtained at the o/p of mixer is very small so 2 or 3

stages of IF signal amplification is necessary.

After amplification they are given to the video detector

Video detector: demodulation takes place and CVS is obtained ,

VIDEO SIGNAL AND AGC SECTION

To produce picture with good contrast about 80 to 100 volts are needed. But

the o/p of video detector is only about 2 volt peak to peak.

Therefore a video amplifier is used at the o/p of video detector.

To adjust the contrast b/w black and white parts of the picture, a contrast

control is provided at the front panel.

The amplified signal is given to cathode or control grid depending upon the

picture tube

Signals received from long distance transmitter are very weak so, maximum

gain is required and the signals received from near by transmitters are

strong and reduced gain for amplifier is needed.

But it is not possible to change the gain by selecting different channels. So an

automatic gain control (AGC) circuit is used to automatically control the gain

of the amplifiers

AUDIO SIGNAL SECTION

Combination of IF amplifier, FM demodulator, audio amplifier and loud

speaker.

Using 5.5 Mhz sound trap circuit, the audio signal is separated from the

video detector section.

The IF (5.5 Mhz) of audio signals are amplified and fed to FM discriminator

for demodulation.

Demodulation signals are given to audio amplifier and finally loud speaker

for the reproduction of sound

SYNC SEPARATOR SECTION

Combination of sync separator, integrator and differentiator.

The o/p of video detector (CVS) is given i/p to the sync separator. This

separates both horizontal and vertical sync pulses.

Integrator(LPF) is used to separate the vertical sync pulses having field freq.

of 50 Hz for triggering vertical oscillator.

Page 24 of 27

Differentiator(HPF) is used to separate the horz. Sync pulses having line

freq. of 15625 Hz for triggering horz. Oscillator

HORIZONTAL AND VERTICAL OSCILLATOR SECTION

Combination of vertical oscillator, o/p amplifier and vertical deflection coil.

Vert. osc is synchronised by the trigger pulses from the integrator.

Then the saw tooth signal is given to deflection coil, after proper

amplification by vert. o/p amplifier

Horz. osc is synchronised by the trigger pulses from the differentiator.

Then the saw tooth signal (15625 Hz) is given to deflection coil, after proper

amplification by horz. o/p amplifier.

To maintain the freq. at 15625 Hz, an automatic freq. control (AFC) circuit is

used

EXTRA HIGH TENSION (EHT)

High voltages needed by final anode are developed.

Voltages of 15 to 20 KV are developed by an auto transformer.

After rectification, the high voltages are applied to final anode and aquadag

coating.

NOTE: Aquadag is a trade name for a graphite based coating commonly

found in cathode ray tubes.The coating can be on the inside of the tube,

where it helps to maintain a constant electrical field near the screen.

31. Draw a block diagram of a monochrome TV receiver and explain the function

of each block? (16)

In a TV Transmitter, we have to transmit both video and audio signal. So we

have 2 types of transmitter within the TV main transmitter. They are

Visual transmitter

Aural transmitter

VISUAL TRANSMITTER is used for transmitting video signals and aural

transmitter is used for transmitting audio or sound signal.

o/p of these two transmitters are combined before transmitting through

common antenna. The combining network is called diplexer.

Before transmission, the video signal is amplitude modulated (AM) and audio

signal is frequency modulated (FM). After modulation and proper power

amplification they are fed to a common antenna through the diplexer.

Initially a camera is used to convert optical image into electrical signal. After

adding sync pulses and blanking pulses we get composite video signal which

is to be AM modulated

Page 25 of 27

BLOCK DIAGRAM OF MONOCHROME TV RECEIVER

Similarly, a microphone (MIC) is used to convert audio signal into electrical

signal.

After pre-emphasis and proper amplification we have to FM modulate the

audio signal

IN THE SIMPLIFIED BLOCK DIAGRAM OF MONOCHROME TELEVISION

TRANSMITTER, THE MAIN SECTIONS ARE GIVEN BELOW:

Video and audio processing unit

Video signal modulation unit and

Audio signal modulation unit

VIDEO AND AUDIO PROCESSING UNIT

In video processing unit, the camera o/p signal having 1 volt p-p amplitude is

converted into standard form by eliminating hum and noise.

If necessary, linearity correction can also be done. This correction is done

during the back porch by clamping the video signal.

Controlling facilities for linearity correction, gain, black setup, sync level,

picture to sync ratio, sync stretching are also available in this unit

Page 26 of 27

Sync pulses and blanking pulses are added to the camera signal to get CVS.

Several cameras are used to telecast the scene from different angles.

In audio processing unit we have microphone, which convert the audio

signals into electrical signal. Here pre-emphasis circuit is used, which will

give necessary frequency compensation for the audio signal.

To pickup audio signals from different locations associated with the program,

large number of microphones are provided

VIDEO SIGNAL MODULATION UNIT OR PICTURE SIGNAL

MODULATION OR VISUAL EXCITER

Amplified CVS signals are fed to the modulation unit through the distributor

and switcher.

The distributer is used to receive the signals from other studios also.

CVS and picture carrier signal are fed as i/p to the modulator unit. Crystal

controlled oscillator is used for generating the picture carrier signal

VISUAL EXCITER

After AM modulation, it is power amplified and fed as i/p to the modulator

unit.

Crystal controlled oscillator is used for generating the picture carrier signal.

After AM modulation, it is power amplified and fed to the combining network.

To save the power, VSB technique is used in TV transmitters.

Before combining with the audio signal, the video signals are passed through

the VSB filters .

AUDIO SIGNAL MODULATION OR AURAL EXCITER

FM modulation is used for audio signals in TV transmitter. This frequency

modulator is a crystal controlled one.

Here the FM modulation of audio signal takes place with a carrier signal

from crystal oscillator.

To have an automatic frequency control (AFC) facility, phase detector and

CPF stages are used.

If there is any change in frequency of carrier signal, then phase detector will

produce a dc correcting voltage. This dc voltage is used to automatically

correct any change in carrier frequency

AURAL EXCITER

To increase the power level, audio power amplifiers are used. To up convert

the frequency level, frequency multipliers are used.

Turnstile antenna array is used to broadcast the TV signals from the

combining network.

The combining network is used to combine the audio and video signal,

without any interference between them

Page 27 of 27