ALL INDIA RADIO, KOTA submitted by: Jitendra Malav 11BEC0082 ECE & 4th year IntroductionAIR,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 kHzThis 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.
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.
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.
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.AM TransmitterAIR,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
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.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 StagesThis 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.
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.
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.
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 CentreThe 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.
Fig. Simplified block schmatic of broadcasting chainBroadcast StudioA 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 providedMixingAs 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.
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 mixingIn low level mixing, there is signal loss of about 10 to 15 dB