TELEPHONES

I. INTRODUCTION  

Telephone, communication instrument designed to transmit speech and other sounds to a distant point by means of electricity, and to reproduce them. The telephone contains a diaphragm, which vibrates when struck by sound waves. The vibrations (wave motion) are converted into electrical impulses and transmitted to a receiver, which converts the impulses back into sound.

In common usage, the term "telephone" is also applied in a much broader sense to the entire system to which an individual telephone set is connected; a system which allows the sending of not only a user’s voice but also data, pictures, or any other information which can somehow be encoded and converted into electrical energy. This information is exchanged between points connected to the network. The telephone network consists of all of the transmission paths between subscriber’s sets and of the switching machinery used to select a particular path or group of paths between subscribers.

II. DEVELOPMENT 

 In 1854 the French inventor Charles Bourseul suggested that vibrations caused by speaking into a flexible disc or diaphragm might be used to connect and disconnect an electric circuit, thereby producing similar vibrations in a diaphragm at another location, where the original sound would be reproduced. A few years later, the German physicist Johann Philip Reis invented an instrument that transmitted musical tones but could not reproduce speech. A form of acoustic communication device had also been developed in the early 1870s by an Italian-American inventor, Antonio Meucci. However, in 1876, having discovered that only a steady electric current could be used to transmit speech, the American inventor Alexander Graham Bell produced the first telephone capable of transmitting and receiving human speech with its quality and timbre. His compatriot Elisha Gray had filed a claim for the invention just hours after Bell, but Bell's patent was upheld by the United States Supreme Court, and he has become widely recognized as the inventor of the telephone.

III. BELL’S MAGNETIC TELEPHONE

The basic unit of Bell’s invention consisted of a transmitter, a receiver, and a single connecting wire. The transmitter and receiver were identical; each contained a flexible metallic diaphragm and a horseshoe magnet with a wire coil. Sound waves striking the diaphragm caused it to vibrate in the field of the magnet. This vibration generated an electric current in the coil that varied in proportion to the vibrations of the diaphragm. The current travelled through a wire to the receiving station, where it produced changes in the strength of the magnetic field of the receiver, causing its diaphragm to vibrate and reproducing the original sound.

In the receiver of the modern telephone the magnet has been flattened into the form of a watch, and the magnetic field acting on the ferrotype iron diaphragm has been made more intense and uniform. The modern transmitter consists of a thin diaphragm mounted behind a perforated grill. At the centre of the diaphragm is a small dome forming an enclosure filled with carbon granules. Sound waves passing through the grill cause the dome to move in and out. When the diaphragm presses in, the granules become densely packed, allowing an increase in the flow of current through the transmitter.

IV. PARTS OF A TELEPHONE SET

A basic telephone set contains a transmitter, receiver, dial, ringer, and antisidetone network as electrical parts. (This use of the word "network" refers to a small assembly of electrical components inside the set and should not be confused with "network" in "telephone network" which refers to the global interconnected system.) If it is a two-piece set, the transmitter and receiver are mounted in the handset, the ringer is typically in the base, and the dial and antisidetone network may be in either the base or handset but are usually together. More sophisticated telephones will have a microphone and speaker in the base in addition to the transmitter and receiver in the handset. In a cordless phone the handset cord is replaced by a radio link between the handset and base but a line cord is still used. A cellular phone is often a one piece unit in which extremely miniaturized components make it possible to combine the base and handset into one handheld unit that communicates with a distant radio station. No line or handset cords are needed, providing the ultimate in portability.

Many early telephones used a single device for transmitter and receiver. Its essential working parts were a permanent magnet with wire wound around it to make an electromagnet and a thin diaphragm made of cloth and metal which was attracted to the magnet. Speech energy in the form of sound waves caused the diaphragm to move which created a tiny AC current in the electromagnet’s wires. Such a device could reproduce speech but only so weakly that it was little more than a toy.

The invention of the carbon telephone transmitter by Emile Berliner was the key to a practical telephone. It is constructed by placing some carbon granules between metal plates called electrodes, one of which is the thin diaphragm and transmits pressure variations to the carbon granules. The electrodes conduct electricity which also flows through the carbon. Variations in pressure cause the electrical resistance of the carbon to vary. A DC voltage is provided by the exchange over the line and applied to the electrodes. The resultant DC current also varies. The fluctuation in DC current through a carbon transmitter can represent more energy than that in the original sound wave. This effect is called amplification and is crucial. An electromagnetic transmitter can only convert energy and will always deliver less electrical energy than the energy contained in the sound wave.

The electrostatic analogue to a permanent magnet is a plastic material called an electret. Just as a permanent magnet, once energized, provides a permanent magnetic field in space, the material in an electret, once energized, provides a permanent electric field in space. Just as an electrical conductor moving in a magnetic field can induce a current, so the movement of an electrode in an electrostatic field can cause a change in voltage between the moving electrode and a stationary electrode on the other side of the electret. While this effect has been known for many years, it remained a laboratory curiosity until the development of materials which could retain an electrostatic charge for years. Telephone transmitters now use this effect rather than the pressure sensitive resistance of carbon granules since an electret microphone can be very small, light, and inexpensive. Electret microphones depend on transistors for the necessary amplification.

Since the carbon transmitter is not useful in converting electrical energy back to sound pressure, telephones evolved with receivers that are separate from the transmitter. This arrangement allows the transmitter to be placed close to the mouth for maximum pick up of sound energy and permits the receiver to be placed in a tight fitting earcup which helps exclude bothersome background noise. The receiver is still made from a permanent magnet wound with wire but now may have an aluminium diaphragm attached to a piece of iron. The details of the design are vastly improved but the original concept continues to yield a rugged and efficient device.

The alerter in a telephone is usually called the ringer, a reference to the fact that for most of the telephone’s history, the alerting function was provided by an electrically actuated bell. Creating an electronic replacement for the bell that could provide a pleasing yet attention getting sound at a reasonable cost was a suprisingly difficult task. For many people, the sound of a bell is still preferable to the sound of an electronic alerter. However, since a mechanical bell requires a minimum physical volume to be effective, the trend to smaller telephones mandates the use of electronic alerters in most telephones. The steady replacement of the bell also will make it possible, at some future date, to change the current method of alerter actuation (the application of 90 volts 20 Hz AC to the line) with lower voltage techniques more compatible with transistorized telephones. A similar change is already in progress with the telephone dialling scheme.

The telephone dial has undergone a major change in its history. Two forms of dialing still exist within the telephone system, dial pulse and multifrequency tone, which is commonly called by its original trade name of "Touch Tone".

The rotary dial was a very clever mechanical design that achieved an electrical result. On the dial the numerals 1 to 9 followed by 0 are placed in a circle behind round holes in a movable plate. The user places a finger in the hole corresponding to the desired digit and rotates the movable plate clockwise until the finger hits the fingerstop, then removes the finger. A spring mechanism causes the plate to return to its starting position and, while turning, open an electrical switch a number of times equal to the desired digit, except 0 gets 10 switch openings since it is the last digit on the dial. The result is a number of "dial pulses" in the electrical current flowing between the telephone set and the central office. Each pulse has an amplitude equal to the voltage provided by the exchange battery, usually about 50 volts, and is about 45 milliseconds (thousands of a second) in duration. Equipment at the central office counts these pulses and thus determines the number being called.

The rotary dial’s output of electric pulses is well suited for controlling step-by-step switching equipment used in the first automatic exchanges. However, mechanical dials were a major source of repair costs in telephones and the rotary dialling process is slow, especially if a long string of digits is dialled. The availability of inexpensive and reliable amplification as provided by the transistor made practical the design of a dialling system based on the transmission of relatively low power tones instead of the higher power dial pulses. Each pushbutton in a multifrequency dial controls the sending of a pair of tones. A "2 out of 7" coding scheme is used in which one tone corresponds to the row of a normal 12-button array and the second tone corresponds to the column (4 rows plus 3 columns need 7 tones).

Today, most telephones have pushbuttons instead of a rotary dial. Because Touch Tone was introduced as an optional premium cost service the exchange has to maintain the ability to receive either pulse or multitone dialling. Since a person buying a telephone might have a line on which multifrequency signals are not accepted by the telephone company, pushbutton telephones usually have a switch which the customer can set to determine whether the telephone will send pulses or tones.

One important functional part of a telephone is invisible to the user: the antisidetone network. Humans continuously monitor the sound of their voice while speaking and adjust their speaking volume accordingly; a phenomena called "sidetone". In early telephones the transmitter and receiver of each set were directly connected to each other as well as to the line. This caused a telephone user to hear their own voice in the ear using the receiver much more loudly than when the receiver was not in place against the ear. The sound was louder than normal because the carbon microphone amplifies the energy of the sound at the same time it converts this energy from acoustic to electrical form. In addition to being unpleasant, this caused the user to speak more softly and made it harder for the listener to hear.

The original antisidetone network contained an electrical transformer along with other components whose characteristics depend on the electrical parameters of the telephone line. The receiver and transmitter were connected to separate "network ports" (in this case, different windings on the transformer) rather than to each other. The antisidetone network has the ability to transfer the energy from the transmitter to the line (with some going also to the other components) without allowing any of this energy into the receiver. This eliminates the sensation of shouting in your own ear. In practice, a small amount of the speech energy is allowed into the receiver for otherwise the connection would sound unpleasantly "dead". Contemporary telephone designs use transistors embedded in integrated circuits to replace the transformer as these are lighter, smaller, and less expensive. Other parts of this integrated circuit function as an automatic volume control to compensate for the varying lengths of wire between different customers and the exchange. Since this variation can be from almost nothing to tens of miles, customers very distant from the exchange would receive too little volume while those close in would experience undesirable loud volumes.

V. CIRCUITS AND EXCHANGES

A telephone call starts with the person making a call lifting the handset off its base and listening for a dialling tone. This closes an electrical switch called the switchhook (originally "hook switch", named after its shape). Closing this switch starts the flow of an electric current over the caller’s line, also called the loop, between the caller’s location and the building containing the automatic exchange, a part of the switching system. This is a DC or direct current which does not change direction of flow although its intensity or amplitude may vary. The exchange detects the current and returns dialling tone, a precise combination of two notes to permit reliable detection by machines as well as by people.

Once the dialling tone is heard, the caller enters a sequence of digits on pushbuttons mounted either on the handset or base. This sequence is unique to one other telephone subscriber, the party being called. The switching equipment in the exchange removes dial tone from the line after the first digit is received and, after receiving the last digit, determines whether the called party is in the same exchange or a different exchange. If the called party is in the same exchange, bursts of ringing current are applied to the called party’s line. Ringing current is 20 Hz alternating current. This alternating or AC current flows in each direction 20 times a second. Each subscriber’s telephone contains a ringer which responds to a ringing current, usually by making a sound which can be heard throughout the room containing the telephone. If the called party answers the telephone by picking up her or his handset, DC current starts to flow in the called party’s line and is detected by the exchange. The exchange then stops applying the ringing and sets up a connection between the calling and called parties that can be used for talking.

If the called party is in a different exchange from the calling party, the calling exchange sets up a connection over the network to the called party’s exchange. As part of this process, the calling exchange must tell the called exchange who the called party is. The called exchange then handles the process of ringing, detecting answering, and notifying the calling exchange and billing machinery whether the call is completed; in telephone terminology a call is completed when the called party answers, not when the conversation is over. When the conversation is over, one or both parties hang up by replacing their handset on the base. This opens the switchhook and stops the flow of DC current. The exchange then initiates the process of taking down the connection including again notifying the billing equipment if appropriate. Billing equipment may or may not be involved as calls within the local calling area may be either flat rate or message rate. The local calling area includes several nearby exchanges. In flat rate service, the subscriber is allowed an unlimited number of calls for a fixed fee each month. Message rate subscribers pay a charge for each call which depends on the distance between the calling and called parties and the duration of the call. A long distance call is a call out of the local calling area and is always billed as a message rate call.

In early telephones the current was generated by a battery. The local circuit included, in addition to a battery and a transmitter, one winding of a transformer called an induction coil; the other winding, connected to the line, stepped up the sound wave voltage. Connections between telephones were made manually, by operators working at switchboards located in central switching offices.

As telephone systems grew, manual switching proved too slow and labour intensive. This provided the impetus for developing a series of mechanical and electronic devices that allowed switching to be done automatically. In the modern telephone, an electronic device transmits either a number of successive impulses of current or a series of audible tones corresponding to the number being called. Electronic equipment at a central switching station automatically translates the signal and routes the call to the receiving party.

Solid-state technology enables these central exchanges to process calls at speeds of one-millionth of a second, so that large numbers of calls can be handled simultaneously. First the input circuit converts the caller’s voice into digital signal pulses. These pulses are then transmitted through the network by high-capacity systems that exchange individual calls by means of computerized mathematical switching operations. Instructions for operating the system are stored in computer memory. Equipment maintenance is facilitated by duplication of components. When a defect becomes manifest, a standby unit automatically begins handling calls. Using computer techniques to handle telephone calls, data messages, and even visual signals, the system can make speed calls, both local and long distance, by swiftly determining the most efficient route.

Today there are no telephones served by manual exchanges in the United States and Britain. All telephone subscribers are served by automatic exchanges. In an automatic exchange switching equipment performs the functions of the human operator. A line current relay in a line circuit replaces the switchboard light and a crosspoint switch replaces the cords. Other relays replace the key. Since computers only now are beginning to be able to understand spoken commands, about a century too late for the earliest automatic exchanges, the dial is used to indicate what number is being called. Incoming registers store the number being dialled and then pass it to the exchange’s central computer which in turn operates the crosspoint switch array to complete the call or route it to a higher level switch for further processing.

VI. TRANSOCEANIC TELEPHONY  

Overseas radio-telephone service was introduced commercially in 1927, but the problem of amplification prevented the laying of telephone cables until 1956, when the world’s first transoceanic submarine telephone cable, extending between Newfoundland Island and Scotland, was placed in service.

VII. CARRIER-CURRENT TELEPHONY  

 Through the use of frequencies above the voice range, extending from about 4,000 to several million cycles per second, or hertz, as many as 13,200 telephone messages can be carried simultaneously over a single conducting medium. Carrier-current telephony techniques are also being used to send telephone messages over the normal distribution lines without interfering with regular service. With the growth in size and complexity of systems, solid-state amplifiers, called repeaters, are used to amplify the messages at regular intervals.

VIII. COAXIAL CABLE  

Developed in 1936, the coaxial cable uses cable conductors to carry a large number of circuits. The modern coaxial cable consists of copper tubes 0.95 cm (0.375 in) in diameter. Each has a thin copper wire held exactly in the centre of the tube by plastic disc insulators about 2.5 cm (1 in) apart. The tube and the wire have the same centre; that is, they are coaxial. The copper tubes shield the transmitted signal from electrical interference and prevent energy losses by radiation. A cable, consisting of up to 22 coaxial tubes arranged in tight rings sheathed in polyethylene and lead, can carry 132,000 messages simultaneously.

IX. OPTICAL FIBRES  

Coaxial cables are increasingly being replaced by optical glass fibres. Messages are digitally coded into pulses of light and transmitted over great distances by these slender fibres. A fibre cable may contain up to 50 fibre pairs, each pair carrying up to 4,000 voice circuits. The basis of the new fibre optics technology, the laser, exploits the visible region of the electromagnetic spectrum, where frequencies are thousands of times higher than in radio and thus able to carry much larger volumes of information. The light-emitting diode (LED), a simpler device, is adequate for most transmission purposes.

One fibre-optic cable, TAT 8, carries more than twice the number of transatlantic circuits that were available in the 1980s. Used in a system that stretches from New Jersey to Britain and France, it can transmit up to 50,000 conversations at once. Such cables also provide channels for high-speed transmission of computer data that are more secure than those offered by communications satellites. Another major advance in telecommunications, TAT 9, which is an even higher capacity fibre cable, came into operation in 1992 and can carry 75,000 calls simultaneously.

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