Training Report on radio broadcasting

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RADIO BROADCASTING

A PROJECT REPORT

Submitted by

ANUJ DHIMAN

In partial fulfilment of the award of the degree

Of

BACHELOR OF TECHNOLOGY

in

ELECTRONICS AND COMMUNICATION ENGINEERING

INSTITUTE OF ENGINEERING AND EMERGING TECHNOLOGIES

BADDI

HIMACHAL PRADESH UNIVERSITY

JUNE-JULY 2010


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CERTIFICATE

Certified that this project report RADIO BROADCASTING is the work of ANUJ

DHIMAN who carried out the project work under my supervision.

SIGNATURE SIGNATURE

HEAD OF THE DEPARTMENT SUPERVISOR


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ACKNOWLEDGEMENT

This project involved the collection and analysis of information from a wide variety of

sources and the efforts of many people beyond me. Thus it would not have been possible to

achieve the results reported in this document without their help, support and encouragement.

I will like to express my gratitude to the following people for their help in the work leading

to this report:

Engg. Head SH. P.S. CHAUHAN(Asst Engineer) and SH. JATINDER GUPTA (Asst.

Engineer)

Er. VIKRAM CHAUHAN, Er. SUMAN KANT and Er. JITENDRA KUMAR YADAV for

their useful comments on the subject matter and for the knowledge I gained by sharing ideas

with them.


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ABSTRACT

Radio broadcasting is considered as powerful way of mass communication. Radio has grown

very quickly. The biggest organisation of RADIO BROADCASTING in INDIA is ALL

INDIA RADIO. Radio has its own special characteristics. Its vast reach covers almost the

entire country, and it has a big audience.

All India Radio has the latest technology used in the field of Radio Broadcasting. We enjoy

the radio programmes and but there is a lot of engineering involved in the back end of that

programme.

First of all the programme is generated in the studio and then passed to the control room,

there it is processed, modulated and then uplinked and after this we receive the programme.

RADIO is the transmission of signals by modulation of electromagnetic waves with

frequencies below those of visible light. Electromagnetic radiation travels by means of

oscillating electromagnetic fields that pass through the air. Information is carried by

amplitude, phase or pulse modulation techniques. And at the receiving end it is again

decoded into the actual signal.


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TABLE OF CONTENTS

CERTIFICATE......................................................................................................................2

ACKNOWLEDGEMENT.....................................................................................................3

ABSTRACT............................................................................................................................4

TABLE OF CONTENTS.......................................................................................................5

1. INTRODUCTION............................................................................................................6

2. STUDIO SETUP..............................................................................................................8

2.1. STUDIO CHAIN........................................................................................................8

2.2. OUTSIDE BROADCASTING...................................................................................20

3. TYPES OF AUDIO..........................................................................................................25

3.1. MONO........................................................................................................................26

3.2. DUAL MONO...........................................................................................................26

3.3. STEREO.....................................................................................................................26

3.4. SURROUND SOUND...............................................................................................26

4. NEED FOR MODULATION..........................................................................................26

5. TYPES OF MODULATION...........................................................................................26

5.1. AMPLITUDE MODULATION..................................................................................26

5.2. ANGLE MODULATION...........................................................................................27

5.3. PULSE MODULATION............................................................................................29

6. CAPTIVE EARTH STATION........................................................................................35

7. TRANSMITTER..............................................................................................................38


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8. REFRENCES....................................................................................................................51

1. INTRODUCTION

RADIO BROADCASTING is a media of mass communication. Radio Broadcasting is

the one of the earliest way of the mass communication. Radio systems used for

communications will have the following elements. With more than 100 years of

development, each process is implemented by a wide range of methods, specialized for

different communications purposes. Each system contains a TRANSMITTER. This

consists of a source of electrical energy, producing alternating current of a desired

frequency of oscillation. The transmitter contains a system to modulate some property

of the energy produced to impress a signal on it. This modulation might be as simple as

turning the energy on and off, or altering more properties such as amplitude, frequency,

phase, or combinations of these properties. The transmitter sends the modulated

electrical energy to a tuned resonant antenna; this structure converts the rapidly

changing alternating current into an electromagnetic wave that can move through free

space. Electromagnetic waves travel through space either directly, or have their path

altered by reflection, refraction or diffraction. Noise will generally alter the desired

signal; this electromagnetic interference comes from natural sources, as well as from

artificial sources such as other transmitters and accidental radiators. Noise is also

produced at every step due to the inherent properties of the devices used. If the

magnitude of the noise is large enough, the desired signal will no longer be discernible;

this is the fundamental limit to the range of radio communications.

The electromagnetic wave is intercepted by a tuned receiving antenna; this structure

captures some of the energy of the wave and returns it to the form of oscillating

electrical currents. At the receiver, these currents are demodulated, which is


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conversion to a usable signal form by a detector sub-system. The receiver is

"tuned" to respond preferentially to the desired signals, and reject undesired signals.

Early radio systems relied entirely on the energy collected by an antenna to produce

signals for the operator. Radio became more useful after the invention of electronic

devices such as the vacuum tube and later the transistor, which made it possible to

amplify weak signals. Today radio systems are used for applications from walkie-talkie

children's toys to the control of space vehicles, as well as for broadcasting, and many

other applications.

2 .STUDIO SETUP

A broadcasting studio is a room in studio complex which has been specially designed and

constructed to serve the purpose of originating broadcasting programs. Whenever any

musician sings and we sit in front of a performing musician to listen to him, we enjoy the

program by virtue of the superb qualities of our sensory organs namely ears. However,

when we listen to the same program over the broadcast chain at our home though

domestic receivers, the conditions are entirely different. Broadcasters are continuously

engaged in the task of ensuring the maximum pleasure for the listener at home when the

artists are performing inside the studios. The science of sound is often called Acoustics.

2.1. STUDIO CHAIN

A STUDIO CHAIN represents how the data transfer takes place from one place to

another place in a studio and how it is transmitted. First of all the programmes are

generated in the different studios with the use of tape deck recorders, computers and

CD players and then that audio signal is passed to the control room. There it is

processed and amplified at different levels. There is a switching console in the control

room which decides which programme to be transmitted. After such processes the

signal is passed to the transmitter. And then after transmission we receive the

programme.


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The

science of sound is often called Acoustics. It would be thus prudent to understandthe field of acoustics as applied to broadcasting.

ACOUSTIC TREATMENT

Good acoustics is a pre-requisite of high quality broadcasting or recording.

Acoustic treatment is provided in studios, control rooms, and other technical

areas in order to achieve the acoustic conditions which have been found fromexperience to be suitable for the various types of programmes. In this section

problems and design aspects of internal acoustics of a broadcast studio are

explained.

A) PROPAGATION OF SOUND WAVES

Sound waves emanating from a sound source are propagated in all

directions. These sound waves are subject to reflection, absorption and

refraction on encountering an obstacle. Extent to which each of these

phenomenon takes place depends upon the structure and shape of theobstacle, and also on the frequency of sound waves. In close rooms, the

sound would be reflected and re-reflected till the intensity weakens and it

dies down.

Physical characteristics of sound waves are thus modified in various ways

before they reach the human ear. These reflected waves can create echo

effect in the room. To achieve the desirable effects of the reflected

sound, the dimensions and shape of the room are decided with due care

and acoustic treatments are also provided on the various surfaces.

b) REVERBERATION TIME(R/T)


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In any enclosed room when a sound is switched off, it takes a finite

length of time to decay to inaudibility.

The hanging-on of the sound in a room after the exciting signal has

been removed, is called reverberation and the time taken for the sound

to decay to one millionth of its initial value, i.e. 60 dB, after the source

has stopped, is termed Reverberation Time(R/T).

c) FACTOR COVERING REVERBERATION TIME

R/T of a room depends upon shape and size of room and on the total

absorption offered on boundary surfaces.

For a room of given volume and surface area, the R/T can be derived by

Eyrings formula

)1(lnS

V049.0T/R

=

where R/T = Reverberation time in seconds

V = Volume in cubic ft.

S = Total surface area of room in Sq.ft.

= Average absorption coefficient

Average absorption coefficient ( ) is given by

n21

nn2211

S.......SS

S.........SS

+++

+++=

Where S1, S2.Sn are the areas (in sq. ft.) of different materials

provided, and 1 , 2 n are the absorption coefficients of these

materials. of acoustic material is defined as the ratio of absorbed sound

to the total incident energy of sound. An open window absorbs/allows to

pass all of the sound energy striking it and reflects none. Thus it has of

unity.

of practically all acoustic materials vary with frequency.

d) EFFECTS OF REVERBERATION ON PROGRAMME

Reverberation is the most important single parameter of a room. Itinfluences the audio programs in following ways:-


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Volume of program increases due to reverberation of sound. This

is a desirable feature, however, too much of reverberation may

impair the quality of program and, therefore, should be controlled.

Reverberation results in prolongation of sound inside the room.

This leads to blending of one sound with the next and produces a

very pleasant continuity in the flow of music. Too much of

prolongation, however, may create loss in intelligibility of programdue to decrease in clarity.

Reverberation time of a room is dependent on frequency.

Therefore, it modifies the frequency characteristics of the total

sound field inside the room. High R/T at mid and high frequencies

lead to increased liveness and that at low frequencies increases

warmth. This effect can be used judiciously for desirable

qualities.

e) OPTIMUM REVERBERATION TIME

R/T value at each frequency of sound is fixed for most desirable

results for different type of programmes .Larger the room size the longer

it takes for the sound to travel to the boundary surfaces and get reflected.

Therefore, optimum R/T increases with the increase in the room size.

Generally Morris & Nixons curve (Fig. 1) is followed for optimum R/T

at 1 kHz as a function of room size.

Fig. Reverberation Time vs. Volume


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Optimum R/T values at other audio frequencies are dependent mainly on

the type of programme for which the studio will be used. These values

have been decided after detailed study and subjective listening tests.

Optimum R/T for talk studio is generally flat, whereas for music, studio,

Morris & Nixons recommendations are followed in AIR. For drama

programmes, the optimum R/T is taken as an average of talks and music

values at each frequency.

Fig. Recommendation MORRIS & NIXON

F) ACOUSTIC ABSORBERS

Acoustic absorbers are provided on the inner surfaces of the room to

achieve optimum R/T characteristics. Different absorbers have different

absorption characteristics. No single absorber generally provides uniform

absorption over the complete frequency spectrum. Some of the commonly

used absorbers are:

i. Porous Materials: Mineral wool, glass wool, etc. are members

of this class. These materials are very good absorber and aremost effective in mid and high frequencies, however, these

cannot be used without some facing material.

Carpets and curtains also fall in this category.

ii. Fibrous Materials: Celotak, insulation boards, perfotiles, jolly-

lowtone tiles etc. fall in this category. Absorption of these

materials depends upon their softness. Absorption efficiency of

these materials depends upon the trapping and dissipation of

sound energy in tiny pores. Absorption gets reduced if the

surface pores are filled with paints etc.These materials have very poor absorption on low frequencies.

However appreciable improvement at these frequencies is

possible by providing air-gap behind.


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iii. Panel Absorbers: Panel absorbers are thin sheets/membranes

with an air cavity behind. The mass of the panel and the

springiness of the air in the cavity resonant at some particular

frequency.

Panel absorbers with 3mm teak ply-facing + 50mm air gap +

25mm mineral wool resonate at about 125Hz. This is generally

used as low frequency absorber (LFA).

iv. Perforated Panel Absorbers: Perforated hardboard (PHB)

spaced from the wall constitute a resonant type of sound

absorber. The absorption can be considerably enhanced by

inserting suitable porous/fibrous damping materials in the air

cavity.

The absorption pattern can be varied by adjusting the front and

rear air gap from the damping material. Absorption coefficient

of this absorber depends on the percentage open area ofPHBs also.

G) DESIGN OF ROOM ACOUSTIC

Design for correct reverberation time consists of estimating the total

absorption which must be present in the studio. This is calculated by

Eyrings Formula, some of the absorption is offered by windows, doors,

flooring and artists inside the studio. For the balance requirement sound

absorbing materials are provided on walls and ceiling surfaces. Calculationsare generally made at six spot frequencies of 125, 250, 500, 1000, 2000 and

4000 Hz. Quantities of materials of known absorption coefficients are

selected by trial and error method so that R/T requirements are met within

+5% of the optimum R/T at all these frequencies. Computer aided design for

the same has also been evolved. Thereafter these acoustic materials are

distributed on various surfaces for proper diffusion of sound in the studio.

Typical acoustic treatment for a studio is given in Appendix.

After completion of acoustic installation as per the theoretical design, R/T

measurements are carried out and if the achieved R/T figures are found to be

very much different than the designed values, then acoustic corrections are

also applied.

SOUND INSULATION

The unwanted sound or noise in the studios spoils the quality of recorded

programmes. Sound insulation of walls doors etc. and layout of the studio building is

therefore, decided for acceptable background noise level in the studios.


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A) ACCEPTABLE BACKGROUND NOISE LEVEL

It is not possible to specify an acceptable background noise level in the

studios as a single weighted figure, because the noise normally present is

spread over a wide range. An excessive noise energy over a small

bandwidth could be very disturbing without very much affecting the

weighted noise figure. Therefore, the acceptable background noise level

is specified as a graph of band level in octave bands against frequency,

usually over the range 68 Hz to 4 kHz. These acceptable limits have

varied widely between different authorities. In AIR NC 20 curve is

followed for studios (Refer Figure 3 for NC Curve), which corresponds to

following values.

Frequency Band (Hz) Noise Level (db above

0.002 dynes/cm2)

37.75.1 54

75.150.1 43

150.300.1 35

300.600.1 28

600.1200.1 23

1200.2400.1 20

2400.4800.1 17

4800-9600 10

Fig. Noise Criteria Curve

B) SOURCE OF NOISE AND SOUND INSULATION

Noise in studios may be either air-borne or structure borne. Background

noise in a studio can originate from


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Outside the building

Inside the studio itself and /or

Outside the studio but within the building

C) NOISE ORIGINATED FROM OUTSIDE THE

BUILDING

Noise from outside the studio building are mostly due to aircraft, roadand rail traffic etc.These noise can be avoided/minimised by locating the

studio building in a quiet environment away from the railway lines

highways and aerodromes. In case studio centre is located in noisy street,

sufficient set-back distance is provided between the street kerb and the

main building. Sometimes a multi-storeyed office building is built in

between the studio building and the sound source to act as a sound barrier

for the studio building.

D) NOISE FROM INSIDE THE STUDIO

Noise from inside the studio itself consist of air-conditioning noise due to

air flow, the noise from fluorescent lights, from cooling fans in tape

recorders etc.

Noise due to airflow in the studios is controlled by creating slow

diffusions of air.

To avoid noise of fluorescent lights, ballast chokes are not mounted with

the light fittings in the studio. These are mounted separately in a ballast

nitch outside the studio.

Cooling fans in tape recorders are generally of low noise type.

E) CONTROL OF AIR-CONDITIONING AND DIESEL

GENERATOR AND LIFT NOISE

Noise due to air-conditioning plants can transfer to the studios as

structural borne noise as well as air borne noise. The structural bornenoise is avoided by providing the a.c. plants in a separate block isolated

from the main studio mook. A structural isolation gap of 75 mm width

right from foundation level up to the roof height is provided between the

two blocks. This gap is filled with damping materials, such as asphalt, to

avoid bridging by stone, cement mortar etc. Wherever required, only

flexible connections are used for linking these blocks for running

electrical cables, duct etc.

These plants are mounted on vibration isolation pads and water pipes for

condenser cooling are also isolated from the walls with resilient packing

materials so that transmission of the vibration to the building is


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avoided.To avoid transferred structural vibration through ducts, the main

supply and return ducts from the plants are connected to the studio ducts

through flexible canvass connection.

To avoid transfer of airborne noise from the a.c. plants, the plenum

chamber and the entire length of supply/return duct is insulated internally

with sound absorbing materials e.g. glass wool. Also speed of the bloweris kept low (about 750 rpm) as the noise at source itself is controlled.

Similarly diesel generator is either installed in this structurally isolated

block or in a separate building away from the studio. The generator is

mounted on anti-vibration mounting so that vibration due to the same is

minimised in the structure.

F) SOUND INSULATION FROM FOOTFALL, DRAGGING OF

FURNITURE ETC.

Noise due to footfall, dragging of furniture, falling of paper weight etc. is

transmitted at long distance as structure borne noise. Transmission of

this noise is much more in steel framed buildings than in load bearing

structure. Therefore, studios are generally made in load bearing single

storied buildings.

In case of steel-framed building and/or multi-storeyed buildings, floating

construction i.e. box within the box is recommended for broadcastingstudios.

G) SOUND INSULATION FROM ADJACENT ROOM/CORRIDOR

NOISE

High level of programme/ monitoring in adjacent rooms and conversation

in corridors may cause leakage of this sound in a studio. This leakage

may be due to poor sound insulation of intervening walls or due to

flanking paths.

Sound insulation of a single solid wall (generally known as transmission

loss, TL) against airborne noise is determined by its mass per unit area.

TL of a 115 mm brick wall, plastered on both sides, is 45 dB. A 225 mm

plastered wall has a TL of 50 dB, which is a very poor return for the extra

mass. Though the TL figures are much better for cavity walls (with air

gaps), however, their construction is very difficult. Therefore these

cavity walls are avoided in AIR, all studio walls have been standardised

as 340 mm thick. Additional insulation, whenever required, is achievedby proper positioning of various sources of noises (at the planning stage)

so that either the high level studio/room is not very close to another


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studio or by providing a buffer room (such as musical instruments room,

store room etc.) between the high level source and the studio.

TL of a specially designed sound proof door is about 30 to 35 dB only.

This is much less than that of a 300 mm wall. Therefore, a sound lock,

heavily treated, is placed at the entrance of the studio so that corridor

noises do not leak to the studio through the entrance door. Similarly theobservation windows are constructed with double glass so that TL of the

wall is not reduced with the provision of this window.

Leakage of sound in a studio may be through cracks in walls, holes made

for running ducts etc. and/or through a.c. ducts and conduits.

To avoid leakage through these flanking paths, all the partition walls in

the studios are erected up to the real ceiling height. Walls are plastered

on both sides without any crevices/gaps. Holes made in walls for a.c.ducts are closed tightly by ramming high density mineral wool into the

hole and applying a layer of plaster to the outer faces. Similarly all holes

made for running conduits are sealed properly. To avoid leakage of

sound through the a.c. ducts, the layout of ducts is decided judiciously

and all the ducts (supply as well as return) are lined internally with

mineral wool after running cables.

CONCLUSION

It hardly needs to be over-emphasised that broadcasting studios should be

free from noise and be designed for optimum R/T requirements. For

international exchange of programmes, it is essential that the condition of

noise and acoustics are as per international standards. These

requirements are duly taken care of at the design and installation stage,

however sufficient precautions should be taken during maintenance i.e.

painting etc. and/or at the stage of making any additions/changes in the

studios so that these characteristics are not altered.


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2.2 OUTSIDE BROADCASTING

Introduction

Outside Broadcasts (abbreviated as OBs) form a substantial portion of

programmes radiated from a Radio Station. Major events that occur at different

parts of a country, such as sports events, important functions of political,

cultural and national important and other such programmes that originate fromoutside the broadcast studio are covered as OBs.


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Different Types of OBs

OBs can be classified into two types :

i) Live Broadcast

Events of national importance such as Independence Day Celebrations, sports

events etc. are generally radiated as Live programme.

ii) Spot Recordings

Most of the OB programmes are recorded at the OB spot with the help of a

portable, battery operated OB amplifier and or an Ultra Portable Tape Recorder

(UPTR) or a cassette tape recorder. Some programmes, depending on their

importance are recorded at the studio end. In this case, it is necessary to book

telephone lines, from the OB spot to CR. Normally three such lines are booked.

One for feeding the programme to CR, one for inter communication betweenthe OB spot and CR using a magneto telephone, and one as a standby

programme line.

Equipments Normally used in OBs

i) OB Amplifier

An OB amplifier is a portable mixing unit. Normally four low level

microphone inputs and one high level input from a PTR or UPTR, can be

mixed and controlled by this unit. The individual channel output levels as well

as the level of the programme after mixing can be controlled by rotary step

attenuators.

A tone generator providing three spot frequencies (75 Hz, 750 Hz or 1 kHz, 7.5

kHz) is incorporated in this unit so that the frequency response of the telephone

line on which the programme is fed can be quickly checked at CR end andequalisation done, if found necessary.

The auxiliary output can be used for random operation or for feeding a public

address system. Thus two OB amplifiers can be cascaded, and nine programme

sources can be controlled. A portable mixer has recently been developed by

M/s Meltron which can be used with Nagra or Meltron UPTRs. This mixer

enables use of three microphones and has a high level input. The main featureof this mixer is that it is of light weight and takes power supply from UPTR

itself.


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ii) Microphones

The choice of the correct type of microphone and its proper handling and

placement is very important for the success of an OB. The microphones used in

OBs must be robust, insensitive to wind noise and popping effects, and having a

good front to back ratio to avoid feedback. Hence, when choosing a

microphone, for OB operations the directional characteristics of the

microphones should be considered carefully. Suitability of differentmicrophones for OB recording is discussed below.

Omni directional Microphones

Omni-directional microphones are sensitive to sound from all directions equally

and hence they are preferred in studio recordings. But dynamic cardiod

microphones are better suited for OB recordings.

Short Gun Microphones

In OB situations such as cricket test match or athletics coverages, the sound is

to be picked up from a distance and hence we require a microphone with a

narrow acceptance angle. Gun microphones are used on such occasions. Its

constructional structure is such that all sounds other than those from the wanted

direction, arrive in such a manner as to produce a very low output from the

microphone. Hence, shot-gun microphones are used when the microphone must

remain at some distance from the sound or good rejection of sound from the

sides and rear is desired.

Radio/Wireless Microphones

In sports coverages, there may be situations such as in a big stadium where

different athletic events take place simultaneously where it is not possible to

lay cables. Radio microphones are best suited for these locations. In a radio

microphone, the microphone is connected to a miniature FM transmitter (held in

hand) and the audio is picked up from the demodulator output of a FM receiver.


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Such radio microphones are used in locations where long cable distances are

involved or where it is not possible to lay the cable.

Use of Wind shields

When microphones are used out of doors, in windly conditions, wind shields are

used. But wind shields tend to have adverse effect upon the frequency and

directional response of the microphones. Hence, they should be selected with

care, and used only when necessary and suitable corrections are to be made to

the frequency response and operational techniques.

iii) Tape Recorders

Spot interview and glimpses of the various events taking place in a particular

city, are covered by spot recordings done with Ultra Portable Tape Recorders

(UPTRs) and cassette tape recorders. They are light weight battery operated

recorders and are provided with only headphone monitoring facility in order to

avoid the drain on the batteries. Generally two sets of either dry cells or

chargeable cells are taken for the OB recordings, so that atleast 30 minutes of

recorded programme is made feasible. Major studio centres such as the BH,

New Delhi are provided with a number of such UPTRs and cassette tape

recorders so that more than twenty different event can be covered with the help

of such UPTRs. The recorded tapes are brought back to the BH, and a

composite news capsule is made with the help of console tape recorders, in the

dubbing room. The edited programme is used in the programmes such as Radio

News Reel, Agricultural Programmes, special features etc.

Important Guidelines for coverage of OBs

Cassette tape recorders in our network are not of uniform quality.Each cassette recorder should be thoroughly tested for satisfactory

quality before sending it for OB recording.

For VIP recordings, Portable tape Recorders (PTRs) are used. A PTR

is mains operated, provides good quality and is also sturdy enough to

withstand continuous operation. PTRs can also be taken to those OB

spots where AC power supply is available. It is preferable to take a

variance to take care of power supply voltage fluctuations.

Where more than one microphone is needed for an OB, proper

phasing, correct placement, proper balancing and mixing of the


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microphones is essential to get the desired sound quality.

Microphones should be so located as to avoid direct pick up from

strong external noise sources e.g. public address loudspeakers,

generators, viscinity of heavy traffic etc. Strong sources of RF

radiation in the viscidity of the equipment will also adversely affect

the quality. The commentators mike should be unidirectional and

should have minimum possible pick-up from external sound sources.It should also be kept away from sound reflecting surfaces.

The connecting cables for microphones and for other equipment

should be carefully laid so that these do not get disturbed during the

progress of the OB. Spare cables should be provided wherever

possible. If the effects microphones are at considerable distance from

the equipments, these may be connected through battery operated

booster amplifiers located near the mikes.

The equipment should be set-up well before the start of the OB and

the entire chain from the microphone to the receiving end should be

thoroughly tested for reliability and satisfactory sound quality. The

equipment should not be disturbed after testing and any last minute

changes and adjustment must be avoided.

For the coverage of various functions and sports events etc. it is

essential to provide adequate sound effects. If the sound effects are

not available in the background of the running commentary, it

becomes an extremely dull coverage, uninteresting to the listeners.

The engineer on duty should ensure that the sound effect do not

override the main commentary and proper balance is maintained.

Proper selection of the microphones for coverage of an OB is very

important. Apart from good quality, the microphone should be rugged

and capable of withstanding transit hazard. It should always be

carried in a proper case to avoid damage due to improper handling.

It is always essential to take standby equipment, spare batteries, spare

components and essential tools for the coverage of an OB. A portable

battery operated receiver should also be taken for monitoring

purposes.

OB Van

AIR has acquired a few OB Vans recently. The vans are of the size of a

matador vehicle and incorporate equipment of latest technology. Each

van has been provided with a 6 channel audio mixer 3 UPTRs and a

Public Address Amplifier. The interior is acoustically treated and air-

conditioned. A portable diesel generator can be housed in the body. It is

possible to record talks and interview inside the van. All microphones

inputs are terminated on a panel and cable drums provided for laying ofthe cables for recording the outside programmes and placement of effects

mikes in the field. Provision is available to meet most of the

requirements of production, recording, editing and dubbing etc. The van


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can also meet the requirements of a live coverage. Provision will be kept

for installing a VHF/FM transmitter and a video camera along with a

monitor inside the van in case these are required for certain types of

coverage.

3. TYPES OF AUDIO

There are various types of audio which can be transmitted from a studio:

3.1MONO

3.2 DUAL MONO

3.3STEREO

3.4SURROUND SOUND

3.1MONO: In mono audio system all the audio is fed at the single channel. We

receive the same audio at all the speakers.

3.2DUAL MONO: In this system two separate speakers are used when signals are to

be fed. When we receive the signal there is stereo effect is produced.

3.3STEREO: In this system very good sound effect is produced. We can even

identify the different- different instruments sound when we are listening to music

in a stereo system.

3.4SURROUND SOUND: In surround sound there is a two or more speakers and a

woofer. A woofer amplifies the low frequency audio signals.

Before the transmission the audio signal is made that much capable by modulation

so that it can be transmitted easily and nicely.

4. NEED FOR MODULATION

Antenna size can be reduced by modulating the signal over higher frequency.

among transmissions (stations)Maximum to minimum frequency ratio can be reduced to minimum by modulating

the signal on a high frequency.


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5 .TYPES OF MODULATION

Modulation is of three types mainly:

i. AMPLITUDE MODULATION

ii. ANGLE MODULATION

iii. PULSE MODULATION

i. AMPLITUDE MODULATION

If the amplitude of the carrier is varied in accordance with the amplitude

of the modulating signal, it is called amplitude modulation. This

modulation has been shown in a figure below

o

o

o

o

E

EEm

E

EEm

ulationofDegreemmm

min

max

mod

=

=

===

+

+

EminEmax

E0

E0

0

RF Carrier

Modulating signal

0

0

AM signal

Fig . 1 Amplitude Modulation.

ii. ANGLE MODULATION

Variation of the angle of carrier signal with time results in angle

modulation. It is of two types:

a. FREQUENCY MODULATION

b. PHASE MODULATION

a. FREQUENCY MODULATION

The type of modulation in which the instantaneous frequency ofthe carrier is varied according to amplitude of modulating signal is

called frequency modulation. Frequency modulation is widely


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used in VHF communication systems e.g. FM broadcasting,

transmission of sound signal in TV, Satellite Communication etc.

Fig. 2 Frequency Modulated wave

The instantaneous frequency varies about the average frequency (carrier

frequency) at the rate of modulating frequency. The amount by which the

frequency varies away from the average frequency (carrier frequency) is called

frequency deviation and is proportional to the amplitude of the modulating

signal.

b. PHASE MODULATION

If the Phase of the carrier is varied in accordance with the

amplitude of the modulating signal (information), it is called phase

modulation.

Analysis of Phase Modulation Signal

Let carrier,

)twCosE)t(V ccc =


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Modulating Signal

tCosE)t(V mmm =

Then, Phase Modulated Signal

tCosEkwhere

)tw(CosE)t(V

mmpo

cc

+=

+=

Phase of carrier is varied as per amplitude of modulating signal.

IndexModulationEkwhere

)tCostw(CosE

)tCosEktw(CosE)t(V

mpd

mdcc

mmpcc

==

+=

+=

Vm

f

FM and PM

fm

f

FM

PM

Fig.3 Phase Modulation

In FM, modulation index is directly proportional to modulating signal

amplitude and inversely to modulating frequency.

In PM, modulation index is directly proportional to modulating signal

amplitude but independent of modulating frequency.

iii. PULSE MODULATION

A set of techniques where by a sequence of information-carrying quantitiesoccurring at discrete instances of time is encoded into a corresponding regular

sequence of electromagnetic carrier pulses. Varying the amplitude, polarity,

presence or absence, duration, or occurrence in time of the pulses gives rise to

the four basic forms of pulse modulation: pulse-amplitude modulation (PAM),

pulse-code modulation (PCM), pulse-width modulation (PWM, also known as

pulse-duration modulation, PDM), and pulse-position modulation (PPM).

ANALOG-TO-DIGITAL CONVERSION

An important concept in pulse modulation is analog-to-digital (A/D)

conversion, in which an original analog (time- and amplitude-continuous)

information signal s(t) is changed at the transmitter into a series of


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regularly occurring discrete pulses whose amplitudes are restricted to a

fixed and finite number of values. An inverse digital-to-analog (D/A)

process is used at the receiver to reconstruct an approximation of the

original form ofs(t). Conceptually, analog-to-digital conversion involves

two steps. First, the range of amplitudes ofs(t) is divided or quantized

into a finite number of predetermined levels, and each such level is

represented by a pulse of fixed amplitude. Second, the amplitude ofs(t) isperiodically measured or sampled and replaced by the pulse representing

the level that corresponds to the measurement.

According to the Nyquist sampling theorem, if sampling occurs at a rate

at least twice that of the bandwidth of s(t), the latter can be

unambiguously reconstructed from its amplitude values at the sampling

instants by applying them to an ideal low-pass filter whose bandwidth

matches that ofs(t).

Quantization, however, introduces an irreversible error, the so-calledquantization error, since the pulse representing a sample measurement

determines only the quantization level in which the measurement falls

and not its exact value. Consequently, the process of reconstructing s(t)

from the sequence of pulses yields only an approximate version ofs(t).

PULSE-AMPLITUDE MODULATION

In PAM the successive sample values of the analog signal s(t) are used to

effect the amplitudes of a corresponding sequence of pulses of constantduration occurring at the sampling rate. No quantization of the samples

normally occurs (Fig. 4a, b). In principle the pulses may occupy the

entire time between samples, but in most practical systems the pulse

duration, known as the duty cycle, is limited to a fraction of the sampling

interval. Such a restriction creates the possibility of interleaving during

one sample interval one or more pulses derived from other PAM systems

in a process known as time-division multiplexing (TDM).


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sine wave.

(a) Analog signal,s(t).

(b) Pulse-amplitude modulation.

(c) Pulse-width modulation.

(d)Pulse-position-modulation.

PULSE-WIDTH MODULATION

In PWM the pulses representing successive sample values ofs(t) have

constant amplitudes but vary in time duration in direct proportion to the

sample value. The pulse duration can be changed relative to fixed leading

or trailing time edges or a fixed pulse center. To allow for time-division

multiplexing, the maximum pulse duration may be limited to a fraction of

the time between samples (Fig. 4c).

PULSE-POSITION MODULATION

PPM encodes the sample values ofs(t) by varying the position of a pulse

of constant duration relative to its nominal time of occurrence. As in

PAM and PWM, the duration of the pulses is typically a fraction of the

sampling interval. In addition, the maximum time excursion of the pulses

may be limited (Fig. 4d).

PULSE-CODE MODULATION


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Many modern communication systems are designed to transmit and

receive only pulses of two distinct amplitudes. In these so-called binary

digital systems, the analog-to-digital conversion process is extended by

the additional step of coding, in which the amplitude of each pulse

representing a quantized sample ofs(t) is converted into a unique

sequence of one or more pulses with just two possible amplitudes. The

complete conversion process is known as pulse-code modulation.

Figure 5a shows the example of three successive quantized samples of an

analog signal s(t), in which sampling occurs every T seconds and the

pulse representing the sample is limited to T/2 seconds. Assuming that

the number of quantization levels is limited to 8, each level can be

represented by a unique sequence of three two-valued pulses. In Fig. 5b

these pulses are of amplitude Vor 0, whereas in Fig. 5c the amplitudes

are Vand V.

Pulse-code modulation.

(a) Three successive quantized samples of an analog signal.

(b) With pulses of amplitude V or 0.

(c) With pulses of amplitude V or V.

PCM enjoys many important advantages over other forms of pulse

modulation due to the fact that information is represented by a two-state

variable. First, the design parameters of a PCM transmission system

depend critically on the bandwidth of the original signal s(t) and the

degree of fidelity required at the point of reconstruction, but are otherwise

largely independent of the information content ofs(t). This fact createsthe possibility of deploying generic transmission systems suitable for

many types of information. Second, the detection of the state of a two-

state variable in a noisy environment is inherently simpler than the


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precise measurement of the amplitude, duration, or position of a pulse in

which these quantities are not constrained. Third, the binary pulses

propagating along a medium can be intercepted and decoded at a point

where the accumulated distortion and attenuation are sufficiently low to

assure high detection accuracy. New pulses can then be generated and

transmitted to the next such decoding point. This so-called process of

repeatering significantly reduces the propagation of distortion and leadsto a quality of transmission that is largely independent of distance.

TIME-DIVISION MULTIPLEXING

An advantage inherent in all pulse modulation systems is their ability to

transmit signals from multiple sources over a common transmission

system through the process of time-division multiplexing. By restricting

the time duration of a pulse representing a sample value from a particular

analog signal to a fraction of the time between successive samples, pulses

derived from other sampled analog signals can be accommodated on thetransmission system.

One important application of this principle occurs in the transmission of

PCM telephone voice signals over a digital transmission system known as

a T1 carrier. In standard T1 coding, an original analog voice signal is

band-limited to 4000 hertz by passing it through a low-pass filter, and is

then sampled at the Nyquist rate of 8000 samples per second, so that the

time between successive samples is 125 microseconds. The samples are

quantized to 256 levels, with each of them being represented by asequence of 8 binary pulses. By limiting the duration of a single pulse to

0.65 microsecond, a total of 193 pulses can be accommodated in the time

span of 125 microseconds between samples. One of these serves as a

synchronization marker that indicates the beginning of such a sequence of

193 pulses, while the other 192 pulses are the composite of 8 pulses from

each of 24 voice signals, with each 8-pulse sequence occupying a

specified position. T1 carriers and similar types of digital carrier systems

are in widespread use in the world's telephone networks.

BANDWIDTH REQUIREMENTS

Pulse modulation systems may incur a significant bandwidth penalty

compared to the transmission of a signal in its analog form. An example

is the standard PCM transmission of an analog voice signal band-limited

to 4000 hertz over a T1 carrier. Since the sampling, quantizing, and

coding process produces 8 binary pulses 8000 times per second for a total

of 64,000 binary pulses per second, the pulses occur every 15.625

microseconds. Depending on the shape of the pulses and the amount of

intersymbol interference, the required transmission bandwidth will fall inthe range of 32,000 to 64,000 hertz. This compares to a bandwidth of

only 4000 hertz for the transmission of the signal in analog mode.


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APPLICATIONS

PAM, PWM, and PPM found significant application early in the

development of digital communications, largely in the domain of radio

telemetry for remote monitoring and sensing. They have since fallen into

disuse in favour of PCM.

After the all these processes the signals are to uplinked to the satellite and

EARTH STATION is used as a uplink center from which from which the

signals are fed to the satellite for distribution in a specified area covered by the

satellite. The signal is uplinked from the earth station and received by many

downlink centers in RADIO broadcasting. Lets study the CES briefly:

6. CAPTIVE EARTH STATION

As mentioned earlier that the captive earth station is meant for the up linking of the

signal to the satellite. It is designed as that it also amplifies and modulate the signal.

The basic components that a captive earth station has are:

PDA (Parabolic dish antenna)

FEED

LNBC

WAVE GUIDE

HPA

UPCONVERTER

MODULATOR

ENCODER

MULTIPLEXER

IRD(Integrated Receiver Decoder)

CES receiving system is used for monitoring of the up-linked

programme .C-BAND uplink frequency range is from 5.850 GHz

to 6.425 GHz & downlink frequency range is from 3.7 GHz to 4.2

GHz.

Transmitted power is 400W.


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Captive Earth Station is utilized to uplink its Radio programs for distribution in its network

through satellite.

Programs up-linked by these Captive Earth Stations are to be received at other ALL INDIA

RADIO stations their RADIO NETWORKING (RN) Terminals and used for recording or

retransmission through their terrestrial transmitters.


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CES uplinks the programs using both analog as well as digital chain. The Transmit chain is

in C BAND while receive chain is in C BAND and S BAND. Each CAPTIVE EARTH

STATIONS has facility to Transmit Two digital carrier and one analog carrier.

Digital Carrier undergoes QPSK Modulation of digital audio while for Analog carrier

Analog FM Modulation of Mono Audio is done.

9. TRANSMITTER

After the whole of these processes the signal is fed to the transmitter. The FM signal is

fed to the FM TRANSMITTER and AM signal is fed to the AM TRANSMITTER.

The AM signal is fed either by the captive earth station to the HIGH POWER

TRANSMITTER or by the STL i.e. STUDIO TO TRANSMITTER LINK. STL is a

microwave link between transmitter and the studio.

In ALL INDIA RADIO SHIMLA transmitter used for MW is THALES transmitterwhich is a digitalized instrument. And also NEC is used for the stand by.

There is a MAST for which is a tower antenna of 120 metre height and it has a

impedance of 50 ohm. The impedance of the feeder lines coming from the various

transmitters is 230 ohm. Therefore for the impedance matching the tuning is to be

done. For that purpose ATU i.e. ANTENNA TUNING UNIT is used so that maximum

power transfer can take place.

A mast is supported by the wires which are grounded and the air cored coils are usedin between the wires. So that when there is a lightening then all the current is fed to

the ground and no damage is done. The RF signal is not grounded by those wires

because that is an AC signal and the coils do not allow the AC pass through them.

Other components which are attached to the mast are spark discharge and austein

transformer.

Spark discharge is used as safety precaution during the lightening and austein

transformer is used to give the supply to the aviation lights.

THE basic structure of a transmitter is same whether it is a SW or MW transmitter.

First of all a crystal oscillator generates a frequency and that is amplified at various

stages and an audio signal is passed and amplified at different stages then both the

audio signal and the RF signal generated by the oscillator are mixed in a modulation

transformer and from the modulation transformer the signal is passed through the

feeder lines of copper to the ATU and then to the MAST and from there it is

transmitted.

The transmitter used nowadays for MW transmission is THALES transmitter. It is

completely a digital device which works on 300 V dc.


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A THALES transmitter has 80 modules of power amplification. Each gives a power of

1.25 KW and so after the 80 modules the power becomes 100 KW. A Thales

transmitter looks like

The SW TRANSMITTER used in ALL INDIA RADIO SHIMLA has the power of 50

KW. The MW transmission is used for the short distance transmission and the SW

transmission is used for the long distance transmission. In AIR SHIMLA the SW

frequency used at the day time is 6020 kHz and in the night time is 4965 kHz.

When the signal from the various transmitters comes in six line transmissions line and

we need to pass that to antenna or mast then a concept of impedance matching is used.

The impedance of the antenna is 50 ohm and that of the transmission lines is 230 ohm.

So we need to match the impedance to transfer maximum power to the antenna.

For that purpose the ATU is used i.e. Antenna Tuning Unit.


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MAST

ANTENNA TUNING UNIT


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Antenna Tuning Unit (ATU) is to match the feeder line impedance to the mast

impedance of MW Transmitters for maximum transmission of power. So ATU

is located between the mast base and the feeder line and is very close to the

mast base. Commonly Feeder Unit which is located in the aerial field, houses

the ATU.

Generally the mast impedance (aerial impedance) is obtained in a complex formi.e. the real part (resistive) and the imaginary part (reactive) component. When

the mast impedance is expressed in polar form then negative angle indicates the

mast is capacitive and positive angle indicates the mast is inductive. Whether

the mast impedance is inductive or capacitive depends on the height of the mast

in terms of wave length (). If the height is less than /4, it will be capacitive

and inductive if more than /4. This can be measured with impedance bridges.

ATU can be designed in a number of ways. The method used may be differentin different conditions. Criteria depend on the requirements. Especially when

directional antenna system is employed by splitting power to different antenna,

the phase angle of the network is the most important parameter. In other cases

mostly, simplicity and safety against lightning is important. One of the methods

adopted in the past was the reactive component of the mast impedance is

neutralised, by putting opposite reactive component of same value in series at

mast end side, to make the mast impedance purely resistive (i.e. for inductive

mast the series reactance should be capacitive and vice versa). Then theresistive part of the mast impedance can be matched to the feeder line

impedance by selecting a suitable matching network. This matching network

can be L, T or network, and can be designed as phase lag or phase lead type.

In these cases if a capacitor is put in series, there is every possibility of

puncturing of capacitors due to lightning. Hence this method is being

discouraged.

The second method, which is most commonly used now, is first to convert the

antenna impedance into a parallel combination. Most of the bridges used to

measure the mast impedance measure it in the series form. This series

impedance can be converted into a parallel impedance using the following

formula: -

( )( )2Rs/Xs1RsRp +=


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( )( )( )

( ) 2

2

2

1

1

1

1

1

Rp/XpXpXs

,Xs/RsXsXp,Xp/Rp

RpRs

+=

+=

+=

Fig. Series to Parallel Conversion

After the conversion we find that the mast impedance has a resistance in parallel with

a reactance which could be either capacitive or inductive. This reactance can be

neutralised with the help of a reactance of same magnitude but opposite in phase.

These two reactances which are equal but opposite in polarity resonate and offer pure

resistance. Further this resistance Rp can be matched to the feeder line with the help

of any network. The advantage of this method is that whenever the mast is capacitive

we can neutralise with a parallel inductive reactance. This reactance in addition to

matching, also provide a static leaks for the lightning. This will eliminate the separate

provision of static leaks. Besides the coils being sturdy will be a more appropriate

solution for lightning protection.


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The third method employed is shunting the mast impedance with a high Q coil

irrespective of whether the mast is inductive or capacitive. This will alter the netimpedance offered by the antenna and can be manipulated to the desired value by

varying the inductive reactance. In effect the coil impedance alters the mast

impedance. This method is used to bring down the higher value of mast impedance to

a manageable level for designing suitable network. This method is often known as

Pre-Tuning

FM TRANSMITTER

One other transmitter is FM transmitter. The AIR SHIMLA has a FM frequency of100.9 MHz. The FM transmitter used here is 1 kW which is 2*500W


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INTRODUCTION

The new generation FM transmitters in the AIR network can be classified according to

their output powers as follows :

3 kW FM Transmitter

2x3 kW FM Transmitter

The Salient features, principles of working and RF block schematic of these three

types of FM transmitters have been outlined in this chapter.

SALIENT FEATURES

a. Completely solid state.


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b. Local/remote operation capability

c. Forced air-cooled.

d. Digitally synthesized crystal oscillator which can be set in steps of 10

kHz in the frequency band of 87.5 to 108 MHz. Frequency can be

selected internally by BCD switches or externally by remote control.

e. Broadband VHF Power Amplifiers require no tuning.

f. Full power output just by pressing a single button.g. Automatic output power reduction in the following cases :-

Mismatch (VSWR > 1.5)

o Excessive heat sink temperature of output RF transistors (> 80oC).

o Absorber temperature 70oC due to failure of one or more power

amplifier units.

h. An automatic switch-over circuit ensures operation in the passive exciter

standby mode. This means that either of the two exciters can be selected

to operate as the main unit and the other exciter waits to be taken over.i. The switching and operating status of the system is indicated by LEDs.

j. RF power transistors of power amplifiers are of screw-in type and no

soldering is required during replacement.

k. Additional information such as SCA or RDS can also be transmitted.

l. Parallel operation of two transmitters in active standby mode is possible

using a combining unit. If one of the transmitters fails, 1/4th of the total

nominal power goes on the air so that continuity in service is maintained.

Fault free transmitter can then be selected manually on antenna during

suitable pause in programme with the help of U-link panels provided onthe combining unit front panel.

m. High overall efficiency of the order of 55 to 60%.

PRINCIPLE OF WORKING

The principle of working of a modern FM Transmitter is given in block diagram in fig

The L and R audio signals are converted into the stereo signal by a stereo coder. The

stereo signal, also called the MULTIPLEXED (MPX) signal, then frequency

modulates the VHF oscillator which is a voltage controlled oscillator (VCO) of the

phase locked loop (PLL). The PLL is an automatic frequency control (AFC) system in

the FM transmitter is maintained within the specified tolerance limits of + 2 kHz. In

this arrangement, the phase of the VHF oscillator is compared with that of a reference

crystal oscillator operating at 10 MHz. The frequency of the reference oscillator is


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divided by 1/1000 with the help of three decade counters in cascade to bring it down

to the audio range (10 kHz). The VHF oscillator frequency is also divided by a factor

N to scale it down to 10 kHz. As the VHF oscillator can operate at any assigned

frequency in the FM Broadcasting band of 87.5 to 108 MHz, the factor N will vary

from 8750 to 10800. the phases of the outputs from the two frequency dividers are

then compared in a phase comparator and the resultant error voltage is amplified,

rectified and filtered to get a DC error voltage of positive or negative polarity which

corrects and drift in the VHF oscillator frequency.

STEREOCODER

VHFOSCILLATOR

AND

MODULATOR

WIDE BAND

POWERAMPLIFIER

RECTIFIERAND FILTER

PROGRAMMABLEDIVIDER 1/N

PHASEDETECTOR

FREQUENCY

DIVIDER1/1000

FRQUENCYCRYSTAL

OSCILLATOR

10 MHz10KHz10KHz

R

L Antenna

Fig. Block Diagram of Modern FM Transmitter

The operating frequency and the variable factor N are synthesised with the help

of digital frequency synthesis techniques. Thus any frequency of high stability

(same as that of the reference crystal oscillator) can be generated by using the

same crystal oscillator of 10 MHz.

The FM signal obtained at the output of VHF oscillator is then amplified in a

VHF Power Amplifier with an output power of 1.5 kW. This amplifier is the


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basic building block in the series of FM Transmitters. It is a wideband

amplifier so that no tuning is required when the operating frequency is changed.

3 kW, 2x3 kW FM TRANSMITTER

I) 3 KW FM TRANSMITTER

RF block schematic of 3 kW FM transmitter is shown in figure 2. low

level modulation of VHF oscillator is carried out at the carrier frequency

in the Exciter type SU-115. The RF output of the exciter is split up into

two halves using a splitter network called input coupler. Thus two VHF

power amplifiers type VU 315 are driven by one Exciter. The RF outputs

of these amplifiers are passed through harmonic filters and combined inthe power coupler to get an output power of 3 kW. RF switch connects

the selected exciter to the input coupler and the standby exciter to dummy

load and AF switch feeds the audio to the selected exciter.

Nominal output power of the Exciter in a 3 kW transmitter is 6 W. All

the modules are mounted in a single rack. Transmitter output is taken

from the top and can be connected either to antenna or dummy load with

the help of a U-link.

VHF

AMPLIFIER

2.5 W

VHF

AMPLIFIER

2.5 W

INPUTCOUPLER

HARMONIC

FILTER

1.5K W

HARMONIC

FILTER

1.5K W

OUTPUTCOUPLER

3K W

50

5 W

5 W

INPUTCOUPLER

10 W

FROMRF SWITCH

Fig. RF signal flow of 3 kW FM Transmitter ( A or B)

II) 2 X 3 KW FM TRANSMITTER

RF block schematic of 2 x 3 kW FM Transmitter is shown in figure

3. It may be seen that the outputs of two 3 kW transmitters are

combined in a combining unit to get an output of about 5.5 kW.


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The nominal output of exciter in this transmitter is about 10 to 12

W because there are four PA units and the input power requirement

of each PA is about 2.5 to 3 W. The exciter output is split into 4

equal parts in two stages of power splitting using three couplers.

These four outputs drive four power amplifiers, each amplifier

developing an output of 1.5 kW which is filtered in a harmonic

filter (low pass filter with cut off frequency of 110 MHz). Two 1.5

kW outputs of each transmitter are then combined in output

coupler to get an output of 3 kW for each transmitter. Both the

transmitter outputs (3 kW each) are then combined in the

combining unit to get an output of about 5.5 kW.

Fig. RF block schematic of 2 x 3 kW FM Transmitter

The modern FM transmitters are compact, versatile, easy to install and operate. Their design

incorporates in-built flexibility to provide different output powers using identical modules.

This also adds to redundancy thereby increasing the reliability of the transmitter.

REFRENCES

The training material provided by ALL INDIA RADIO.


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Training Report on radio broadcasting