All India Radio, Kota Internship Presentation

1. ALL INDIA RADIO, KOTA
submitted by:
Jitendra Malav
11BEC0082
ECE & 4th year

2. Introduction
• AIR,Kota is situated at Jhalawar Road,Kota (Rajasthan).It started working on 4th
january 1987. Initially there was only 1KW MW transmitter. Programmes are
broadcast mainly in two languages Hindi and Hadoti.1 KW transmitter cover 50 km
and 20 KW transmitter covers 150 km .
• 1)1KW medium wave transmitter
Operating frequency : 1584 KHz
This frequency channel is also called as Chambal chanel of Akashvani Kota. For this
channel many programs are locally produced here in Akashvani Kota studio.These
programs may be sent to 20 KW SW transmitter situated in Ummedganj,Kota via STL
used for coupling of transmission.
• 2) 20 KW medium wave Transmitter(Ummedganj,Kota)
Operating frequency : 1413 kHz
• This transmitter works on principle of sky wave propagation. It has two operating
frequencies because of change in thickness of ionosphere layer. It has very high
coverage i.e. programmes are listen all over cities because of its transmission. Its
frequency response is almost flat on entire audio range.

3. AM Modulation:
If the amplitude of the carrier is varied in accordance with the amplitude of the
modulating signal(information), it is called amplitude modulation. This modulation has
been shown in figure.
o
o
o
o
E
EE
m
E
EE
m
ulationofDegreemmm
min
max
mod








Emin
Emax
E0
E0
0
RF Carrier
Modulating signal
0
0
AM signal

4. FM 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.

5. ADVANTAGES OF FM OVER AM MODULATION:
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.

6. AM Transmitter
• AIR,Kota has one 1KW medium wave transmitter(AM Tx)
having operating frequency1584 KHz. A.M. Transmitter of any
power in general will have a separate HF and AF stages. In the
conventional transmitters, vacuum tubes are used right from
the first stage to the final stage and the preliminary stages are
solid state devices. A brief description of RF and AF stages
and Power Supply of a 20 KW AM transmitter(AM Tx) is given
below

7. • RF Section
RF section consists of crystal oscillator, buffer, intermediate power Amplifier,
Exciter and power amplifier. The crystal oscillator with buffer stage is
generally kept together and is shielded by a metal cover to isolate from other
circuits.
• IPA Stage
This stage employs an indirectly heated beam power tube BEL 25 and it
operates as a class C amplifier.
• Exciter
This stage is operated as a class - C amplifier. This stage is modulated about 10
to 20%. A small secondary tap from the modulation transformer supplies the
necessary audio and super-imposes on the DC Plate supply. When the triodes
are anode modulated, the grid must be overdriven in the carrier condition in
order that the drive level will be adequate to sustain the peak anode current
at 100% modulation.

8. • Power Amplifier Stage
This is a class - C power amplifier obtaining the required output by means of
three parallel connected forced air cooled, directly heated triode tubes type
BEL 3000. As a triode tube is used in this stage, neutralization technique is
adopted to neutralize, the grid-plate capacitance.
• AF Circuits
The audio frequency amplifier consists of two voltage amplifiers, a cathode
follower which serves as a driver to the modulator and the modulator is a
class B push pull power Amplifier.
• First and Second AF Amplifier Stages
This stage is operated as a class A push pull connected amplifier employing
two indirectly heated pentode type 4P55 or its near equivalent which
provides about 30 dB gain. The output from the first AF stage is coupled to
the second stage through the coupling condensers. Plate supply is obtained
from the neutral of the HT. (Plate) Transformer.

9. • Sub Modulator Stage
This stage employs two 4B 85 (or its equivalent with modifications) in
push-pull mode to excite the modulator. The sub-modulator is a
cathode follower. As the grid current flows in the modulator tube, the
input impedance varies widely with different input levels and hence a
cathode follower which possesses low output impedance, very small
non linear distortion for load impedance variations and good
frequency and phase shift characteristics is used. The DC. potential of
the cathodes of sub-modulator and the grid of the modulator stages
are kept nearly at the same negative voltage of about 200 volt.
• Modulator Amplifier
This is the final stage audio frequency power amplifier which supplies
the RF power amplifier, the required modulating power. The HT and
the superimposed audio signals are connected to the plate of the PA
valves. It may be noted that the negative feedback Network is
connected in the primary of the modulation transformer.

10. • Power Supply
Filament Supply
For PA and modulator valves, there is a separate filament transformers with centre tap
arrangement. The centre tap will be grounded through metering current shunt
resistance for the measurement of a cathode currents and an overload coils in parallel
with a resistance.
Low Tension : 3 phase 220 V AC is stepped up to 3 phase 520 V AC using a
Delta/Star connected transformer. It is rectified using silicon diodes and filtered using
L C components. It gives DC voltage to the following.
1. Plate and screen of 1st AF, 2nd AF, oscillator and Buffer.
2. Screen grid of sub modulator
3. Sub modulator plate and IPA plate.
Bias : 3 phase 400 V AC is stepped up to 3 phase 470 V using Delta/Star
connected transformer and rectified using silicon diodes in two sets SE 2 and SE3 and
filtered using L-C components. SE 2 output supply is connected to the cathode Bias of
sub modulator. The out put of SE 3 is connected to control grid of Exciter and Grid of
P.A.
High Tension : 3 phase 400 V AC is stepped up to 2300 V 3 phase and rectified
using silicon diodes assembly SE4 and filtered using L-C components. Full HT is
supplied to plate of modulator and PA valves. The filtered DC from the star point of
the HT transformer is connected to the plate of 2nd AF and plate and screen grid of
Exciter.

11. 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”.
• Studio Centre
The Studio Centre comprises of one or more studios, recording and dubbing
room, a control room and other ancilliary rooms like battery room, a.c.
rooms, switch gear room, DG room, R/C room, service room, waiting room,
tape library, etc. . The studio centres in AIR are categorised as Type I, II, III
and IV. The number of studios and facilities provided in each type are
different. For example a type I studio has a transmission studio, music studio
with announcer booth, a talks studio with announcer booth, one
recording/dubbing room and a Read Over Room. Type II has one additional
drama studio.
A simplified block schematic showing the different stages is given in Fig.

12. Fig. Simplified block schmatic of broadcasting chain

13. • Broadcast Studio
A broadcast studio is an acoustically treated room. It is necessary that the
place where a programme for broadcast purposes is being produced should
be free of extraneous noise. This is possible only if the area of room is
insulated from outside sound. Further, the microphone which is the first
equipment that picks up the sound, is not able to distinguish between wanted
and unwanted signals and will pick up the sound not only from the artists and
the instruments but also reflections from the walls marring the quality and
clarity of the programme. Outside of every studio entrance, there is a
warning lamp, which glows ‘Red’ when the studio is ‘ON-AIR’ The studios have
separate announcers booths attached to them where first level fading, mixing
and cueing facilities are provided
• Mixing
As already mentioned, various equipments are available in a studio to
generate programme as given below:
• Microphone, which normally provides a level of –70 dBm.
• Turntable which provides an output of 0 dBm.
• Tape decks which may provide a level of 0 dBm.
• CD and R-DAT will also provide a level of 0 dBm.

14. Audio mixing is done in following two ways:
1. Required equipments are selected and then outputs are mixed before
feeding to an amplifier. This is called low level mixing . This is not commonly
used now days.
2. Low-level output of each equipment is pre-amplified and then mixed. This
is called high level mixing.
Fig. High level mixing Fig. Low level mixing

15. • In low level mixing, there is signal loss of about 10 to 15 dB in mixing
circuits. Therefore, the S/N ratio achieved in low level mixing is 35 to 40
dB only.
• High level mixing system requires one pre-amplifier in each of the low
level channels but ensures a S/N of better than 50 dB. All India Radio
employs High level mixing.
Announcer Console
Most of the studios have an attached booth, which is called transmission
booth or Announcer booth or play back studio. This is also acoustically
treated and contains a mixing console called Announcer Console. The
Announcer Console is used for mixing and controlling the programmes that
are being produced in the studio using artist microphones, tape playback
decks and turn tables/CD players. This is also used for transmission of
programmes either live or recorded. The technical facilities provided in a
typical announcer booth, besides an Announcer Console are one or two
microphones for making announcements, two turn tables for playing the
gramophone records and two playback decks or tape recorders for recorded
programmes on tapes. Recently CD and Rotary Head Digital Audio Tape
Recorder (R-DAT) are also included in the Transmission Studio.

16. • Control Room:
For two or more studios set up, there would be a provision for further
mixing which is provided by a control console manned by engineers.
Such control console is known as switching console. Broad functions of
switching console in control room are as follows:
• Switching of different sources for transmission like News, O.Bs.
other satellite based relays, live broadcast from recording studio.
• Level equalisation and level control.
• Quality monitoring.
• Signalling to the source location.
• Communication link between control room and different studios.
• Swicthing Console
This switching console has been specially designed for broadcast
applications keeping in view the necessity for high reliability and
continuous operation. The design of this Console has been evolved
after close interaction between the P&D Unit of AIR, ER & DC and M/s.
Keltron.

17. • MICROPHONES:
A microphone is an acoustic-to-electric tranceducer (sensor) that converts
sound into corresponding electrical signals. Its short names are mike and mic.
In broadcasting, a mic is the most basic device in any studio from where an
audio chain begins. Broadly, broadcast-quality of sound largely depends upon
the conversion & pre-amplification. Being users, it is important for us to
understand how they function, how to use them and care. In AIR &
Doordarshan, only professional- grade-microphones are used.
Special type of Mics used in Broadcasting
Clip on Mic Lavalier microphone LIP MIC

18. Instrument Mics
SHURE PG52 SENNIESER e 901 condenser mic SENNIESERe 904
Shotgun microphones
Digital Recording Mic

19. HDBR(HARD DISK BASED RECORDING) SYSTEM
The computer system deals mainly with texts and digits. These texts and
digits are in digital form and are recorded on the hard disk of the computer.
So, if the analog audio or video is converted into digital form, it can be
recorded on the hard disk of the computer. Then the computer can
manipulate the audio/video in the similar way as it manipulates texts and
digits. This is what is known as hard disk based recording system
• There are two types of hard disk based (audio) recording system :
1. Dedicated system and
2. Networked Hard Disk Based System
• Dedicated System
In dedicated system, there is only one computer terminal for recording or
play. This is mainly used for editing purpose with a special key board (edit
controller).

20. Networked Hard Disk Based System
In networked hard disk based system a number of work stations (computer
terminals) are connected together to the main server (central server). They
work in a LAN environment. This system facilitates the following :-
• Integrated studio automation system
• Simplified operational task
• Reduced handling cost
• Elimination of monotonous repeat works.
• Instant and random access to all audio clips.
• Detailed logging of on-air events
Function of the System: The hard disk based system performs the
following functions :
• Playback
• Recording & Editing
• Storage and archiving
• Scheduling
• Transmission and Distribution

21. AUDIO COMPRESSION :
The ultimate goal of compression is the bit-rate reduction for storage and
transmission. Bit rate is the product of the sampling rate and the number of
bits in each sample and this is generally constant. However, the information
rate of a signal varies. The difference between the bit rate and the
information rate is known as the redundancy.
(1)MPEG-1 audio : Total bit rate of 1.5 Mbit/sec for CD quality multimedia
storage, 1.2 Mbits/sec is for video, and 256 Kbits/sec is for audio, up to two
channels of audio are accommodated .
(2) MPEG-2 audio : HDTV applications. In its audio part, two to five full
bandwidth audio channels are accommodated.
(3) MPEG-4 audio : mobile access, low complexity multimedia terminals to
high complexity multichannel sound systems.

22. SATELLITE COMMUICATION
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.

23. • Architecture of a Satellite Communication System
Basically it comprises two elements :
• Ground Segment
• Space Segment
The Ground Segment
The ground segment consists of all the earth stations ; these are most often
connected to the end-user’s equipment by a terrestrial network or, in the
case of small stations (Very Small Aperture Terminal, VSAT), directly
connected to the end-user’s equipment. The largest are equipped with
antenna of 30 m diameter (Standard A of the INTELSAT network). The
smallest have 0.6 m antenna (direct television receiving stations).Some
stations are both transmitters and receivers. Others are only receivers.

24. • Satellite Transponder
• 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). The local oscillator frequency of down converter is 2225 MHz for C
band and Ex-C band transponders
• STUDIO TRANSMITTER LINK
• The high quality sound programs from AIR studio centers are normally
transported to the AIR transmitting centers with the help of Department
of Telecommunications land lines. These gave way to VHF-FM
transmit/receive systems in some of the AIR centers. Now, AIR has
introduced the new generation microwave studio-transmitter link (STL) for
better reliability and quality.

25. • Studio Transmitter Link - Transmitter (STL-TX)
• It consists : A single audio input transformer which splits the audio input
into two equal audio outputs .
• The base band unit consisting of a music amplifier , and a base band
interface unit which is a 15 kHz low pass filter.
• The radio frequency unit which generates the carrier, FM modulates and
generates microwave (RF) power.
• An antenna change over unit which selects one of the RF outputs for
feeding to the antenna.
• A low loss cable connected to a microwave dish antenna at suitable height
above the ground.
• Two identical dc power supplies
• One each of logic and parameter control card which selects one of the RF
outputs to be connected to the transmitting antenna

26. • Studio Transmitter Link – Receiver (STL-RX)
• The STL receiver essentially consists sections similar to STL transmitter.
• 2 m dia microwave dish antenna mounted on a tower of suitable height
and a low loss cable connects the received RF power into the receiving
system.
• Antenna filter and RF hybrid divider unit.
• The radio frequency receiver unit which recovers the base band signal
from the modulated RF carrier.
• A single audio line transformer which provides the audio output.
• Two identical DC power supply units.
• One each of Logic and parameter control card which selects one of the
audio outputs
The receiver has a threshold sensitivity of the order of –80 dBm. But it is
advisable to use the system at –40 to –60 dBm.

27. SAFETY MEASURES
• FIRE EXTINGUISHERS
• Soda Acid Type
• Chemical Foam Type
• Carbon-tetra-chloride
• CO2 Gas Type ,Sand bucket
Earthing
• The term earthing means connecting the neutral point of a supply
system or non-current carrying parts of electrical apparatus to the
general mass of earth in such a manner that at all times an
immediate discharge of electrical energy takes place without
danger.
• The function of earthing is two fold
• It is for ensuring that no current carrying conductor rises to a
potential with respect to general mass of earth than its designed
insulation.
• It is for the safety of the human beings from the electric shocks.

28. • CONCLUSIONS
• The training at RADIO broadcasting station indeed adds
knowledge of wide concept of wireless communication
especially for broadcasting purpose. The study of
operation and maintenance of studio consoles and the
medium wave high power transmitter helps in relating
theoretical concept of communication with the
practical.
• Also, the measuring instrument used there give better
understanding of the monitoring and control of audio
signal as well as modulated signal" It was very
satisfactory period of training where I gained very
useful overall training. It also provided me an
opportunity to develop myself as an engineer with
Competitive edge.