In British India broadcasting started in June 1923 with programs by the Radio Club of Bombay. Then, by an agreement of 1926 the private Indian Broadcasting Company (IBC) was granted permission to operate two radio stations; the Bombay station was inaugurated on July 23, 1927, the Calcutta station followed on August 26, 1927. But already on March 1, 1930 the Company went into liquidation. Thus the Government took over the broadcasting facilities, starting the Indian State Broadcasting Service (ISBS) on April 1, 1930 (on experimental basis for two years, but continued in May 1932). On June 8, 1936 the ISBS was renamed All India Radio (AIR; also known as Akashvani since 1956).
A radio wave carries information signal; Signals are converted into electrical signals. A carrier wave is then produced from the modulation. The wave is then amplified, and sent to the antenna that then converts signal into an E.M. wave.
An antenna on receiving the signal send it to the receiver this then converts the electrical signal sends it to the amplifier either a speaker/headphones jack this is then converted into a sound wave.
Methodology of Radio Modulating signal Carrier Wave Modulated Wave Microphon e Broadcast Tower How radio electromagnetic waves are changed to Radio Waves - Radio
In Realty it works like this: Phase 1 : Baseband Or the Original signal message signal Phase 2 : After carrier being sent then modulating siganl
Microphone is a device which converts acoustical energy into electrical energy . In the professional broadcasting field microphones have primarily to be capable of giving the highest fidelity of reproduction over audio bandwidth .
The first type of studio microphone is the condenser microphone.
The second type of studio microphone is dynamic microphone.
The third major type of studio microphone is the ribbon microphone .
Pressure Operated Type : In such microphones only one side of the diaphragm is exposed to the sound wave. The output voltage is proportional to the sound pressure on the exposed face of the diaphragm with respect to the constant pressure on the other face. Moving coil, carbon, crystal and condenser microphones are mostly of this type. In their basic forms, the pressure operated microphones are Omni-directional.
Velocity or Pressure Gradient Type : In these microphones both sides of the diaphragm are exposed to the sound wave. Thus the output voltage is proportional to the instantaneous difference in pressure on the two sides of the diaphragm.
Microphones can be designed either to respond equally to sounds from an angle or to discriminate those arriving from specific directions. Microphones which respond equally at all angles are called Omni-directional. The microphones which pick up equally from front and rear and have very little pick up equally from sides are called Bi-directional and have a polar diagram as figure of eight. The microphones which pick up maximum from the front with slight reduction in the sides and very less pick up from the rear are called C
These are the kind of microphones that generate signals by a conductor which is in magnetic field being in motion.
The majority of microphones that fall under the dynamic microphone category have a light, very thin diaphragm that gets in motion responding to the pressure of the sound it is exposed to . The motion creates voice coils which are suspended to magnetic fields, making the fields to move which in turn creates electric currents in small quantities.
Majority of dynamic studio microphone can operate without external power supply.In many occasions dynamic microphones are used in studios to record drums, electric guitars and the like.
a velocity microphone , which means that such usually respond to the velocity of the air that is produced by the sound rather than the level of the pressure caused by the sound which many other microphones respond to.
Ribbon microphones operate by suspending loosely small elements in a magnetic field that is quite strong . The “ribbon” is put to motion by the moving air thus cutting through the magnetic fields which in turn creates audio signals . Typically, have pick up patterns making a figure 8 .
The microphone must have proper impedance and a balanced or unbalanced output suited to the pre-amplifier . In the broadcast chain the microphone lines cover long distances, therefore, the impedance is chosen in the range of 50 ohms to 60 ohms at the microphone terminals
Sensitivity : The ability to pick up weak sound and to deliver more electrical signal determines the sensitivity . It is measured in dBs below 1 volt as the electrical output from a microphone when a standard sound pressure of one microbar i.e. 1 dyne per sqr. cm. is applied at the diaphragm of the microphone
Placement of microphone has important bearing on the quality of its output. A few general guidelines given in the following paragraphs should help in improvement of programme production.
Microphones should be placed with its 0 o axis facing the source of sound to avoid off axis coloration.
Phasing of Microphone : Whenever two or more microphones are used with their outputs mixed together, it should be ensured that their outputs are in phase.
Working Distance : Whenever a directional microphone is kept fairly close to the source of sound low frequencies in the output of the microphone may get disproportionately boosted thereby giving rise to boom sound . This effect known as proximity effect. This effect should be normally avoided by placing the microphones fairly away (30-45 cm) from the source of sound.
The magnetic material used in recording is magnetic oxide of iron Fe 2 O 3 and Fe 2 O 4 .This is mixed with suitable adhesives, plasticizers, fillers etc. and applied in the form of an extremely smooth, even and thin coating (0.4 to 0.6 mils) on to a PVC backing (1.0 to 1.5 mils thick ) . This tape gets magnetized when it comes in contact with a recording head with audio frequency signal currents flowing through the head windings, and as it passes on forward, retains the magnetism induced, due to the magnetic properties of remanance and coercivity. Thus if the tape is moved across the head at a constant speed of V cm/sec. and the signal current is of frequency "f" Hz , the signal current variations in time, will be recorded as magnetic intensity variations along the tape length . Thus a single cycle will be recorded on a tape length V/f cm. This is called recorded wavelength
The magnetized tape when passes over the play back head, which is almost similar to the record head, will induce tiny voltages in the coil depending upon the intensity of magnetization of the tape, which itself is representative of the signal current fed to the recording head at the time of recording. Thus, the signals recorded magnetically are reproduced from the tape.
Method for this is to feed the erase head with a high amplitude signal of about 100 kHz and the tape passes over this erase head before it passes on to the record head. In this arrangement every part of the tape passes the erase head gap (about 15 mil) and is subjected to about 200 cycles of alternating magnetic field, starting from low value at the start of the gap, increasing to saturation value in the middle of the gap and again steadily dropping to low value of the field, as the tape leaves the gap . These repeated magnetizing cum de-magnetizing cycles erase the signal completely and leave the tape in completely demagnetized form similar to a virgin tape without a magnetic history.
Magnetic recording is made possible due to magnetism remaining behind after the magnetizing force is removed. The curve showing the relation between remanant magnetism (Br) and magnetizing field (H), indicates that the relation is non-linear in the beginning. Therefore a signal recorded as such will have a high component of harmonic distortion of the wave form as shown in the Fig.8 and is not useful. This can be partially improved by using D.C. bias to avoid the instep of the initial Magnetization curve (I.M.C) and shifting the operating point to a more linear middle region as shown in the fig. 9. This method of D.C. bias does give a fairly satisfactory quality of recording but the noise and distortion in the reproduced signal leaves much to be desired and thus only part of Br-h curve is utilized.
when the high frequency bias, a tap off from the erase head was also applied to the record head along with the audio signal to be recorded. This H.F. bias current agitates the magnetic particles on the tape, sufficiently that they settle down to the flux value, corresponding to the superimposed signal current, overriding the in step non-linear portion of the I.M.C. Both halves of the Br-h curves are utilized and thus flux recorded is high, signal to noise ratio is good and distortion is low as a result of H.F. bias. It may be noted that the bias signal is not recorded or reproduced. It only acts as a vehicle to carry the audio signal beyond the non-linear portion .
The rewind and forward motors are ordinary induction motors. Their torque depends on the voltage applied. The rewind and F/F motors freely rotate in opposite direction . In fast forward mode, the F/F mode is supplied full voltage and the R/W motor, a fraction of it, so the tape moves forward under a small reverse drag to keep the tape taut. I n rewind mode, the R/W motor is given full voltage and the F/F motor a small voltage. The tape rewinds fast under a small reverse drag . In play back mode, the motors are given equal voltage and about half the full voltage . The tape as such should not move in any direction. During record/replay mode the tape must move with constant speed, as otherwise the frequency reproduced will be different from the one recorded This is achieved by the capstan servo control motor.
The Recording Chain : A correct amount of HF boost is provided to pre compensate for the HF record process losses as described earlier. It converts constant voltage input into constant current output . This is required because the record head is a current operated device and the magnetic flux is proportional to the current flowing in the record head coil.
The Playback Chain : The output of the PB head is rather low and rising with frequency. Great care is taken in electrically and magnetically shielding the P.B. Head to avoid hum pick up.
With a constant current to the recording head at various frequencies, the flux recorded on the tape will be less for higher frequencies. The most important ones are:-
Head losses , due to hysteresis and eddy current being more at higher frequency.
Self demagnetization losses also are higher at higher frequencies as the recorded wave length and the associated half wave bar magnets become smaller and smaller in length and are closely packed and try to get demagnetized by mutually canceling effect of the bar magnets.
High frequency flux is not able to penetrate the full depth of the magnetic coating on the tape. Only the layer near the surface effectively contributes towards recorded flux. This also causes loss of recorded flux as the signal frequency increases
Radio networking terminal RNT acts as the ground terminal for satellite signal reception
Passive transmitting unit : The passive translator splits the combined nominal IF of 70 MHz into 52 MHz and 92 MHz components
Active frequency translator : The function of FTA is to boost the RN carriers and translate them all to 52 MHz band .
The 52 MHz band signals coming from FTP are amplified in IF amplifier having a gain of 50 dB. Then the output is divided into three outputs using power divider . The 92 MHz band signals coming from FTP are also amplified in 50 dB. IF amplifier then converted into 52 MHz band after beating with 40 MHz oscillator. The output of mixer is passed through a band pass filter and then amplified and further divided into three outputs.
A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is related to the phase of an input "reference" signal. It is an electronic circuit consisting of a variable frequency osci llat or and a phase detector . This circuit compares the phase of the input signal with the phase of the signal derived from its output oscillator and adjusts the frequency of its oscillator to keep the phases matched. The signal from the phase detector is used to control the oscillator in a feedback loop .
Frequency is the derivative of phase. Keeping the input and output phase in lock step implies keeping the input and output frequencies in lock step. Consequently, a phase-locked loop can track an input frequency, or it can generate a frequency that is a multiple of the input frequency. The former property is used for demodulation , and the latter property is used for indirect frequency synthesis .
Synthesizer unit This unit consists of six modules of synthesized frequency translator . Each module takes the 52 MHz IF. Synthesizer is used as a variable local oscillator. It consists of a VCO and a PLL. In PLL synthesizer a reference signal is generated using a crystal of 2.048 MHz
The demodulator has a band pass filter in the first stage and is tuned to 5.5 MHz. After BPF the sub carrier is amplified and then demodulated in a phase locked loop frequency demodulator circuit. The audio signal retrieved after the PLL demodulator are amplified in operational amplifier which incorporates de-emphasis circuit. This is followed by a expander. The expansion is done by an IC. The audio is amplified and filtered. The unbalance to balance is done by a repeat coil. The output is balanced 600 ohms and is providing +9 dBm .
AM broadcast bands gives shrinkage in the night-time service area due to fading, interferenc e, etc. FM broadcasting offers several advantages over AM such as uniform day and night coverage, good quality listening and suppression of noise, interferenc e, etc. All India Radio has gone in for FM broadcasting using modern FM transmitters incorporating state-of-art technology.
Working Of an FM Transmitter L R Stereo coder VHF oscillator & modulator Wide Band Power Amplifier Frequency crystal oscillator 10Mhz Frequency divider 1/1000 Phase detector Rectifier and filter Programmable divider 1/N antenna
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 . 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 .
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 synthesized with the help of digital frequency synthesis techniques. 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
A tower of good height is required for mounting the FM antenna since the coverage of the transmitter is proportional to the height of the tower . For a 100 m height, the coverage is about 60 km . Wherever new towers were to be provided, generally they are of 100 m height since beyond this height; there is steep rise in their prices because of excessive wind load on the top of the tower.
The main requirements of the antenna to be used for FM transmitters are :
Wide-band usage from 88 to 108 MHz range.
Omni-directional horizontal pattern of field strength.
Circular polarization for better reception.
High gain for both vertical and horizontal signals.
The pole type antenna is mounted on one of the four faces of the tower. This system will give a field pattern within a range of 3 dB. The antenna is mounted in such a direction in which it is required to enhance the signal.
The other important features are:
Very low power radiation towards Transmitter building.
Spacing between dipoles is 2.6 m and all the dipoles are mounted one above the other on the same face.
Lengths of feed cables of dipoles will be different and has been calculated to give a beam tilt of 2 o below horizontal.
The feed point of the antenna is looking towards ground so as to avoid deterioration of the insulating flange. This flange consists of high density PVC. The life of this is expected to be about 7 to 10 years
The panel type antenna is to be used on TV tower. 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.
Each panel consists of:
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. 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.
It is for connecting the output power of the transmitter to the dipoles through the power divider. Enough safety margins have 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 cable and the antenna system should be fed with dry air by means of a dehydrator provided with the transmitter.
variable tuning capacitor (150pF) coil aerial Our tuner uses a coil of wire (called an inductor) and a capacitor. The combination of inductor and capacitor makes something called a “ resonator ”-- it is a circuit that throws away all the unwanted signals, and keeps only the one that we want . The resonator resonates at a particular frequency that is determined by the size of the inductor and capacitor. The charge sloshes back and forth in the tank circuit at a certain frequency and the station being tuned in to must be very near that frequency In my radios, I will use a capacitor that has a fixed size, and i will “tune” the radio to different stations by changing the size (the "inductance") of the inductor. Working of tuner circuit Phase 1
Working of tuner phase 2 The tuner coil has about 200 turns of wire with a thin red insulation on it. The insulation is scraped away on the top. To tune the radio, we will use a brass strip with a contact on the end that will make electrical contact at different points along the coil, where the insulation was scraped away . By pivoting the brass strip about a screw, the sliding contact will move across the coil, changing its inductance . When the contact is near one end the inductance will be very small , and the circuit will tune in radio stations whose carrier frequency is very high . When the contact is near the other end, the inductance will be large and the circuit will tune in radio stations whose carrier frequency is very low. pivot
. The detector is something called a “germanium diode”. The germanium diode lets electricity flow in one direction but not in the opposite direction. The germanium diode looks like a little glass cylinder with metal wires coming out each end. It is recovery of baseband signal that can be done by coherent detection or synchronous detection i.e. by producing the same carrier signal at receiving end with same generating or centered frequency and phase angle to avoid any discrepancy
The signals that we pick up with the antenna and tuner are very small -- maybe only a few thousandths of a volt. (A regular flashlight battery is 1 ½ volts). So, the amplifier makes the signal bigger. In our radios, we will two little things called “integrated circuits”, or “chips”, to make the signal bigger. There are a lot of different kids of integrated circuits. The kind that we will use are called “op-amps”, or “operational amplifiers”. Each op-amp looks like little spiders with 8 legs! We will use 2 of them in each radio. It simply produces a more powerful version of the audio signal.