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 INDIAN RADIO,
BHOPAL (M.P)
ON
RADIO BROADCASTING
Training Under:
Submitted By:
Ravindra Goel(Station Engineer)
Subedar Singh(0103EC071108)
Department of Electronics & Telecommunication
Engineering
Lakshmi Narain College Of Technology, Bhopal (M.P.)
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
and 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.
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 programmes. In this section problems and design aspects of
internal acoustics of a broadcast studio are explained.
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 rereflected 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:y Volume of program
increases due to reverberation of sound y 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. y
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.
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
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 y Outside the building y
Inside the studio itself and /or y 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 quality telephone lines or studio transmitter
microwave/VHF links. A simplified block schematic showing the
different stages is given in Fig. 1.
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, certain basic minimum technical facilities
are required for smooth transmission of programmes and for proper
control. These are as follows:
y 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. y
Facility to fade in/fade out the programme smoothly and control the
programme level within prescribed limits. y Facility for aural
monitoring to check the quality of sound production and sound
meters to indicate the intensity (VU meters). y 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. y 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) y Visual signaling
facility between studio announcer booth and control room should
also be provided. y If the programs from various studios are to be
fed to more than one transmitter, a master switching facility is
also required.
FACILITIES IN STUDIO CENTRE Introduction 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
y For recording of programmes originating from any studio. y For
recording of programmes available in the switching consoles in
control room. y For dubbing of programmes available on cassette
tape. y For editing of programmes y 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.
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
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 wave-length 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 : 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.
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. The maintenance
schedule suggested below should therefore be carried out very
regularly.
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. 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 y Marblex
floors should be protected from heavy point loads. y Furniture and
other heavy articles should not be dragged on the marblex flooring
y Kerosene, petrol, turpentine or any polish containing spirit,
should not be used for cleaning marblex flooring. y Hard scrubbing
must be avoided. y A door mat should invariably be used near the
entrance to keep the dust away. y 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.
WALLS 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.
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.
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. iv) Studio
Fixtures : All studio fixtures such as signal lights, designation
plates, clocks and light fittings should be dusted and cleaned once
a week with a clean soft cloth. Special care is needed in cleaning
these fittings since they are generally of a fragile nature.
EQUIPMENT IN STUDIO i) 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. ii) Fader Boxes : These should be
dusted and cleaned every day. iii) 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. iv) 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
y y y y
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.
y y y y y y y y y y y .
Muslin Cloth Vacuum cleaner Dry distemper (Ivory) Some small
pieces of clean Celotax Sand paper number 100. 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
BROADCASTIN: There 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. 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 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.
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.
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 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-inunit (KA 033A1), also ensures the
parallel operation of transmitter B. Combining unit is housed in a
separate rack.
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 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), 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 supply
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.
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) 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. 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.
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. 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 3 kW 5100 W
5320 W Power Consumption 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
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. 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 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
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.
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.
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: (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 adjacent- channel 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
stereo Signal
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 up-linking 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 down-linking from satellite at radio station and
then broadcasting.