Written by Rajinder Soni. Posted in New Technologies on 30 October 2009.
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In 1667, Robert Hooke invented the telephone cord. Almost everyone has ever two cans connected with string tie and sent messages through it. However, a device is impractical and besides no good for long distances. In 1854 the French inventor Charles Bourseul raised the possibility of using vibrations caused by the voice on a floppy disk or diaphragm, in order to activate and deactivate an electrical circuit and produce similar vibrations in a diaphragm located in a remote location, which would reproduce the original sound. Some years later, the German physicist Johann Philip Reis invented an instrument conveying musical notes, but could not reproduce the human voice. In 1877, having discovered that to transmit voice could only be used direct current, the Scottish-born American inventor Alexander Graham Bell built the first phone capable of transmitting and receiving human voice in all its quality and timbre.

Magnetic Phone Bell

Two Americans, working independently, invented the telephone almost simultaneously. Recall that Alexander Graham Bell invented the telephone because I get to the patent office only hours before Elisha Gray on February 14, 1867. Bell support the findings of several scientific and based his system on the electromagnet's core set of his invention consisted of a transmitter, a receiver and a single cable. The sender and recipient were identical and contained a flexible metal diaphragm and a horseshoe shaped magnet inside a coil. The sound waves that impinged on the diaphragm is vibrated within the field of the magnet. This vibration induces an electric current in the coil, which varied according to the vibrations of the diaphragm. The traveling oriented cable to the receiver, where fluctuations generated magnetic field strength of it, making the diaphragm vibrate and reproduce the original sound. Bell actively promoted your phone. He shows throughout the United States and also in England and France, which unlike other countries were not too excited and continue to serve as messengers. The phone's sound quality was poor, the system no good for long distances and very expensive.

On the other hand, had invested heavily in the telegraph and investors clearly would not support an invention that could ruin their businesses.

The older phones using a single device as transmitter and receiver. Its basic components were a permanent magnet with a coiled cable that made him and a thin coil of cloth and metal diaphragm subjected to the force of the magnet. The strength of the voice, as snide waves, caused a movement of the diaphragm, which in turn generated a very small alternating current in the wires of the electromagnet. These teams were able to reproduce the voice, though so faintly that were little more than a toy.

The invention of the carbon telephone transmitter by Emile Berliner is key in the development of useful phone. It consists of a carbon granules placed between a sheet metal called electrodes, one of which is the diaphragm, which transmits pressure variations to these granules. The electrodes conduct electricity flowing through the carbon. The pressure variations in turn give rise to a variation of the electrical resistance of carbon. Through the line current is applied to the electrodes, and the resulting current also varies. The fluctuation of the current through the carbon transmitter results in a higher power that's inherent to the original sound wave. This effect is called amplification, and is of crucial importance, since until then an electromagnetic transmitter was only capable of converting energy, and always produced a lower power than that contained in the sound wave.

Mobile post

In the recipients of the latest phones, the imam Step to be flat as a coin and the magnetic field acting on the diaphragm of iron was of greater intensity and consistency. The transmitters were very thin diaphragm mounted beneath a perforated grid. In the center of the lens had a small container with carbon granules. The sound waves passing through the grid cause a swing of the receptacle. In the downswing, the granules are compacted and produce an increase in current through the transmitter.

Since the carbon transmitter was not practical when converting electrical energy into sound pressure, the phones have evolved into separate receivers to the transmitters. This provision allows you to place the transmitter near the mouth to collect the maximum sound energy, and the receiver in the handset, which eliminates the annoying background noise. In these phones, the receiver was still a permanent magnet with a coil of wire, but with an aluminum diaphragm subject to a metal part. The design details have made dramatic improvements, but the original concept continues to allow robust and efficient equipment.

Current Phones

The electrical equivalent of the permanent magnet is a plastic substance called electret. As a permanent magnet produces a permanent magnetic field in space, an electret generates a permanent electric field in space. As an electrical conductor moving within a magnetic field induces a current, the movement of an electrode within an electric field can produce a change in voltage between a mobile and a stationary electrode on the opposite side of the electret. Although this effect was known of old, it was only a laboratory curiosity until the appearance of materials capable of retaining an electrostatic charge for years. Current telephone transmitters are now based on this effect, rather than the change in resistance of carbon granules in terms of pressure.

Today, carbon microphones have been replaced by electret microphones, which are smaller, cheaper, better reproduce sound and are more robust than the former. The signal amplification is achieved using electronic circuits (transistors and / or integrated circuits). The receiver is normally a small diameter speaker, either vibrating diaphragm or cone.

Parts of telephone apparatus

The handset consists of a transmitter, a receiver, an audible alarm, a device and a circuit marker suppressor local effects. If this is a two-piece device, the transmitter (microphone) and receiver (receiver) are mounted on the handset, the ringer is in the base and the markup and the local effects suppressor circuit can be in any of the two parties, but usually go together. More complex phones can lead to a microphone and a speaker in the base part, apart from the transmitter and receiver in the handset. In wireless phones, the handset cord is replaced by a radio link between it and the base, although still a cable to the line. The cell phones are usually one piece, and miniature components allow you to combine the base, the headset microphone and a portable component that communicates with a remote station radio. No online or cables needed for the handset.

The audio alarm phone ring is often referred to, reference to the fact that during most of the history of these teams the alarm function is provided by an electric bell. The creation of an electronic substitute for the bell, capable of creating a pleasant sound while distinctive at a reasonable cost, was a surprisingly difficult task. For many, the sound of the bell is still preferable to an electronic buzzer. However, since the mechanical bell requires some physical volume to be effective, the trend towards smaller teams increasingly demand the use of electronic alarms in most phones. The gradual replacement of the bell will also change in the near future, the current method of activating the alarm 90 volts alternating current (V) and 20 hertz (Hz) to the line-by techniques of lower voltages, more compatible with transistorized phones. Something similar is occurring with the marking scheme phones.

The dial has gone through a development over its history. There are two ways of marking, and pulse or tone multifrequency. The pulse system is based on a hard marker. The dial has a very ingenious mechanical design, consists of the numbers 1 to 9 followed by 0, arranged in a circle below the hole and drilled a moving disk. Place the finger in the hole corresponding to the number chosen and the disc is rotated in the direction of clockwise until you reach the top and then release the disc. A spring forces the disk to return to its starting position while rotating, opening and closing an electric switch as many times as you turn the disk to dial the number chosen. In the case of 0 is made 10 starts, since it is the last number of the disk. The result is a series of pulses called the electrical current flowing between the telephone and the switchboard. Each pulse has an amplitude equal to the voltage supplied by the switchboard, usually 50 V, and lasts about 45 ms (milliseconds, thousandths of a second). Switchboard equipment have these pulses and determine the number you want to dial.

The electrical pulses produced by the rotating disc are ideal for controlling switch gear step-by-step of the first automatic switching centers. However, mechanical marking is a major source of maintenance costs, and the disk marking process is slow, especially in the case of long numbers. The availability of cheap and reliable amplification transistor advised that brought the design of a marking system based on transmission power of tones rather small, rather than the pulses of high power marking. Each button on a multi-keyboard controls the sending of a pair of shades. It uses a coding scheme "2 7" on the first pitch corresponds to the normal row of a matrix of 12 buttons and the second column (4 rows 3 columns need 7 tones).

Today, most phones are hard buttons instead of dialing and uses a system of tones. The modern telephone exchanges are designed to receive ringtones, but for many years because the system pulses the only one available and there are still phones of this type, the plants can continue to receive pulses. As a user who buys a phone may have an old line not yet support multi-frequency signals, the phones have a switch button to select the sending of pulses or tones.

There is an important functional element of the phone that is invisible to the user: the local effects suppressor circuit. The people control the tone of voice when speaking and adjust the volume accordingly, a phenomenon called "local effect". In the first phones, the receiver and transmitter were connected directly to each other and the line. This meant that the user could hear his own voice through the receiver with much greater intensity than when it was not pressed to his ear. The sound was much stronger than normal because the carbon microphone amplifies the sound energy while the converts to electrical noise. In addition to being unpleasant, this meant that the user go down the voice volume when speaking, making it difficult to listen to the receiver.

The first circuit containing a transformer suppressor along with other components whose characteristics depend on the electrical parameters of the telephone line. The receiver and transmitter were connected to different "ports of the circuit" (in this case, different windings of the transformer), but not each other. The suppressor circuit transfers energy from the transmitter to the line (although some also to other components), but nothing happens to the receiver. This eliminates the feeling that one cries out in his own ear. Currently, the transmitter and receiver are isolated from each other, separated by electronic circuits that completely eliminate the "local effect".

Telephone exchange

At first, the phone simply plugs into two homes or offices to one another; He bought the phone and consequently a line was purchased privadaa own. There was no way to connect a phone to pair another pair. After centrals were built and installed to stop allowing such connection: lass centrals manuals, run by people called operators.

Central manuals

In the first cell, the current was generated by a battery. The local circuit also had the battery and transmitter, a transformer winding, which is called the induction coil, the other winding, connected to the line, raised the voltage of the sound wave. The connections between phones were either manual, by switchboard operators who worked at headquarters located in switching or exchanges.

As telephone systems were developed, the manual connections began to be too slow and laborious. This was the trigger for the construction of a series of mechanical and electronic devices that enable automatic connections (see Electronics).

Automatic Power

Almon B. Strowger, an undertaker, automatic design a cental which was patented in 1891. But the big question was how could the phone to indicate the central number. The doctor came to settle the question. By dialing the number will produce a series of electrical impulses that reach the central and trigger a special switches (selectors) that connect to the desired number

The first automatic central install the world of La Porte, Indiana, in 1892, but the wonderful invention of Strowger spread slowly, since it took 16 years before the inauguration of the first in Europe, particularly in Munich. In England we had to wait until 1958 for the first plant was installed automatic intercity service in Bristol, because then I could not dial directly from one city to another, but needed to call the operator to establish communication.

Crossbar switching

The centrals have undergone considerable improvements in telephones and tested many different systems. One of the most successful which is the crossbar switching. It is also an electromechanical system, but can handle multiple calls simultaneously.

Existing plants

Currently, there are virtually no longer seen by switchboards phone manuals. All subscribers are served by automatic exchanges. In this type of plant, the functions of human operators are performed by switching equipment. A relay of line current of a circuit replaced the switchboard light of the switchboard, and a crossover switch serves as the cables.

Electronics equipment in central switching the handle to automatically translate the dialed number, either by pulses or system of tones, and route the call to its destination.

The phone call begins when the person lifts the handset and wait for the dial tone. This causes the closing of an electric switch. The closing of the switch activates an electrical current flow through the line of the person making the call, between the location of it and the building housing the PABX, which is part of the switching system. This is a current that does not change its flow direction, although it can do its intensity or amplitude. The plant detects this current and returns a dial tone, a particular combination of two notes to make it perfectly detectable by both teams as by people.

Having heard the ringing tone, the person makes a series of numbers using the buttons on the handset or the base team. This sequence is unique to another subscriber, the person called. The switching equipment of the plant removes the dial tone of the line after receiving the first number and, after receiving the last, determines whether the number you want to connect central part of the same or a different one. In the first case, apply a series of intervals current line call to the call recipient. The current call is 20 Hz alternating current, which flows in both directions 20 times per second. The phone user has an audible alarm that meets the current call, usually with a perceptible sound. When the phone is answered by lifting the handset, begins to circulate a steady stream of line that is detected by the plant. It fails to apply the current call and establishes a connection between the caller and the call, which is what allows us to talk.

Call Centers are a hierarchical network. If the code of the dialed number does not belong to the same plant, but belongs to another center at the same level and geographic area, establishing a direct connection between the two stations. However, if the dialed number belongs to a distinct branch of the hierarchy have to make a connection between the first central switching center that higher-level common to both and between it and the second facility. Switching centers are designed to find the shortest path available between the two stations. Once the connection between the two stations is established, the second central alarm activates the corresponding receptor as if it were a local call.

Automatic relay stations are being replaced by computer-controlled digital exchanges. Solid state technology has allowed these plants to process calls in a time of one millionth of a second, so you can process large amounts of calls simultaneously. The input circuit becomes, first, the caller's voice to digital pulses. These impulses are then transmitted through the network via high capacity systems, which connect the different calls based on computerized mathematical operations switching. The instructions for the system are stored in computer memory. The equipment maintenance is simplified thanks to the duplication of components. When a failure occurs, operation automatically enters a standby unit to handle calls. Thanks to these techniques, the system can make quick calls, both local and long distance, quickly finding the best available route.

Routes of transmission

Early telephone systems used by heavy steel cables or copper to keep the intensity to transmit the electrical signal. However, one kilometers of cable weighing 170kg, so very solid poles were needed to sustain them.

According increased the LENGTH of the lines, would that received attenuated to become completely unintelligible.

To avoid this, PROMPTED century began to settle in the circuitry telephones some devices called loading coils. These amplify the signal could not keep to a minimum but if lass lost along the line.

Obviously, he must locate a way to amplify signals lass credibility and convey them with the maximum intensity. This will repeaters, by which were no longer needed and these cables so thick they could lay under the soil surface.

However, as call volume and the distance between switching centers grew, it became necessary to use other routes of transmission. The most commonly used are the submarine coaxial cable, by radio (either by microwave or satellite) and fiber optics today. The connection between telephone exchanges and subscribers are still performed using a pair of copper wires to each subscriber. However, in some large cities have already started replacing them by fiber optics.

Telephony carrier wave

Using frequencies above the range of voice, ranging from 4,000 to several million cycles per second, or hertz, can be transmitted up to 13,200 telephone calls simultaneously by the same driver (coaxial cable, submarine cable, and microwave). Techniques telephony carrier wave are also used to send phone messages through normal distribution lines without interfering with the regular service. Due to the growing size and complexity of systems, using solid state amplifiers, called repeaters, to amplify the signal at regular intervals.

Coaxial cable

The coaxial cable, which appeared in 1936, uses a series of drivers to support a large number of circuits. The modern coaxial cable is manufactured with copper tubes of 0.95 cm in diameter. Each of them has, right in the center of the tube, a fine copper wire attached with insulating plastic disks spaced about 2.5 cm. The tube and wire have the same center, ie are coaxial. Copper tubes protect the transmitted signal of possible electrical interference and prevent energy loss by radiation. A cable, consisting of 22 rings arranged in coaxial tubes embedded in polyethylene and lead, can carry 132,000 simultaneous telephone conversations.

Submarine Cables

Transoceanic telephone service was introduced commercially in 1927 using radio transmission. However, the problem of amplification stopped laying telephone cables until 1956, when it first entered service on transoceanic submarine telephone cable in the world, connecting Newfoundland and Scotland using coaxial cables.

Microwave Mobile

In this method of transmission, radio waves are at the very high frequency band, and microwave are called, are used as carriers of telephone signals and transmitted from station to station. Since the microwave transmission requires an expeditious way between source and receiving station, the average distance between relay stations is about 40 km. A microwave channel can transmit up to 600 telephone conversations.

Satellite Telephony

In 1969 he completed the first global telephone network based on a series of satellites in stationary orbits at a distance from Earth of 35,880 km. These satellites are powered by solar cells. Calls transmitted from a terrestrial antenna on the satellite are amplified and relayed to ground stations away. The integration of satellites and ground equipment allows direct calls between different continents as easily as between close proximity. With digitization of broadcasting, the satellites in the Intelsat global series can broadcast up to 33,000 calls simultaneously, and different TV channels.

A single satellite phone will not serve to make a call, for example, between New York and Hong Kong, but two other. Even taking into account the cost of a satellite, this route is cheaper to install and maintain the route channel equivalent using coaxial cables lying on the seabed. Consequently, for large distances are used in every possible way satellite links.

However, the satellites have a major drawback. Due to the large distance from the satellite and the limited speed of radio waves, there is a significant delay in spoken responses. So many calls only use the satellite in one direction of transmission (eg New York to Madrid) and a terrestrial microwave link or coaxial cable in the other direction. A satellite link for both directions would be irritating for two people chatting between New York and Hong Kong, and that could only make breaks, something very common in conversation, and also would be affected by the long delay (more than one second) the response of the other person.

Optical fibers

One of the great advances in communications has been the use of digital signals. In telephony, the signal is digitized to reach the PBX. Communication between telephone exchanges is digital, thus reducing noise and distortion and improving the quality and capacity.

Coaxial cables are being replaced gradually by glass optical fibers. Messages are digitally encoded in light pulses that are transmitted over long distances. A fiber cable can be up to 50 fiber pairs, each pair supports up to 4,000 voice circuits. The foundation of the new optical fiber technology is the laser that uses the visible region of electromagnetic spectrum where the frequencies are thousands of times higher than those of radio and therefore can carry a much larger volume of information. The light emitting diode (LED), a simpler device, also used as appropriate for most of the functions of transmission.

A fiber optic cable, TAT 8, carries more than double the existing transatlantic circuits in the 1980s. As part of a system that extends from New Jersey to England and France, can transmit up to 50,000 conversations at once. Such cables also serve as channels for high-speed transmission of computer data, to be safer than that provided by communications satellites (see satellite communications). Other important developments in telecommunications, TAT 9, a fiber cable with greater capacity, became operational in 1992 and can transmit 75,000 simultaneous calls.

Most major cities are now linked by a combination of microwave links, coaxial cable, fiber optics and satellites. The capacity of each of the systems depends on their age and territory covered (the undersea cables are designed in a very conservative and have less capacity than the cable surface), but in general, can be classified as follows: simple scanning through a couple dozen parallel circuits provides pair, the coaxial circuit allow hundreds and thousands per pair cable, satellite and microwave circuits are thousands of links, and fiber optics allows up to tens of thousands fiber circuit. The capacity of each type of system has increased significantly since its inception due to the continuous improvement of engineering.


Today, it is possible to receive phone calls even while driving a car. In certain areas there is a radio system connected to the network automatically. Dialing a special code and then the desired number, you may contact a person who is in motion.

Indeed, the radio links are possible for over 55 years. Radio waves in the high frequency band (4-30MHz) are already used in international communications. With the development and improvement of submarine cables and satellites, have come to use in high frequency radio links.

Telephony and broadcasting

Teams of long distance telephony can also be used to transport radio and television to spread great distances between stations to broadcast simultaneously. In some cases, the audio portion of television programs can be transmitted through cable circuits or audio frequency carrier frequencies used to transmit telephone conversations. Television pictures are transmitted via coaxial cable, microwave and satellite circuits.


The first two-way videophone was introduced in 1930 by the American inventor Herbert Eugene Ives in New York. The videophone can be connected to a computer to view reports, charts and diagrams in remote locations. Likewise allows face to face meetings of people in different cities and can act as a liaison between meeting centers within a network of cities. The videophones are already commercially available and can be used on domestic lines for callers face to face. Similar functions also exist in the computer or computers connected to the telephone network and equipped for that purpose.


Voicemail allows you to record the received messages for later playback when the call is not heeded. In more advanced versions of voice mail, the user can record a message to be broadcast later during the day.

Voice mail is available in the telephone company as a service switching or by purchasing an answering machine. In general, ordinary telephone equipment is equipped with recording functions, playback and automatic detection of call. If the incoming call is answered on any telephone line rings before a certain number of times, the answering machine does not work. However, the number of calls completed, the answering machine pick up and proceeds to play back a previously recorded message informing the subscriber can not answer the call at that time and invited to leave a recorded message.

The owner of the answering machine is notified of the presence of recorded messages through a light or an audible beep and can retrieve the message later. Most answering machines and operator services all likewise allow the user to retrieve the recorded messages from a remote location by dialing a particular code when they have received response from your computer.

Mobile or cellular

The cell phones are essentially a low-power radios (see Radio mobile phone). The calls go through radio transmitters placed within small geographic units called cells. The cells cover almost the entire territory, but particularly residential areas and communication routes (such as roads and railways) from where you do most of the calls. Radio transmitters are connected to the telephone network, allowing communication with regular phones or to each other.

Adjacent cells operating at different frequencies to avoid interference pear. Since each cell signals are too weak to interfere with those of other cells operating in the same frequencies, you can use a larger number of channels in the transmission with high power radio frequency. When a user moves from one cell to another, the transmission has to change its transmitter and frequency. This change must take place at high speed for a user traveling in a moving car or train can continue their conversation without interruption.

Frequency Modulation Narrowband is the most common method of transmission and each message is assigned an exclusive carrier for the cell from transmitting. Today it multiband mobile phones that can use two or three carriers at once, thus reducing the chance that the phone loses the signal.

The digital mobile phones can be used anywhere in the world using the same mobile telephone system. There are also mobile phones that allow access to the Internet, fax transmission and reception, and even videophone.

Technology Trend

The replacement of coaxial cables for transoceanic optical fiber cable continues today. Advances in integrated circuit technology and semiconductor designing and marketing have enabled phones and switching centers that not only produce high-quality voice fidelity, but offer a range of functions such as stored numbers, call forwarding, call multiuser, call waiting and caller ID number.

Traditionally, the phone has been used to transmit voice, however, is increasingly being used more for other types of transmissions. You can transmit images by telephone using the fax. Two computers can communicate with each other by telephone using the modem. This type of communication is becoming popular because it allows access to the Internet using just a modem connected to the telephone line.


Rajinder Soni

Author: Rajinder Soni

173 14151 9
I am working as a senior assistant editor for Electronics For You magazine.
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