Telephone

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 1850s 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.