Training Report on radio broadcasting
-
Upload
dhawal-kirti-vasudev -
Category
Documents
-
view
230 -
download
1
Transcript of Training Report on radio broadcasting
-
7/27/2019 Training Report on radio broadcasting
1/44
1
Radio Broadcasting
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
-
7/27/2019 Training Report on radio broadcasting
2/44
2
Radio Broadcasting
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
-
7/27/2019 Training Report on radio broadcasting
3/44
3
Radio Broadcasting
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.
-
7/27/2019 Training Report on radio broadcasting
4/44
4
Radio Broadcasting
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.
-
7/27/2019 Training Report on radio broadcasting
5/44
5
Radio Broadcasting
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
-
7/27/2019 Training Report on radio broadcasting
6/44
6
Radio Broadcasting
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
-
7/27/2019 Training Report on radio broadcasting
7/44
7
Radio Broadcasting
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.
-
7/27/2019 Training Report on radio broadcasting
8/44
8
Radio Broadcasting
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)
-
7/27/2019 Training Report on radio broadcasting
9/44
9
Radio Broadcasting
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:-
-
7/27/2019 Training Report on radio broadcasting
10/44
10
Radio Broadcasting
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
-
7/27/2019 Training Report on radio broadcasting
11/44
11
Radio Broadcasting
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.
-
7/27/2019 Training Report on radio broadcasting
12/44
12
Radio Broadcasting
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.
-
7/27/2019 Training Report on radio broadcasting
13/44
13
Radio Broadcasting
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
-
7/27/2019 Training Report on radio broadcasting
14/44
14
Radio Broadcasting
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
-
7/27/2019 Training Report on radio broadcasting
15/44
15
Radio Broadcasting
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
-
7/27/2019 Training Report on radio broadcasting
16/44
16
Radio Broadcasting
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.
-
7/27/2019 Training Report on radio broadcasting
17/44
17
Radio Broadcasting
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.
-
7/27/2019 Training Report on radio broadcasting
18/44
18
Radio Broadcasting
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.
-
7/27/2019 Training Report on radio broadcasting
19/44
19
Radio Broadcasting
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.
-
7/27/2019 Training Report on radio broadcasting
20/44
20
Radio Broadcasting
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
-
7/27/2019 Training Report on radio broadcasting
21/44
21
Radio Broadcasting
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
-
7/27/2019 Training Report on radio broadcasting
22/44
22
Radio Broadcasting
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.
-
7/27/2019 Training Report on radio broadcasting
23/44
23
Radio Broadcasting
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
-
7/27/2019 Training Report on radio broadcasting
24/44
24
Radio Broadcasting
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 =
-
7/27/2019 Training Report on radio broadcasting
25/44
25
Radio Broadcasting
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
-
7/27/2019 Training Report on radio broadcasting
26/44
26
Radio Broadcasting
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).
-
7/27/2019 Training Report on radio broadcasting
27/44
27
Radio Broadcasting
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
-
7/27/2019 Training Report on radio broadcasting
28/44
28
Radio Broadcasting
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
-
7/27/2019 Training Report on radio broadcasting
29/44
29
Radio Broadcasting
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.
-
7/27/2019 Training Report on radio broadcasting
30/44
30
Radio Broadcasting
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.
-
7/27/2019 Training Report on radio broadcasting
31/44
31
Radio Broadcasting
-
7/27/2019 Training Report on radio broadcasting
32/44
32
Radio Broadcasting
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.
-
7/27/2019 Training Report on radio broadcasting
33/44
33
Radio Broadcasting
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.
-
7/27/2019 Training Report on radio broadcasting
34/44
34
Radio Broadcasting
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.
-
7/27/2019 Training Report on radio broadcasting
35/44
35
Radio Broadcasting
MAST
ANTENNA TUNING UNIT
-
7/27/2019 Training Report on radio broadcasting
36/44
36
Radio Broadcasting
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 +=
-
7/27/2019 Training Report on radio broadcasting
37/44
37
Radio Broadcasting
( )( )( )
( ) 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.
-
7/27/2019 Training Report on radio broadcasting
38/44
38
Radio Broadcasting
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
-
7/27/2019 Training Report on radio broadcasting
39/44
39
Radio Broadcasting
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.
-
7/27/2019 Training Report on radio broadcasting
40/44
40
Radio Broadcasting
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
-
7/27/2019 Training Report on radio broadcasting
41/44
41
Radio Broadcasting
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
-
7/27/2019 Training Report on radio broadcasting
42/44
42
Radio Broadcasting
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.
-
7/27/2019 Training Report on radio broadcasting
43/44
43
Radio Broadcasting
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.
-
7/27/2019 Training Report on radio broadcasting
44/44
44
Radio Broadcasting
www.sci-tech.com.
http://www.sci-tech.com/http://www.sci-tech.com/