MAJOR TRAINING REPORTA Report Submitted in partial fulfillment
of the requirement for the award of Degree of Bachelor of
Engineering in Electronics & Communication AT ALL INDIA RADIO,
BHOPAL (M.P)
ON RADIO BROADCASTING Guided By : Ravindra Goyal (Station
Engineer) Submitted By: Shaival Chatterjee (0131EC081098)
Department Of Electronics & Communication Engineering Jai
Narain College Of Technology, Bhopal (M.P.)
1
Acknowledgement
I deem it my privilege to extent my profound gratitude and
appreciation towards all those who have directly or indirectly
involved themselves in making this training a great success. My
sincere appreciation and thanks to my training in charge, G.P KHARE
(Assistant Station Engineer) and various other engineers for
guiding us and explaining how the AM and Fm broadcast takes place
.
For their diligent attention towards my training through its all
stages. I wish to express my gratitude to honourable Station
Engineer, Ravindra Goyal for their sincere guidance. They stood by
us for explaining the procedure, explaining each and every
component in detail all along the of my training. They also
provided us with additional ideas with painstaking attention to
details of each and every concept. Their comments and criticism
have been invaluable.
Shaival Chatterjee (0131EC081098)
2
INDEX
Page No.
Company Profile. 5
Studio Section.. Studio Transmission... Acoustic Treatments..
Facilities In Studio Center.. Maintenance... Broadcasting Section
Principle of working.. Frequency Modulation... Amplitude
Modulation... Comparison Between FM & AM...
8 8 8 14 18 25 26 28 43 43
Satellite Communication.. 46
Conclusion... 50
3
CHAPTER 1 Introduction of the company
4
Company ProfileFor 75 years, AIR has been distinctive part of
the Indian way of life. From a small beginning, in the form of
radio clubs in 1927. With one of the largest network of SW/MW/FM
transmitters, AIR, reaches the remotest corners of the country to
serve the people. People living in far flung areas with modest
means can also have access to its programmes if a small transistor
radio set run on dry batteries is available with them. All India
Radio (abbreviated as AIR), officially known as Akashvani
(Devanagari:, kshavn) is the radio broadcaster of India and a
division of Prasar Bharati Act provides for establishment of April
1930 Broadcasting was placed under the direct control of Government
under the title 'Indian State Broadcasting Service' (ISBS) to be
known (Broadcasting Corporation of India), an autonomous
corporation of the Ministry of Information and Broadcasting,
Government of India. Established in 1936, today, it is the sister
service of Prasar Bharati's Doordarshan, the national television
broadcaster. Today AIR has a network of 213 broadcasting centers
covering 90% of the area & almost reaching the entire
population of one billion. All India Radio is one of the largest
radio networks in the world. The headquarters is at the Akashwani
Bhavan, New Delhi. Akashwani Bhavan houses the drama section, the
FM section and the National service. The Doordarshan Kendra (Delhi)
is also located on the 6th floor of Akashvani Bhavan.
Services: AIR has many different services each catering to
different regions/languages across India. One of the most famous
services of the AIR is the Vividh Bharati Seva (roughly translating
to "Multi-Indian service"). Vividh Bharati celebrated its Golden
Jubilee on 3 October 2007. Vividh Bharati has the only
comprehensive database of songs from the so termed "Golden Era" of
Hindi film music (roughly from 1940s to 1980s). This service is the
most commercial of all and is popular in Mumbai and other cities of
India. This service offers a
5
wide range of programmes including news, film music, comedy
shows, etc. The Vividh Bharti service operates on different MW band
frequencies for each city. Some programs broadcast on the Vividh
Bharti:
Hawa-mahal - Skit (Radio Play) based on some novels/plays.
Santogen ki mehfil - Jokes & humour.
Various Region Services: East regional service North reginal
Service North-east regional service West regional service South
regional service
External Services: The External Services Division of All India
Radio broadcasts in 27 languages to countries outside of India,
primarily by high powered short wave broadcasts although medium
wave is also used to reach neighbouring countries. In addition to
broadcasts targeted at specific countries by language there is a
General Overseas Service which broadcasts in English with 8 hours
of programming each day and is aimed at a general international
audience. Yuv-vani: The voice of youth News-on-phone service
6
CHAPTER 2 Studio Section
7
STUDIO SECTION
Studio Transmission: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.
We as 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. In order to achieve our goal we must
thoroughly understand the characteristic of the different
components involved in the broadcast chain, and in this process we
must preserve the original quality of sound produced by the artists
inside the studio. The science of sound is often called Acoustics.
It would be thus prudent to understand the 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 from
experience to be suitable for the various types of programs. In
this section problems and design aspects of internal acoustics of a
broadcast studio are explained.
8
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 the obstacle, 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. Reverberation Time(R/T) 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).
R/T of a room depends upon shape and size of room and on the total
absorption offered on boundary surfaces. Reverberation is the most
important single parameter of a room. It influences the audio
programs in following ways: Volume of program increases due to
reverberation of sound 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 program due 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.
9
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 ii) Fibrous Materials iii) Panel Absorbers
iv) Perforated Panel Absorbers
10
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 Noise
in studios may be either air-borne or structure borne. Background
noise in a studio can originate from Outside the building Inside
the studio itself and /or Outside the studio but within the
building Studio chain in a typical air station The broadcast of a
programme from source to listener involves use of studios,
microphones, announcer console, switching console, telephone lines
/ STL and Transmitter. Normally the programmes originate from a
studio centre located inside the city/town for the convenience of
artists. The programme could be either live or recorded. In some
cases, the programme can be from OB spot, such as commentary of
cricket match etc. Programmes that are to be relayed from other
Radio Stations are received in a receiving centre and then sent to
the studio centre or directly received at the studio centre through
RN terminal/telephone line. All these programmes are then selected
and routed from studio to transmitting centre through broadcast
11
quality telephone lines or studio transmitter microwave/VHF
links. schematic showing the different stages is given in Fig.
1.
A simplified block
Fig. 1 Simplified block schematic of broadcasting chain
Studio Operational Requirements Many technical requirements of
studios like minimum noise level, optimum reverberation time etc.
are normally met at the time of installation of studio. However for
operational purposes,
12
certain basic minimum technical facilities are required for
smooth transmission of programmes and for proper control. These are
as follows: Programme in a studio may originate from a microphone
or a tape deck, or a turntable or a compact disc or a R-DAT. So a
facility for selection of output of any of these equipments at any
moment is necessary. Announcer console does this function. Facility
to fade in/fade out the programme smoothly and control the
programme level within prescribed limits. Facility for aural
monitoring to check the quality of sound production and sound
meters to indicate the intensity (VU meters). For routing of
programmes from various studios/OB spots to a central control room,
we require a facility to further mix/select the programmes. The
Control Console in the control room performs this function. It is
also called switching console. Before feeding the programs to the
transmitter, the response of the program should be made flat by
compensating HF and LF losses using equalized line amplifiers.(This
is applicable in case of telephone lines only) Visual signaling
facility between studio announcer booth and control room should
also be provided. If the programs from various studios are to be
fed to more than one transmitter, a master switching facility is
also required.
13
FACILITIES IN STUDIO CENTRE: In addition to control room and
studios, dubbing/recording rooms are also provided in a studio
complex. Following equipments are generally provided in a
recording/dubbing room :
i) ii) iii)
Console tape recorders Console tape decks Recording/dubbing
panel having switches jacks and keys etc.
The above equipments can be used for the following purpose
For recording of programmes originating from any studio. For
recording of programmes available in the switching consoles in
control room. For dubbing of programmes available on cassette tape.
For editing of programmes For mixing and recording of programmes
Recording Room A block schematic of a typical recording room is
shown in figure 1. Two numbers of CTRs and two numbers of Push
Button switches have been shown. Outputs from various studios and
switching consoles have been given to multiple pads 1,2,3 and 4.
Outputs from the multiple pads are wired to PB switches. Three
numbers of receptacles for cassette outputs have been provided.
Transformers T1 and T2 transform the output impedance of the
cassette recorder to 600 ohm. The output of CTR # 1 is wired to PB
switch # 2 through MP # 6. With this arrangement output of CTR # 1
can be recorded on CTR # 2. Please carefully note the impedances
and levels at various points. Red and green lamps are provided on
the control panel for indications from and to control room and
studios.
14
Dubbing Room A block schematic of a typical dubbing room is
shown in figure 1. The arrangement is similar to the recording room
except that an additional tape deck and a mixer unit have been
provided. This arrangement allows mixing of programmes.
Fig. 1 Block schematic of Recording / Dubbing Room
15
Loud Speakers A loudspeaker performs an opposite function to a
microphone, i.e. it converts electrical signal into sound wave.
Moving Coil or dynamic loudspeaker It consists of a permanent
magnet and a voice coil for carrying audio signals. Voice coil is
having a few turns of wire, wound on paper, plastic or aluminum
former. It is attached to a paper that radiates sound. The coil is
suspended with the help of spider, made of flexible material.
Spider permits forward backward motion but no lateral motion. When
audio currents from an amplifier flow through the coil, it produces
a magnetic field around the coil. This field is at right angle to
the field of permanent magnet. The two fields attract or repel each
other, depending on the position of the permanent magnet. The voice
coil and the cone assembly move corresponding to the audio
currents. The resulting cone vibrations produce air pressure
variation in correspondence with the audio signal. In hi-fi
applications two or more speakers are used to cover the full audio
range. To reproduce high frequencies, it is common to attach a dome
of fabric or plastic material to the coil than to the cone, thus
forming a dome tweeter. Low frequency speakers known woofers are of
large size. The middle range speakers are called squeakers. Baffles
To avoid air escaping round the edges, the diaphragm should be at
least one half wavelength across. If we want to radiate 50 Hz, the
wave length is 1100/50 = 22 feet, it means the diaphragm would be
11 ft in diameter. One way to avoiding this large size is to mount
the speaker in the baffle. The purpose of baffle is to prevent any
access from back to front around the edges. Putting the speaker in
a closed box except for the diaphragm does solve the problem. But
this arrangement has one defect, the air inside the box is
compressed and rarified and places a loading on the diaphragm
movement. This results in the wastage of most of the energy.
Therefore two main problems in designing loudspeakers are :
16
i. To get any radiation of power at low frequencies, the
diaphragm has to move a lot of air. ii. The diaphragm moves air on
both sides of it, however, movements is in opposition. To overcome
this problem and make use of the radiation from the rear, bass
reflect enclosure is used. This box has one or two small holes
reducted ports and this is tuned to the fundamental resonance of
the drive unit. Then not only resonance of the loudspeaker is a
damped but low frequency can be extended. The port in effect,
reverses the phase of the rear wave and uses it to reinforce output
at the front. Hence this form of enclosure is often called a phase
invertor.
Horn A horn is a specialized form of baffle, its cross-sectional
area expands exponentially. It behaves as an acoustic transformer
and improve the efficiency of the speaker. By using horn low
frequencies get extra emphasis. For low frequencies, folded horns
are used. Speaker Impedance Normally such speakers are designed
with impedances of 2,3,5,8,9,16,32 ohms. When several speakers are
connected in parallel as in the case of column units then their
phase must be checked. This is done by feeding currents from a
Torch cell through a switch. While switching it on every time the
position of the cone is watched whether it is moving inwards or
outwards. In fact all the cones should behave identically so that
their outputs are together. Whenever any cone movement is to be
reversed its connections at the terminals may be interchanged to
get the sound output in phase. The matching of the loud-speakers
impedance with the output impedance of a monitoring amplifier is
important. This is done by suitable series parallel combinations in
the speakers to approach the amplifiers impedance. If the permanent
magnet has become weak or the paper cone is torn off, the
loudspeaker may be replaced. By listening to poor quality, the ears
lose the discrimination of good and bad quality programme.
Therefore, monitoring speakers should be the best available.
17
Headphones Headphones basically work on the same principles
which are applicable to loudspeakers. However, with headphones the
acoustical loading is achieved by intimacy of the ear units to the
ears. Thus even very small units are capable of providing very good
bass performance. Most headphones used for high quality
applications are either moving coil or electrostatic. Headphone
impedances range from 4 to 1000 ohms. Specifications of a stereo
headphone type EM 6201 (Philips) are given below :
Frequency range Matching impedance Maximum input
20 to 20 kHz 4 to 32 ohms 0.1 watt.
For checking levels on a studio chain headphones with higher
impedance should be used. Headphones are classified into mono,
stereo and four channel headphones according to the number of
channels.
MAINTENANCE OF STUDIOS: It is important that all studios be
maintained in the best condition at all times. maintenance schedule
suggested below should therefore be carried out very regularly.
The
FLOORING
i) Where marblex is provided it must be swept to remove dirt and
soil. If any liquids are spilled, they should be mopped up
immediately.
18
Every fortnight, the floor must be washed with soap and water,
then wiped with damp cloth or mopped with clean water. Detergents,
harsh soaps and chemical cleaning agent should be avoided. Soft
soaps will give best results. To remove stub born marks, scrub with
a soft coir brush or fine plastic wire brush bright look. The
following precautions may be followed Marblex floors should be
protected from heavy point loads. Furniture and other heavy
articles should not be dragged on the marblex flooring Kerosene,
petrol, turpentine or any polish containing spirit, should not be
used for cleaning marblex flooring. Hard scrubbing must be avoided.
A door mat should invariably be used near the entrance to keep the
dust away. The exposed edges of marblex must be protected to by
fixing aluminium or wooden strips or angles. ii) Where Linoleum is
provided, it should be mopped with cloth soaked in soft soap
solution and thereafter polished with a floor polish. It is
generally not possible to clean all studio floors every day, and
therefore a schedule should be drawn up indicating the studios that
are to be attended to every day so that, at least over the period
of one week, all the studio floorings are attended to. Where
druggets have been provided, those should be cleaned at least once
a week with a vacuum cleaner. The druggets should also be rolled up
on this occasion and the floor cleaned. If coir matting has been
provided, it should be cleaned at least once a week with a vacuum
cleaner. Coir matting should be removed from the studio once a
month and taken out side and given a thorough dusting. Before
replacing the matting the floor should be cleaned.
Ivory finished celotex surface : Such surfaces are likely to
collect dirt marks, finger marks and scratches, mild damage of this
nature can be attended to by gently sand papering the affected area
with number 100 sand paper, and thereafter rubbing dry distemper
(ivory) and finishing off by rubbing with the ivory surface of a
small piece of celotex. The entire celotex surface of the studio
should be cleaned at least once a month with muslin cloth.
19
Distempered Surface : When a distempered surface collect dirt
marks it can be given a coat of distemper of matching shade. It is
better to spray the distemper rather than use a brush or
particularly on celotex surfaces as provided in the older studios.
Oil Painting Surfaces : Such surfaces can be cleaned by gently
scrubbing with a piece of cloth which has been soaked in soap
solution. If the dirt is removed, the surface should be wiped dry
with a piece of clean dry cloth. Similar process should be adopted
for the cleaning of cement wall finished with oil paint, walls and
surfaces finished with transit board, and any other studio fittings
that have been finished with oil paints.
TEAK WOOD WALL PANELS These should be cleaned daily with piece
of clean soft cloth. These should be wax polished once a month and
a fresh coat of polish should be applied once a year. i) Skirting :
Teak wood skirtings should be dusted daily with a piece of cloth.
They should be wax polished once a month and fresh coat of polish
be applied once a year.
CEILING Studio ceilings may be finished with celotex transite
board oil paint or distemper. Cleaning processes suggested in the
previous para will apply to ceilings also. During the process of
weekly dusting and cleaning of a studio, care should be taken to
remove all cobwebs which adhere to edges and corners and cobweb
brush should be used very carefully so that in the process of
cleaning, walls and ceiling do not get damage.
20
GENERAL i) Door : Door finished with enamel paints should be
cleaned daily with a piece of clean damp cloth. It had been
observed that in use, the doors, particularly the area near the
door handle, get soiled rapidly due to finger marks. Such marks can
be removed by gently scrubbing the area with a piece of cloth
soaked in soap solution. The surface should then be wiped dry with
a piece of clean cloth and finished by polishing with some good
furniture polish. Very frequently polishing is not advised since
the paint gets gradually removed in the process. The sides of the
doors finished with French polish should also be cleaned and dusted
daily. Normally, it is necessary to repolish such surfaces once a
year. But if there are any dirt marks, the application of furniture
polish will be found helpful in removing stains. Doors handles and
hinges are generally heavily chromium plated. These, in use, get
dull, and polishing with French chalk will be found helpful in
restoring their brightness. The metal ring of the spy hold should
also be cleaned with French chalk. The spy hole glass should be
cleaned with a piece of cloth soaked in methylated spirit, or by
the use of cleaning powder, at least once a week. ii) Observation
Windows: All observation windows should be dusted daily by mains of
a soft cloth. The glass portions, should be cleaned by using a
piece of cloth soaked in methylated spirit or a cleaning powder.
The teak wood frame or breeding of the window should be attended to
as explained earlier. In some cases breathers have been provided in
a small recess near the window to absorb moisture that may get into
the observation window space. Breathers are generally small
containers of anhydrous calcium chloride. They should be examined
periodically especially during the rainy season and replaced if
found damp or full of dissolved chemical. iii) Furniture : All
furniture provided in the studios should be cleaned every day with
a piece of soft cloth. The wood work of the tables should be
polished with wax once a month, and once a year fresh coat of
polish should be given. Steel furniture should be dusted every day
and polished with French chalk once a month.
21
EQUIPMENT IN STUDIO
Turntable : These should be cleaned and dusted every day. All
bright metal portion should be polished once in a week with metal
polish. The turn-tables should be checked for satisfactory
operation every day. i) Fader Boxes : These should be dusted and
cleaned every day. ii) Microphones and stands : These should be
lightly dusted and cleaned every day. Careful handling of the
microphones is necessary as the slightest carelessness may damage
it considerably. The mechanical operation of the microphone stands
and boom should be checked up daily and the fixing screws
tightened. All highly polished parts of the stands should be
polished with French Chalk once a month. Dull finished surfaces
should be cleaned with wax polish at the same time. iii) Panels :
All equipment panels should be cleaned and dusted every day with
soft cloth. Painted surfaces should be cleaned and wax polished
once a week. Crinkle finished surfaces can not be wax polished and
should be attended to be rubbing with a piece of soft cloth lightly
soaked in some thin oil. ITEMS NEEDED FOR GENERAL STUDIO
MAINTENANCE
Some good brand of furniture and floor polish. A good brand of
cleaning powder. Washing soap. Rough duster for scrubbing linoleum
with cleaning powder and water. Muslin Cloth Vacuum cleaner Dry
distemper (Ivory) Some small pieces of clean Celotax Sand paper
number 100.
22
.
Oil paints and distempers of different shades as used in the
studios. French Chalk Cobweb Brushes Calcium chloride for
Breathers. Metal Polish Some thin oil and V-seline for light
machines
23
CHAPTER 3 Broadcasting Section
24
BROADCASTING SECTIONThere is too much over-crowding in the AM
broadcast band and shrinkage in the night-time service area due to
fading, interference, etc. FM broadcasting offers several
advantages over AM such as uniform day and night coverage, good
quality listening and suppression of noise, interference, etc. All
India Radio has gone in for FM broadcasting using modern FM
transmitters incorporating state-of-art technology. 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 2x5 kW FM Transmitter
SALIENT FEATURES a. Completely solid state. 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.
25
g. Automatic output power reduction in the following cases :
Mismatch (VSWR > 1.5) Excessive heat sink temperature of output
RF transistors (> 80 C). Absorber temperature 70 C 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, 1/4 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 on the combining
unit front panel. m. High overall efficiency of the order of 55 to
60%.th o o
PRINCIPLE OF WORKING:
The principle of working of a modern FM Transmitter is given in
block diagram in figure 1. 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
26
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 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.
27
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 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.
Frequency Modulation
The type of modulation in which the instantaneous frequency of
the carrier is varied according to amplitude of modulating signal
is called frequency modulation. Frequency modulation is widely used
in VHF communication systems e.g. FM broadcasting, transmission of
sound signal in TV, Satellite Communication etc.
28
Bandwidth in FM In FM, the BW is based on the number of
significant sidebands, which depends uponmodulation index mf. In
practice, the number of significant sidebands is determined by
acceptable distortion. These contain about 98% of the radiated
power. By way of best approximation, the Carson s Rule (rule of
thumb) gives a simple formula for bandwidth as BW = 2(1+mf)fm = 2(
f + fm)
For modulation index of 5 and maximum modulating frequency of 15
kHz, we have: BW = 180 kHz A guard band of 20 kHz (10 kHz on each
side) is provided to prevent adjacent
29
channel interference. Thus the maximum permissible BW in FM
broadcasting is 200 kHz. For narrow band FM (mf20), then the BW
becomes 2 f i.e. 150 kHz. For example, if fm = 100 Hz and f = 75
kHz.
2 x 3 kW FM Transmitter Simplified block diagram of a 2 x 3 kW
FM transmitter is shown in Fig.2. 2 x 3 kW Transmitter setup, which
is more common, consists of two 3 kW transmitters, designated as
transmitters A and B, whose output powers are combined with the
help of a combining unit. Maximum of two transmitters can be housed
in a single rack along with two Exciter units. Transmitter A is
provided with a switch-on-control unit (GS 033A1) which, with the
help of the Adapter plug-in-unit (KA 033A1), also ensures the
parallel operation of transmitter B. Combining unit is housed in a
separate rack.
30
Fig.2 Block Diagram Of 2x3 Kw Fm Transmitter
Low-level modulation of VHF oscillator is carried out at the
carrier frequency in the Exciter type SU 115. The carrier frequency
can be selected in 10 kHz steps with the help of BCD switches in
the synthesizer. The exciter drives four 1.5 kW VHF amplifier,
which is a basic module in the transmitter. Two such amplifiers are
connected in parallel to get 3 kW power. The transmitter is forced
air-cooled with the help of a blower. A standby blower has also
been provided which is automatically selected when the pre-selected
blower fails. Both the blowers can be run if the ambient
temperature exceeds 40oC. Power stages are protected against
mismatch (VSWR > 1.5) or excessive heat sink temperature by
automatic reduction of power with the help of control circuit.
Electronic voltage regulator has not been provided for the DC
supplies of power amplifiers but a more efficient system of
stabilization in the AC side has been provided. This is known as
AC-switch over. Transmitter
31
operates in the passive exciter standby mode with help of
switch-on-control unit. When the pre-selected exciter fails,
standby exciter is automatically selected. Reverse switch over,
however, is not possible.
A simplified block diagram of a 2 x 5 kW FM Transmitter is also
given in Fig. 3.
Fig.3 RF Block Schematic of 2x5 kW FM Transmitter
Exciter (SU 115) The Exciter (SU115) is, basically, a
self-contained full-fledged low power FM Transmitter. It has the
capability of transmitting mono or stereo signals as well as
additional information such as traffic radio, SCA (Subsidiary
Channel Authorisation) and RDS (Radio Data System) signals. It can
give three output powers of 30 mW, 1 W or 10 W by means of internal
links and switches. The output power is stabilized and is not
affected by mismatch (VSWR > 1.5),
32
temperature and AC supply fluctuations. Power of the transmitter
is automatically reduced in the event of mismatch. The 10 W output
stage is a separate module that can be inserted between 1 W stage
and the low pass harmonics filter. This stage is fed from a
switching power supply which also handles part of the RF output
power control and the AC stabilizations. In AIR set up this 10 W
unit is included as an integral part of the Exciter. This unit
processes the incoming audio signals both for mono and stereo
transmissions. In case of stereo transmission, the incoming L and R
channel signals are processed in the stereo coder circuit to yield
a stereo base band signal with 19 kHz pilot tone for modulating the
carrier signal. It also has a multiplexer wherein the coded RDS and
SCA signals are multiplexed with the normal stereo signal on the
modulating base band. The encoders for RDS and SCA applications are
external to the transmitter and have to be provided separately as
and when needed. supply
Frequency Generation, Control and Modulation The transmitter
frequency is generated and carrier is modulated in the Synthesiser
module within the Exciter. The carrier frequency is stabilized with
reference to the 10 MHz frequency from a crystal oscillator using
PLL and programmable dividers. The operating frequency of the
transmitter can be selected internally by means of BCD switches or
externally by remote control. The output of these switches
generates the desired number by which the
programmable divider should divide the VCO frequency (which lies
between 87.5 to 108 MHz) to get a 10 kHz signal to be compared with
the reference frequency. The stablised carrier frequency is
modulated with the modulating base band consisting of the audio
(mono and stereo), RDS and SCA signals. The Varactor diodes are
used in the synthesizer to generate as well as modulate the carrier
frequency.
Switch-ON Control Unit (Type GS 033 A1)
33
The switch-on-control unit can be termed as the brain and
controls the working of the transmitter A. It performs the
following main functions: 1. It controls the switching ON and OFF
sequence of RF power amplifiers, rack blower and RF carrier enable
in the exciter. 2. Indicates the switching and the operating status
of the system through LEDs. 3. Provides automatic switch over
operation of the exciter in the passive exciter standby mode in
which either of the two exciters can be selected to operate as the
main unit. 4. It provides a reference voltage source for the output
regulators in the RF amplifiers. 5. It is used for adjusting the
output power of the transmitter. 6. It evaluates the fault signals
provided by individual units and generates an overall sum fault
signal which is indicated by an LED on the front panel. The fault
is also stored in the defective unit and displayed on its front
panel. 7. It controls the switching ON and OFF sequence of RF power
amplifiers, rack blower and RF carrier enable in the exciter. 8.
Indicates the switching and the operating status of the system
through LEDs. 9. Provides automatic switch over operation of the
exciter in the passive exciter standby mode in which either of the
two exciters can be selected to operate as the main unit. 10.It
provides a reference voltage source for the output regulators in
the RF amplifiers. 11.It is used for adjusting the output power of
the transmitter. 12.It evaluates the fault signals provided by
individual units and generates an overall sum fault signal which is
indicated by an LED on the front panel. The fault is also stored in
the defective unit and displayed on its front panel.
34
Adapter Unit (KA 033A1) Adapter Unit is a passive unit which
controls transmitter B for its parallel operation with transmitter
A in active standby mode. The control signals from the Switch-on
control unit are extended to transmitter B via this Adapter unit.
If this unit is not in position the transmitter B can not be
energized.
1.5 kW VHF Amplifier (VU 315) This amplifier is the basic power
module in the transmitter. It has a broad band design so that no
tuning is required for operation over the entire FM Broadcast band.
RF power transistors of its output stages are of plug in type which
are easy to replace and no adjustments are required after
replacement. Each power amplifier gives an output of 1.5 kW.
Depending on the required configuration of the transmitter, output
of several such amplifiers is combined to get the desired output
power of the transmitter. For instance, for a 3 kW set-up two power
amplifiers are used whereas for a 2 x 3 kW set-up, 4 such
amplifiers are needed. The simplified block diagram of 1.5 kW Power
Amplifier is given in Fig. 4.
35
Fig. 4 Block Diagram of 1.5 kW Amplifier VU 315 This amplifier
requires an input power of 2.5 to 3 W and consists of a driver
stage (output 30 W) followed by a pre-amplifier stage (120 W). The
amplification from 120 W to 1500 W in the final stage is achieved
with the help of eight 200 W stages. Each 200 W stage consists of
two output transistors (TP 9383, SD1460 or FM 150) operating in
parallel. These RF
transistors operate in wide band Class C mode and are fitted to
the PCB by means of large gold plated spring contacts to obviate
the need for soldering. The output of all these stages is combined
via coupling networks to give the final output of 1.5 kW. A monitor
in each amplifier controls the power of the driver stage depending
on the reference voltage produced by the switch-on-control unit.
Since this reference voltage is the same for all the VHF amplifiers
being used, all of them will have the same output power.
36
Each amplifier has a meter for indicating the forward and
reflected voltages and transistor currents. Also a fault is
signaled if the heat sink temperature or the VSWR exceed the
prescribed limits. In both cases, the amplifier power is
automatically reduced to protect the transistors.
Power Supply System The FM transmitter requires 3-phase power
connection though all the circuits, except the power amplifiers,
need only single phase supply for their operation. An AVR of 50 kVA
capacity has been provided for this purpose. Power consumption of
the transmitters under various configurations is as follows :
Frequency of operation 87.5 to 100 MHz 100 to 108 MHz
Power Consumption 3 kW 5100 W 5320 W 5 kW 8500 W 8860 W 2 x 3 kW
10200 W 10640 W 2 x 5 kW 17000 W 17720 W
These figures do not include the power consumption of blowers
which is 200 W for each blower. For each transmitter, there is a
separate power distribution panel (mounted on the lower portion on
the front of the rack). Both the distribution panels A&B are
identical except for the difference that the LEDs, fuses and relays
pertaining to switching circuit of blowers and absorber are mounted
on the A panel. Panel Type Antenna The panel type antenna is to be
used on TV tower. Doordarshan have provided an aperture for FM
antenna on their towers. The size of this section is 2.4 x 2.4
mtrs. and its height is different at different places. The antenna
system envisaged for FM broadcasting consists of a total of 16
37
panels. For omni-directional pattern 4 panels are mounted on
each side of the tower. Ladders for mounting these panels have
already been provided on the four sides of the tower.
FM Transmitter STI(T) Publication 165 004/IC(Radio)/2004
Reflector panel Two numbers of bent horizontal dipoles and Two
numbers of vertical dipoles The capacity of each dipole is 2.5 kW.
Therefore, each panel is able to transmit 10 kW power. The
reflector panels are constructed of GI bars whereas the dipoles are
made out of steel tubes. Since each panel consists of 4 dipoles,
there are a total of 64 dipoles for all the 16 panels. Therefore
the power divider has 64 outlets to feed each of the dipoles. The
power divider will be mounted inside the tower. This antenna gives
an omni-directional pattern when the panels are mounted on all the
four faces. Feeder Cable For connecting the output power of the
transmitter to the dipoles through the power divider, a 3 dia
feeder cable has been used. This cable is of hollow type
construction and has to be handled very carefully. From the
building to the base of the tower, the cable is laid on horizontal
cable tray. Along with the tower this is fixed on the cable rack
provided for this purpose. The cable is clamped at every 1.5 m and
the minimum radius of bending of this cable is about 1 m. The cable
has been provided with two numbers of EIA flange connectors of 3
1/8 size on both ends. Both the connectors are of gas-stop type.
The cable connector on the antenna end i.e. on top of the tower is
made gas-through before hoisting. This is achieved by drilling a
hole through the Teflon insulator inside the connector. A dummy
hole (drilled only half way) is already provided by the
38
manufacturer for this purpose. The weight of the cable is about
2.7 kg per meter and the power handling capacity is about 27 kW.
Since enough safety margin has been provided in the power handling
capacity, no standby cable has been provided. This cable can be
used later for two transmitters by diplexing. The attenuation loss
of the cable is about 0.44 dB per 100 meter length. The cable and
the antenna system should be fed with dry air by means of a
dehydrator provided with the transmitter.
Noise Considerations In FM FM offers the advantage of a much
better noise performance as compared to AM, the reasons for which
are analysed here. The main parameter of interest at the input to
the FM detector is the carrier-to-noise ratio (C/N). Since both the
carrier and the noise are amplified equally by the various stages
of the receiver from antenna input to the detector input, this gain
can be ignored and the input to the detector can be represented by
the voltage source Es, which is the carrier rms voltage as shown in
fig 6(a). Also the thermal noise is spread over the IF bandwidth at
the input to the FM detector.
Noise in Narrowband FM
39
Noise In Wideband FM In AM, the maximum permissible modulation
index m= 1, but in FM there is no such limit. It is the maximum
frequency deviation that is limited to 75 kHz in wideband VHF sound
broadcasting service. At the highest audio frequency of 15 kHz the
modulation index in FM is 5. It will be much higher at lower audio
frequencies e.g. if modulating frequency is 1 kHz, the maximum
value of modulation index in FM will be 75. It may be seen from
figure 11 that as the modulation index is increased from mf =1 to
mf = 4, the signal-to-noise voltage ratio will increase
proportionately. Thus the S/N power ratio in a FM receiver is
proportional to the square of the modulation index. For mf = 5 and
modulating frequency of 15 kHz, there will be a 25:1 (14 dB)
improvement for FM, as compared to when mf = 1. No such improvement
is possible in AM. For an adequate C/N ratio at the detector input,
an overall improvement of 18.75 (4.75 + 14) dB is achieved with
wideband FM as compared with AM.
40
Pre-emphasis and De-emphasis According to noise triangle, the
noise output of FM detector increases linearly as the modulating
frequency increases. Also we know that in a complex audio signal,
the higher audio frequencies are weaker in amplitudes. Thus it is a
double tragedy for the high audio frequencies, their amplitudes are
small but they have to face higher noise levels as compared to
lower audio frequencies. To overcome this problem, the higher audio
frequencies are given an artificial boost at the transmitter in
accordance with a pre-arranged curve. This process is called
pre-emphasis. In the FM receiver, the higher audio frequencies are
restored to their normal levels through a reverse process called de
emphasis.
41
42
Advantages Of FM over AM
1. Amplitude and hence power of FM wave is constant and
independent of depth of modulation. But in AM, modulation depth
determines the transmitted power. Thus additional energy is not
required as modulation is raised. 2. FM is more economical than AM
due to following reasons:
43
(a) It is possible to have Low Level Modulation in FM as the
intelligence is in the frequency variations only and the modulated
signal can be passed through class C amplifiers. But since the AM
signal contains information in amplitude variations, so only high
level modulation is possible in an AM transmitter. (b) All the
transmitted power in FM is useful whereas in AM most of it is in
the carrier which contains no useful information. (c) Antenna gain
is possible in FM due to the reason that directive antennas are
used in VHF range where the physical dimensions of the antenna are
very easy to manage. 3. Better Noise Performance Amplitude
variations caused by noise are removed by having limiter in FM
receiver. This makes FM reception lot more immune to noise than AM
reception. Noise can further be reduced in FM by increasing the
frequency deviation. This is not possible in AM as modulation
cannot exceed 100 % without causing severe distortion. Less
adjacentchannel interference due to better planning as the
commercial FM broadcasts began in 1940s (much later than AM) ------
a guard band has been provided as per CCIR standard frequency
allocations. FM broadcasts operate in the VHF and UHF ranges in
which there happens to be less noise than in the MF and HF ranges
occupied by AM bands. Due to the use of space wave propagation in
which the range of operation is limited to slightly more than line
of sight, it is possible to operate several independent
transmitters with much less co-channel interference. 4. Stereo
transmission is possible with FM due to its wider bandwidth 5.
Additional information such as RDS, SCA can be sent along with the
stereosignal
44
CHAPTER 4 Satellite Communication
45
SATELLITE COMMUNICATION
Introduction Satellite Communication is the outcome of the
desire of man to achieve the concept of global village. Penetration
of frequencies beyond 30 Mega Hertz through ionosphere force people
to think that if an object (Reflector) could be placed in the space
above ionosphere then it could be possible to use complete spectrum
for communication purpose
Advantages of satellite Communication The following are the
advantages of satellite communication - This is only means which
can provide multi access two way communication. Within the coverage
area, it is possible to establish one way or two way communication
between any two points. - The cost of transmitting information
through satellite is independent of distance Involved. - Satellite
can be used for two way communication or broadcast purpose with the
covered area. - Satellites are capable of handling very high
bandwidth. Normally any satellite can accommodate about 500 MHz in
C Band. For example the bandwidth of
46
INSAT-I is 480 MHz in C Band and 80 MHz in S Band. INSAT-II has
a bandwidth of 720 MHz in C Band and 80 MHz in S Band. - It is
possible to provide large coverage using satellite. For example
Geostationary satellite can cover about 42% of earth surface using
global beam. - Satellite can provide signal to terrestrial
uncovered pockets like valleys and mountainous regions. -
Satellites can provide uniform signals for urban areas or rural
areas unlike terrestrial service which will lay more signal to
urban areas (where the transmitters are located) as compared to
rural areas. - It is easy and quicker to establish new satellite
link using SNG terminal or VSAT terminal from any point to any
other point as compared to any other means.
TVRO System Presently Doordarshan is up linking its national,
metro and regional services to INSAT2A (74oC) and INSAT-2B (93.5oE)
and INSAT 2E (83o C). Down link frequency bands being used are
C-Band (3.7-4.2 GHz) and Ex-C Band (4.5-4.8 GHz). A simple block
diagram of a satellite earth station uplink/down link chain is
shown in fig. 2(a).
47
Transmission of base band to Satellite The base band signal
consists of video (5 MHz), two audio subcarriers (5.5 MHz &
5.75 MHz) and energy dispersal signal (25 Hz). After modulation (70
MHz) and upconversion (6 GHz) the carrier is amplified and uplinked
through Solid Parabolic Dish Antenna (PDA). Down link signal can be
received through same PDA using Trans-Receive Filter (TRF) and Low
Noise Amplifier (LNA). After down conversion to 70 MHz, it is
demodulated to get audio and video. Satellite Transponder As shown
in fig. 2(b), the uplinked signal (6 GHz) at satellite is received,
amplified and down converted to 4 GHz band and sent back through
filter and power amplifier (TWT).
48
The local oscillator frequency of down converter is 2225 MHz for
C band and Ex-C band transponders.
49
CHAPTER 5 Conclusion
50
Conclusion
I conclude that I have completed my training successfully on
Radio Broadcasting. I have learnt about studio station working,
studio transmission, short wave broadcast, Medium wave broadcast,
and FM broadcast (VIVIDH BHARATI CHANNEL).I have also learnt about
uplinking and down-linking of audio data with the help of a
satellite. I learnt how we are able to receive various radio
channels from a radio station and also how we are able to receive
channels from Delhi and other metropolitan cities on our radio by
downlinking from satellite at radio station and then
broadcasting.
51
CHAPTER 6 Remarks on Training
52
Remarks
53