TELEPHONE LINKS:

ABSTRACT:

Telecommunication is the assisted transmission of signals over a distance for the purpose of communication. In telephone links there are three main components involves i.e. electronic transmitters, medium and receivers. Telecommunications involves the use of electronic transmitters such as telephone, television, radio or computers. The objective of this paper is to discuss the basic elements of telephone links in communication system.

INTRODUCTION:

Telecommunication is the process of sending and receiving the electrical signals over all large distances this is the data and information such as voice, data, video and graphics. A telecommunication network is a network of nodes and links and the communication signals passes through one line to another line

A telecommunication system is defined by the engineers. There are three main components involves in the telecommunication system i.e. transmitter, medium and receiver. A transmitter sends the information in the form of a electrical signals through a medium and receiver takes the information and converts in the readable form. In the early times, this may have involved the use of smoke signals, drums and flags. In modern times, telecommunication typically involves the use of electronic transmitters such as the telephone, television, radio and computers.

TELEPHONE LINKS: The telephone can be used to pass a message quickly and clearly to almost any part of the world. But sound energy cannot travel far along, so it is converted into electrical energy. Simple Telephone: A telephone is one of the devices you have in your house. It is so simple because the telephone connection to your house has not changed in nearly a century. The very simplest working telephone would look like this inside: · Microphones : A microphone is a device that transforms sound pressure into electrical energy. In most types of microphone, the sound pressure acts upon a thin plate or diaphragm, setting it into vibration and this mechanical motion is then utilized to produce electrical effects. The chief types are described below. The various designs are also built to incorporate desired directional pickup properties. TYPES OF MICROPHONE: Carbon grain microphone Crystal microphone Dynamic microphone 4. Ribbon microphone 1.Carbon - Grain Microphone Carbon-grain microphone : One of the earliest microphones, and the type still most generally used in telephone practice, depends for its action on the fact that the electrical resistance between carbon granules in contact with each other varies with the contact pressure. The above figure shows a simplified sectional view of a single-cell or single-button carbon microphone, as is used in some telephone sets. A small brass cup contains two polished carbon discs, one fastened solidly in the cup and the other attached to the diaphragm. The space between the discs is partly filled with carbon granules, and as the diaphragm vibrates in response to the sound waves striking it, the varying pressure on the granules causes changes in the electrical resistance between the buttons. The microphone circuit is shown in the same figure, and from this it is seen that variation of microphone resistance will alter the current through the transformer primary, and so will set up induced voltages in the secondary. By proper choice of diaphragm stiffness and mass, the moving system can be made to resonate near the m1ddle of the speech range of frequencies. When this is done, the electrical output is large enough to operate a receiver over a considerable length of line. Without requiring amplification the frequency response is then not very uniform, although it is entirely adequate for speech reproduction. By using a very light diaphragm, tightly stretched, the frequency response is greatly improved, but at the expense of sensitivity. Carbon microphones with this type of construction, and having two buttons, of carbon cells, were used extensively in early broadcast. 2.Crystal Microphone: Another type of microphone, widely used in public-address systems, depends for its action on the piezoelectric effect possessed by certain crystals, for example Rochelle salt. The term piezoelectric effect refers to the fact that when pressure is applied on the crystal in the proper direction, electrical potentials are produced between opposite faces of the crystal. The sound-cell type of microphone contains an assemblage of small crystals of this type, so connected that their piezoelectric potentials are in series. The sound falls on the crystals and vibrates them directly. The electrical output is quite small, but the frequency range and uniformity of response are excellent. 'In another type of crystal microphone, a metal diaphragm is coupled mechanically to a crystal of Rochelle salt in such 'a way that vibration of the diaphragm causes a twisting of the crystal, and thereby the generation of a voltage at the terminals. This type has much greater, output than the sound cell, but the frequency response is limited by the inertia and stiffness of the diaphragm and the associated driving members. The crystal microphone has high output impedance, making it well suited to direct connection to vacuum tube amplifiers. 3.Dynamic Microphone: Several types of microphones depend for their action upon the voltage induced in a conductor moving in a magnetic field. The dynamic or moving-coil micro- phone contains a small coil attached to a diaphragm, so arranged that when the diaphragm vibrates, the coil moves back and forth in a radial magnetic field. It thus generates the output voltage. By careful design of the moving element, and by making use of air-chamber resonance, it is possible to obtain a nearly uniform response from 40 cycles to 10,000 cycles. An incidental advantage is that the output impedance of the microphone is low, and the microphone cable is less sensitive to hum pickup than in the case of the crystal microphones. A transformer is required to couple such a low-impedance source to the high-impedance input of a vacuum tube grid. 4.Ribbon Microphone: In this type, the moving element is a very thin and flexible aluminum ribbon, upon which the sound waves act directly. It vibrates in a transverse magnetic field and generates an electromotive force on the ribbon. The ribbon impedance is so low that a small step-up transformer is included in the microphone mounting to raise the impedance to a level suitable for transmission over a line. Most ribbon microphones respond to air-particle velocity. In the sound wave, rather than to sound pressure. These are referred to as velocity microphones. They can be made to have excellent frequency characteristics. Reproducers: A reproducer is a device for converting electrical energy into sound. As in the case of microphones, this transformation usually involves an intermediate mechanical motion. Telephone Receivers: Telephone receiver The ordinary telephone receiver is the most commonly known acoustic device. A modified form, the watchcase type used in radio operators’ headsets, is shown in the above figure. Two small coils are wound on soft-iron pole pieces, which are attached to the poles of a permanent magnet. The pole pieces attract the steel diaphragm with a steady pull caused by the permanent magnet, and with an alternating force set up by the voice currents flowing in the coils. The diaphragm is set into vibration, and sets up sound waves in the air in contact with it. The permanent magnet is necessary to avoid distortion in the output, since the diaphragm would be attracted twice in each cycle if only the a.c. attraction were present. The ordinary receiver used with telephone instruments. Telephone receiver is wound for about 70 ohm resistances, and it has a definite resonance peak near 1,000 cycles, for the sake of sensitivity. By winding with many turns of fine wire, the sensitivity to weak currents can be greatly increased, and such receivers are very useful as indicators in a.c. bridges and for radio communication systems. · Loudspeakers: Cone loudspeaker: The most common type of loudspeaker is shown schematically in the above figure. The moving coil, situated in a powerful radial magnetic field, carries the operating current. The reaction of the signal current with the magnetic field causes the coil to move back and forth along its axis. In this motion, it carries with it the paper cone radiator. The cone is supported at its outer edge by a flexible suspension, and at least at the lower frequencies, it moves as a rigid piston, without appreciable bending or deformation. The result is a very effective transformation of the electrical input into sound energy radiated from the surface of the cone. The radiation from the rear surface of the cone is opposite in phase to that from the front surface, and it is the function of the baffle shown in the figure to prevent these two effects from canceling each other. The baffle will be effective at any frequency for which the distance from the front of the cone, around the edge of the baffle, to the rear edge of the cone, is greater than a half wavelength of the sound. For example, at 100 cycle the wavelength is l = 1,100/100 ft = 11 ft and the distance from front to back of the cone around the baffle edge should not be less than about 5.5 ft. Various means for meeting this requirement, employing curved baffles or horns, are used to return the rear radiation in phase with that from the speaker front. This is important because speaker efficiencies falloff at low frequencies and by adding the radiation from the rear to that from the front, an increased efficiency can be obtained to improve low-frequency response. One way in which this is done is to provide a planned path, usually involving a port or hole in the baffle, which will return radiation in the proper phase at some low frequency. Cone speakers are also inefficient at high frequencies because of their large mass. This difficulty is often overcome in high-fidelity systems by employing two speakers, a cone and baffle to provide the low frequencies up to possibly 1,000 cycles to 1,600 cycles, and a small high frequency horn to carry the frequencies above. The amplifier output is divided into two frequency ranges by a dividing network or "crossover network" employing filter principles. Thus, the low-frequency speaker or "woofer" receives only the low frequency band and the high-frequency speaker or "tweeter" receives only the higher frequencies. Some of the imperfections of the system are due to imperfect division or phase shift in the crossover network. But the net effect is still beneficial. Systems of unusual perfection may employ three or four speakers with a corresponding number of frequency bands.

· Telephone Circuits: Two-way operation is essential for satisfactory telephone service, and many of the problems of the industry arise from this fact. A simplified circuit for obtaining two-way operation is shown in the below figure. This is called a local-battery system because a separate battery is required at each end of the line, and in such a telephone system each subscriber must have a battery in his house equipment. The operation of the circuit is simple, since speaking into either microphone will set up voice currents in both receivers. The transformers isolate the direct current required for the operation of the microphone, and also improve the efficiency by stepping' up the voltage and reducing the line current, or more nearly matching the microphone impedance to that of the line. Local - battery telephone circuit Common-Battery Circuits Common - battery telephone circuit The obvious advantage of removing the batteries from the subscribers' premises to the central office led to the development of the common-battery system. This system uses one large battery to supply microphone current to all subscribers' sets. This led to a new difficulty, however, in that the voice currents of all circuits in use flow through the same battery, and because of its internal impedance there is a possibility that some of these currents will find their way into other circuits. This effect is known as cross talk, and it may be avoided by use of a transformer called the repeating coil. A repeating coil and its manner of use are shown in the above figure. It may be recognized as essentially a one-to-one transformer with primary and secondary windings split at the battery. Other repeating coils, connecting other pairs of subscribers, may be tied in at the points BC, B'C', and when G, connected will offer extremely high impedance to the flow of voice current from one channel to the other, but practically none to the flow of voice current in its own channel. The talking circuit of the subscriber's set is different in the case of the common-battery telephone. 'transformers T1 and T2 usually called induction coils, and capacitors C1 and C2, are connected as auto-transformers, that is, the lower portion of the winding serves as primary and also as pan of the secondary to step up the a.c. component of the microphone voltage. · Telephone Lines: The transmission lines used in telephony are of two kinds: open- wire and cable. The open-wire lines are gradually being superseded by cable construction, either overhead or underground, for two principal reasons. In the first place, many more circuits can be accommodated-there is a standard cable containing more than 2,100 pairs of wires, and it is common practice to run 900 pairs in overhead cables on a single pole line; while a pole line carrying 50 pairs of open-wire construction would be a monstrosity. The other reason for preferring cable is it affords much better protection against weather hazards and against electrical interference, both noise and cross talk. The decibel losses and gains in the telephone lines and other equipment are frequently stated in terms of a logarithmic unit. The decibel is defined as Where W1 is the input power and W2 the output power. If W W2 exceeds W1, the value is positive and is spoken of as again. When W1 exceeds W2 the expression will be negative and indicates a loss. The decibel is also used as a measure of the amount of power absorbed or furnished by any device, by comparing it with a standard power. This standard is referred to as zero level, and in telephone practice, the power at zero level is 1 mill watt. For example, if the power output of an amplifier is 4 W, its power level expressed in decibel is 4. When so used, with 1 mill watt as the zero or reference level, the unit is sometimes referred to as the dbm. When used in the broadcast industry and measured by instruments of specified dynamic characteristics, the readings in dbm are sometimes referred to as being in volume units. The decibel is a very important unit, and it is used extensively in all branches of electrical communications. The fact that it is logarithmic in nature makes it possible to obtain over-all effects, resulting from a combination of lines, amplifiers, and other equipment connected in sequence, by simply adding or subtracting their respective gains or losses. It is also true that the sensitivity of the ear is nearly a logarithmic function of sound intensity, and a change in sound level of 1 db is just barely perceptible to the average person, which shows further that the size of the unit is well chosen. · Losses in Telephone Line: The smallest conductor used in standard open-wire telephone circuits is No. 12B and S. gauge. A line of such construction will have a loss of approximately 0.06 db per mile. Cable circuits, on the other hand, make use of conductors not larger than No. 19, and a typical cable circuit with wire of this size will show a loss of about 1.0 db per mile. In this respect, cable circuits are at a disadvantage as compared with open lines, especially for long distances, although recent improvements in the efficiency of telephone instruments have greatly extended the useful range of cable circuits. · Repeaters: In long-distance lines, losses are so high that satisfactory operation becomes impossible without the use of amplification. Amplifiers for telephone service are known as repeaters, and they must of course function in both directions along the line. Repeater stations are ordinarily installed at 50-mile intervals along the line, except in the "newer wide-band transmission systems, for some of which repeater spacing is as little as 5 miles. Because of the total number of stages of amplification in a long-distance transmission, the performance requirements of telephone repeaters are very severe. Any distortion is cumulative, and any appreciable amount would soon result in unintelligible speech. The demands of high-quality programmer’s transmission for broadcast networks are even more exacting. One method for obtaining two-way repeater service is shown in the figure. Two amplifiers are employed, one for each direction of transmission. Feedback and oscillation are prevented by the hybrid coils, which are essentially three-winding transformers of balanced construction. A signal traveling from west to east encounters the first hybrid coil, where part of the power enters the west-east amplifier by way of the centre taps on the main windings. Type 22 telephone repeater The amplified power is fed into the third winding of the other hybrid coil, and divides there into two equal portions, one passing out on the line to the east, the other into the artificial network. This network is constructed to have impedance properties equal to those of the line over the entire frequency band, so that the combination of line and artificial network is equivalent to a balanced bridge. As a result of this balance, none of the output of the west-east amplifier reaches the input terminals of the east-west amplifier, and feedback around the loop is avoided. It will be noted that power is diverted and lost at both hybrid coils. Raising the gain of the amplifier by an equivalent amount, which turns out to be 6 db in power, compensates for this loss.

REFERENCES: · http://www.Tutorvista.com/content/physics/physics-IV/communication system/telephone-links.php · http://www.Tutorvista.com/content/physics/physics-IV/communication system/telephone-circuits.php · http://www.Tutorvista.com/content/physics/physics-IV/communication system/telephone-lines.php · http://www.networktutorials.info/telecom-network.html