Uint III Notes

7/28/2019 Uint III Notes

1/42

Television and Video Engineering

Unit-III Television Transmission system, Propagation And Antennas

Department Of ECE School Of Engineering1

VSA GROUP OF INSTITUTIONSVSA SCHOOL OF ENGINEERINGSALEM 10

DEPT OF ELECTRONICS AND COMMUNICATION ENGINEERING

LECTURE NOTES

TELEVISION AND VIDEO ENGINEERING. (VI I SEM / IV YEAR - ECE)

Compiled by

Mr.V.NAGARAJ, LECTURER / ECE

VSA School of Engineering

STAFF INCHARGE HOD/ECE PRINCIPAL


2/42




UNIT I I I

TELEVI SION TRANSMISSION SYSTEM , PROPAGATION AND

ANTENNAS

REQUIREMENTS OF TV Broadcast Transmission, Design

principle of TV Transmitters, IF Modulation, power output

stages- Block diagram of TV transmitters, visual exciter,- Aural

Exciter- Diplexer- Transmitting antennas-Radio wave

Characteristics- propagation phenomenaspace wave

propagation- Line of sight range- space wave reception oversmooth terrain- distance

reception- Shadow zones- co channel interference- Ghost

imagesinterference problems. Receiving antenna requirements,

characteristics and types- parasitic elements- Yagi aerials-

Feeders- matching- booster amplifiers.

CONTENT:


3/42




1.


4/42




Yagi aerials:

Yagi-Uda antenna:

The antenna widely used with television receivers for locations within 40 to 60 km from the

transmitter is the folded dipole with one reflector and one director. This is commonly known as

Yagi-Uda or simply Yagi antenna. The elements of its array as shown in Fig.(a) are arranged

collinearly and close together. This antenna provides a gain close to 7 db and is relatively

unidirectional as seen from its radiation pattern drawn in Fig. 9.18(b). These characteristics are

most suited for reception from television transmitters of moderate capacity. To avoid pickup

from any other side, the back lobe of the radiation pattern can be reduced by bringing the

radiators closer to each other. The resultant improvement in the front to back ratio of the signal

pick-up makes the antenna highly directional and thus can be oriented for efficient pick-up from

a particular station. However, bringing the radiators closer has the adverse effect of lowering

the input impedance of the array. The separation is shown in Fig. (a) is an optimum value.

Antenna length:

As mentioned earlier, it is not necessary to erect a separate antenna for each channel because the

resonant circuit formed by the antenna is of low Q (quality factor) and as such has a broad

band response. For the lower VHF channels (Band Ichannels 2 to 4) the length of the antenna

may be computed for a middle value. while this antenna will not give optimum results at other


5/42




frequencies, the reception will still be quite satisfactory in most cases if the stations are not

located far away.

Though the antenna length used should be as computed by the usual expression:

but in practice it is kept about 6 per cent less than the calculated value. This is necessary

because the self- capacitance of the antenna alters the current distribution at its ends. The smal

distance between the two quarter wave rods of the driver, where the lead-in line is connected

can be taken as too small and hence neglected. Note that this gap does not affect the current

distribution significantly.

Antenna mounting:

The receiving antenna is mounted horizontally for maximum pick-up from the transmitting

antenna. As stated earlier, horizontal polarization results in more signal strength, less reflection

and reduced ghost images. The antenna elements are normally made out of 1/4 (0.625 cm) to

1/2 (1.25 cm) diaaluminium pipes of suitable strength. The thickness of the pipe should be so

chosen that the antenna structure does not get bent or twisted during strong winds or occasional

sitting and flying off of birds. A hollow conductor is preferred because on account of skin effect,

most of the current flows over the outer surface of the conductor.

The antenna is mounted on a suitable structure at a height around 10 metres above the ground

level. This not only insulates it from the ground but results in induction of large signal strength

which is free from any interference.

The centre of the closed section of the half-wave folded dipole is a point of minimum

voltage, allowing direct mounting at this point to the grounded metal mast without shorting the

signal voltage. A necessary precaution while mounting the antenna is that it should be at least

two metres away from other antennas and large metal objects. In crowded city areas close to thetransmitter, the resultant signal strength from the antenna can sometimes be very low on account

of out of phase reflections from surrounding buildings.


6/42




In such situations, changing the antenna placement only about a metre horizontally or vertically

can make a big difference in the strength of the received signal, because of standing waves set

up in such areas that have large conductors nearby.

Similarly rotating the antenna can help minimize reception of reflected signals, therebyeliminating the appearance of ghost images. In areas where several stations are located nearby,

antenna rotators are used to turn its direction. These are operated by a motor drive to set the broad

side of the antenna for optimum reception from the desired station. However, in installations

where a rotating mechanism is not provided, it is a good practice to connect the antenna to the

receiver before it is fixed in place permanently and proceed as detailed below:

(i) Try changing the height of the antenna to obtain maximum signal strength.

(ii) Rotate the antenna to check against ghost images and reception of signals from far offstations.

(iii) When more than one station is to be received, the final placement must be a

compromise for optimum reception from all the stations in the area.

In extreme cases, it may be desirable to erect more than one antenna.

Sound signal transmission:

Audio signals from different microphones are received at the sound panel in the productioncontrol room. After due amplification all the outputs are fed into a switcher, where they are

mixed and desired output is selected.

The final output goes into the distributor in the master control room, where the picture and

signals are received. This distributor is switched to select corresponding picture and sound

signals from the desired studio.At the transmitter the audio signals are frequency modulated and

transferred to assigned sound carrier frequency by the use of multipliers. It is amplified through

power amplifier to raise the power output to desired level.

Picture signal and sound signal modulation in TV transmissions:


7/42




PICTURE SIGNAL:

A dc restorer circuit (DC clamp) is included before the modulator. Because of

modulation at a relatively low power level, an amplifier is used after the modulated RF amplifier

to raise the power level.

Accordingly this amplifier must be a class-B push pull linear RF amplifier

Both the modulator and the power amplifier sections of the transmitter employ specially

designed VHF triodes for VHF channel and klystrons in transmitters that operate in UHF

channels.

The modulated output is fed to a filter designed to filter out part of the lower side band

frequencies. This results in saving of band space. The filter output feeds into a combining

network where output from the FM sound transmitter is added to it.

This network is designed in such a way that while combining, either signal does not interfere

with the working of the other transmitter.

SOUND SIGNAL:

Audio signals from different microphones are received at the sound panel in the

production control room. After due amplification all the outputs are fed into a switcher, where

they are mixed and desired output is selected.

The final output goes into the distributor in the master control room, where the

picture and signals are received. This distributor is switched to select corresponding picture and

sound signals from the desired studio.

At the transmitter the audio signals are frequency modulated and transferred to

assigned sound carrier frequency by the use of multipliers. It is amplified through power

amplifier to raise the power output to desired level.


8/42




Operation of TV transmitter with neat block diagram:

A television transmitter consists of the visual transmitter for the picture and Aural

transmitter for sound.

Outputs of both the transmitters are connected to a common transmission line feeder to the

transmitting antenna through a combining unit called Diplexer.

Block diagram of Monochrome TV transmitter:

A TV transmitter transmits both audio and video signals using a channel bandwidth of 7 MHz

Two transmitters are employed. One for the picture transmission which is amplitude modulated

and uses a vestigial side band. Other is used for audio signal which is frequency modulated.

PICTURE SIGNAL MODULATION:

A dc restorer circuit (DC clamp) is included before the modulator. Because of

modulation at a relatively low power level, an amplifier is used after the modulated RF amplifier

to raise the power level.

Accordingly this amplifier must be a class-B push pull linear RF amplifier

Both the modulator and the power amplifier sections of the transmitter employ specially

designed VHF triodes for VHF channel and klystrons in transmitters that operate in UHF

channels.

The modulated output is fed to a filter designed to filter out part of the lower side

band frequencies. This results in saving of band space. The filter output feeds into a combining

network where output from the FM sound transmitter is added to it.

This network is designed in such a way that while combining, either signal does

not interfere with the working of the other transmitter.

SOUND SIGNAL MODULATION:

Audio signals from different microphones are received at the sound panel in the

production control room. After due amplification all the outputs are fed into a switcher, where

they are mixed and desired output is selected.


9/42




The final output goes into the distributor in the master control room, where the picture and

signals are received. This distributor is switched to select corresponding picture and sound

signals from the desired studio.

At the transmitter the audio signals are frequency modulated and transferred toassigned sound carrier frequency by the use of multipliers. It is amplified through power

amplifier to raise the power output to desired level.

A coaxial cable connects the combined output to the antenna system mounted on a

high tower situated close to the transmitter. A turnstile antenna array is used to radiate equal

power in all directions. The antenna is mounted horizontally for better signal to noise ratio.

Visual Exciter:

This visual exciter consists following,

1. Video processing unit2. Visual modulator3. VSB filter4. Phase compensator or delay equalizer5. Frequency converter.

Aural exciter:


10/42




The audio signal is given pre-emphasis by a high-pass network of time constant of

50s, and is amplified in the audio processing unit. It is then fed to crystal controlled

oscillator modulator for frequency modulation. The audio signal voltage is applied to a

varactor diode in parallel with the tuned circuit of an LC oscillator that generates a center

frequency equal to the sound IF carrier.

The center frequency of the LC oscillator is stabilized by an automatic frequency

control (AFC) circuit that compares this frequency with a crystal drived stable frequency.

Diplexer: Standard practice in television transmission is to combine the outputs of both

the visual and aural transmitters by means of a diplexer unit for feeding a common

broadband transmitting antenna system.

Constant impedance notch (CIN) is the commonly used diplexer.

Need for notch filter and delay line in the path of the Y signal:


11/42





12/42




Shadow zones:


13/42





14/42





15/42




Visualexciter:


16/42





17/42




Antenna feeding systems:


18/42





19/42





20/42





21/42





22/42





23/42





24/42




Impedance matching and quarter wave transformer act as a impedance

matching device:


25/42





26/42





27/42





28/42





29/42




Fundamentals of digital TV transmission and reception:


30/42





31/42





32/42





33/42




Interference suffered by TV signal

In addition to thermal and man-made noise, the carrier signal must compete with

various other forms of interfering signals originating from other television stations, radio

transmitters, industrial radiating devices and TV receivers. When the interfering signal has a

frequency that lies within the channel to which a TV receiver is tuned, the extent of

interference depends only on the relative field strengths of the desired signal and the

interfering signal. If the interfering signal frequency spectrum lies outside the desired

channel, selectivity of the receiver aids in rejecting the interference.

a)Co-Channel interference:


34/42




Two stations operating at the same carrier frequency, if located close by, will interfere with

each other. This phenomenon which is common in fringe areas is called co-channel

interference. As the two signal strengths in any area almost equidistant from the two co-channel

stations

become equal, a phenomenon known as venetian-blind interference occurs. This takes the

form of horizontal black and white bars, superimposed on the picture produced by the tuned

channel. These bars tend to move up or down on the screen. As the strength of the interfering

signal increases, the bars become more prominent, until at a signal-to-interference ratio of 45 db

or so, the interference becomes intolerable. The horizontal bars are a visible indication of the

beat frequency between the two interfering carriers. Figure 9.3 shows the bar pattern that

appears on the screen. The frequency of the beat note, which is equal to frequency separation

between the two carriers, is usually of the order of a few hertz. This is so because most

transmitters operate almost at the correct assigned frequencies. Motion of the bars, upwards or

downwards occurs whenever the beat frequency is not an exact multiple of the field frequency

Co-channel interference was a serious problem in early days of TV transmission, when thechannel allocation was confined to VHF band only. This necessitated the repetition of channels

at distances not too far from each other. Now, when a large number of channels in the UHF

band are available such a problem does not exist. The sharing of channel numbers is carefully

planned so that within the service area of any station, signals from the distant stations under

normal conditions of reception are so weak as to be imperceptible. However, during a period of


35/42




abnormal reception conditions (often during spring) when the signals from distant VHF stations

are received much more strongly, co-channel interference can occur in fringe areas. The use of

highly directional antennas is very helpful in eliminating co-channel interference.

b) Adjacent channel interference:

Stations located close by and occupying adjacent channels, present a different interference

problem. Adjacent channel interference (see Fig. 8.5) may occur as a result of beats between any

two of these frequencies or between a carrier and any sidebands. A coarse dot structure is

produced on the screen if picture carrier of the desired channel beats with sound carrier of the

lower adjacent channel. The beat pattern is more pronounced if the lower adjacent sound carrier

is relatively strong and is not sufftciently attenuated in the receiver.

The next most prominent source of interference is the one produced by picture sidebandcomponents of the upper adjacent channel. The beat frequency between adjacent picture carrier

is 7 MHz. Since this is far beyond the video frequency range, the resultant beat pattern is not

discernible. However, the picture sidebands of the upper adjacent channel may beat with the

desired channel carrier and produce an interfering image. To prevent adjacent channel

interference, several sharply tuned band eliminator filters (trap circuits) are provided in the IF

section of the receiver. This was explained in Chapter 8 while discussing desired IF response

characteristics of the receiver. In addition to this, the guard band between two adjacent channels

also minimizes the intensity of any adjacent channel interference. A space of about 150 km

between adjacent channel stations is enough to eliminate such interference and is normally

allowed.

c) Ghost interference:

Ghost interference arises as a result of discrete reflections of the signal from the surface of

buildings, bridges, hills, towers etc. Figure shows paths of direct and reflected electromagnetic

waves from the transmitter to the receiver. Since reflected path is longer than the direct path, the

reflected signal takes a longer time to arrive at the receiver.


36/42




The direct signal is usually stronger and assumes control of the synchronizing circuitry

and so the picture, due to the reflected signal that arrives late, appears displaced to the right.

Such displaced pictures are known as trailing ghost pictures. On rare occasions, direct signal

may be the weaker of the two and the receiver synchronization is now controlled by the reflectedsignal. Then the ghost picture, now caused by direct signal, appears displaced to the left and is

known as leading ghost picture. Figure shows formation of trailing and leading ghost pictures

on the receiver screen.

The general term for the propagation condition which causes ghost pictures is multipath

transmission. Ghost pictures are particularly annoying when the relative strengths of the two

signals, vary such, that first one and then the other assume control of the receiver synchronism

In such cases the ghost image switches over from a leading condition to a trailing one or vice

versa at a very fast rate. The effect of such reflected signals (ghost images) can be minimized by

using directional antennas and by locating them at suitable places on top of the buildings.


37/42




TERMS&DEFINITION:

Advantages of IF modulation:

The main function of this section is to amplify modulated IF signal over

its entire bandwidth with an input of about 0.5mV signal from the mixer to deliver about

4V into the video detector.

IF section is used to equalize amplitudes of sideband components , because of vestigia

side band transmission.

IF section is used to reject the signals from adjacent channels.

Interference problems in TV reception

a) Man-made impulsive interference

b) RF interference

c) Co-channel interference

d) Atmospheric noise interference

Differentiate Co-Channel interference and adjacent channel interference

Co-Channel Interference Adjacent Channel Interference

Two channels operating at thesame frequency are called as

Co-Channel stations

If the co channel stations arelocated close to each other,

then they interfere with each

other. This is known as cochannel interference

Stations located nearby andoccupying adjacent channels

are known as adjacent channel

stations

The inference caused due tothe beat between these two

frequencies or between a

carrier and any other side band

is called adjacent channelinterference


38/42




Preferring horizontal polarization for television receiving antenna?

We prefer horizontal polarization for television receiver antenna because it

results in more signal strength, less reflection and reduced ghost images.

Indoor receiver antennas:

In strong signal areas it is sometimes feasible to use indoor antenna provided the

receiver is sufficiently sensitive.

Yagi antenna:

A yagi antenna with a large number of directors is commonly used with

success in fringe areas for stations in the vhf band.

Diplexer:

The outputs of both the video and the audio transmitter are

combined by the diplexer circuit and given to a common broadcast

transmitting antenna.

Co-channel interference:

If two stations are operating at the same carrier frequency and located

nearby then they will interface with each other. This is called co-cannel

interference and it is common in fringe areas.

Adjacent channels:

It is the interference due to stations located nearby and allocated as

adjacent channels.

Guard band:

Guard band can be defined as a small frequency band introduced

between two consecutive channels in order to reduce interference .


39/42




Ghost interference:

Ghost interference arises as a result of discrete reflections of the signal

from the surface of hills, bridges, buildings, towers etc.

Requirements of high level modulation:

In high level modulation, the video signal has to be modulated by

the picture carrier in the final power amplifier which has a high power

level. Grid bias modulation is employed.

Ground waves:

Vertically polarized electromagnetic waves are radiated at zero or

small angles with ground. They are guided by the conducting surface of

the ground along which they are propagated. Such waves are known as

ground or surface waves. As the ground waves travel along the surface of

the earth, their attenuation is proportional to frequency. The attenuation is

reasonably low below 1500KHz.Therefore, all medium wave broadcast and long

wave telegraph and telephone communication is carried out by ground wave

propagation.

Sky waves:

In ground wave propagation ,frequencies above 1600 KHz does not serve

any useful purpose as the signal gets very much attenuated within a short distance of its

transmission .Therefore ,most radio communication in short wave bands upto 30MHz is

carried out by sky waves. When these waves are transmitted high up in the sky , theytravel in the straight line until the ionosphere is reached. This region begins about 120Km

above the surface of the earth. The region consists of large concentrations of charge

gaseous ions, free electrons and neutral molecules. The ions and free electrons cause to

band all passing electromagnetic waves.


40/42




Space wave propagation.

Propagation of radio waves above about 40MHz is not possible through

either sky wave or surface wave propagation .Therefore ,the only alternative for

transmission in the VHF and UHF bands, despite large attenuation is by radio

waves which travel in a straight line from transmitter to receiver. This called

space wave propagation.

AM preferred over FM broadcasting the picture signal:

If FM is adopted for picture transmission ,the changing beat frequency

between the multiple paths delayed with respect to each other would develop a

bar interference in the image with a shimmering effect as the bars continuously

changes as the beat frequency changes therefore ,no study picture is

produced. Apart from that ,circuit complexity and BW requirements are much less

in AM than FM. Hence AM is preferred to FM for broadcasting the picture signal.

Dipole array:

Dipole antenna is used for band I&III transmitters. It consists of dipole

Panels mounted on the four sides at the top of the antenna tower. Each panel has an array

of full wave dipoles mounted in front of reflectors. To get an unidirectional pattern, the

four panels mounted on the four sides of the tower are so fed that the current in each lags

the two alternate panels and by reversal of polarity of the current.

Image rejection ratio:

Image rejection ratio is defined as the output due to desired station divided

by output due to image signal.


41/42




Yagiuda Antenna:

This is a widely used antenna for television receivers .Generally ,for

locations within 40 to 60Km from the transmitter is the folded dipole with one

reflector and one director. This is commonly called Yagi antenna or Yagi-Uda

antenna.

Essential parts of TV transmitter:

The essential parts of TV transmitter includes a video processing unit . A

Visual modulator which is a diode bridge modulator, phase compensator or delay

Equalizer and frequency converter.

LOS range:

The shortest distance between the transmitting and receiving antenna is known as LOS.

Venetian blind interference:

If two stations are operating at the same carrier frequency and located

Nearby then they will interface with each other. This interference may take the form of

regular horizontal bars moving up and down the picture is known as venetian blindinterference.

Types of interference problems:

1. Man-made impulsive interference

2. RF interference

3. Co-channel interference

4. Atmospheric noise interference


42/42



Elliptical polarization:

The electric vector may rotate at the angular frequency of the wave about the line

of propagation, the tip of the vector tracing an ellipse, in which case it is referred to as

Elliptical polarization.

Ghost images:

The transmitting signal gets reflected by large objects and arriving a little later with

respect to the direct ray, due to path difference, produces an additional image displayed

horizontally by a distance proportional to the time delay between the two paths.

Antenna gain:

The increase in the signal received by an antenna with respect to a reference antenna

usually the /2 dipole antenna, is termed as antenna gain.

Types of antenna feeders:

Twin wire feeders

Coaxial cable feeder

parasitic element:

The rod elements placed on either side of a dipole antenna and parallel with it can

make the dipole unidirectional in response. These rod elements that have no electricalconnection as such are called parasitic elements.

Uint III Notes

Documents

Transcript of Uint III Notes