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.)

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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)

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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

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CHAPTER 1 Introduction of the company

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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

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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

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CHAPTER 2 Studio Section

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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.

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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.

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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

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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

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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,

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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.

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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.

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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

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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 :

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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.

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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.

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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.

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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.

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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.

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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.

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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

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CHAPTER 3 Broadcasting Section

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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.

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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

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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.

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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.

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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

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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.

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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

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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),

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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)

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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.

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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.

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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.

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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

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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

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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

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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.

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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.

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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:

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(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

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CHAPTER 4 Satellite Communication

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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

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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).

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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).

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The local oscillator frequency of down converter is 2225 MHz for C band and Ex-C band transponders.

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CHAPTER 5 Conclusion

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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.

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CHAPTER 6 Remarks on Training

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Remarks

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