LESSON 4

TELEGRAPH

I. Consider the following questions in the group of four. A spokesperson will report on your discussion to the whole group.

1. Why is telegraph still popular now?

2. Are there any private wire systems in Belarus?

3. What can you say about modern telegraph services in our country?

II. Read the text below and check its comprehension.

Telegraph

Telegraph is a system of communication employing electrical apparatus to transmit and receive signals in accordance with a code of electrical pulses. Originally the term telegraphy referred to any form of communication over long distances in which messages were transmitted by signs or sounds.

The Morse Telegraph

The first electrical instruments for telegraphic transmission were invented in the United States by the American inventor Samuel F. B. Morse in 1837 and in Britain the same year by the British physicist Sir Charles Wheatstone in collaboration with the British engineer Sir William F. Cooke. Morse used a simple code in which messages were transmitted by electric pulses passing over a single wire. Morse's apparatus, which sent the first public telegram in 1844, resembled a simple electric switch. It allowed current to pass for a pre-scribed length of time and then shut it off, all at the pressure of a finger. The original Morse receiver had an electro-magnetically controlled pencil that ma-de marks on paper tape moving over a clockwork-operated cylinder. The marks varied with the duration of the electric current passing through the wires of the electric magnet and took the written form of dots and dashes.

While experimenting with his instrument, Morse found that signals could be transmitted successfully for only about 32 km. Beyond that distance the sig-nals grew too weak to be recorded. Morse and his associates there-fore developed a relay apparatus that could be attached to the telegraph line 32 km from the signal station to repeat signals automatically and send them an additional 32 km. The relay consisted of a switch operated by an electro-magnet. An impulse entering the coil of the magnet caused an armature to rotate and close an independent circuit actuated by a battery. This action sent a fresh pulse of current into the line, and this pulse in turn activated successive relays until the receiver was reached. A few years after Morse developed his receiving instrument and demonstrated it successfully, tele-graph operators discovered that it was possible to distinguish dots and dashes by sound alone, and the Morse recording apparatus was therefore discarded.

Because telegraphy was too expensive for widespread use, several means of sending some messages simultaneously over a single line were developed. In duplex telegraphy, the earliest advance of this kind, one message can be transmitted simultaneously in each direction between two stations. In quadruples telegraphy, invented in 1874 by the American engineer Thomas Edison, two messages were transmitted in each direction simultaneously. In 1915 multiplex telegraphy came into use, permitting the transmission of eight or more messages simultaneously. Because of this and the development of teleprinting machines during the mid-1920s, the Morse manual telegraph system of code and key was gradually discontinued for commercial use and replaced by automatic wire and wireless radio-wave methods of transmission.

Automatic Telegraph Systems

There are two basic systems of modern telegraphic communication: the teleprinting system (teletype), which is still in use, and the facsimile repro-duction system, which became obsolete in the 1980s.

Teleprinting

In teleprinting, the message is received in the form of typed words on a pa-per form. In this system each letter of the alphabet is represented by one of 31 combinations of five equal-interval electronic impulses, with the sequence of used and unused intervals determining the letter. The start-stop printing code uses seven pulses for each character, the first pulse indicating the begin-ning and the seventh pulse the end of the letter.

The transmitter or teletypewriter consists of a typewriter like keyboard and may or may not record the message on tape before it is transmitted. The recei-ver is basically like a typewriter without a keyboard that prints the message on a tape or a paper form. Most machines in the start-stop system are both transmitters and receivers. News organizations were among the major users of the teletype and similar communications systems. By the early 1990s, press associations and broadcast media transmitted both text and pictures electronically via satellite.

Facsimile Reproduction

 Facsimile telegraph systems, which send and receive images and text, have been rendered largely obsolete by facsimile transmission, commonly referred to as fax.

Telegraph Carrier Media

The electrical impulses that make up telegraph messages may be carried through wire circuits or may be broadcast as radio waves.

When Morse invented the telegraph, the only way that a message could be carried from one point to another was by wires strung directly from the transmitting device to the receiver, regardless of the distance. The wire could carry only one message at a time, and reamplification and signal correction devices had to be set up at regular points along the line. By utilizing carrier currents, which are alternating currents of a number of different frequencies, a single pair of wires can simultaneously transmit hundreds of messages, for each frequency represents a transmission channel. The various channels are combined at the sending station into the carrier current transmitted by the telegraph wires. At the receiving end the carrier current is passed through electrical filters, each of which transmits only a particular frequency to an appropriate receiving device. Thus, a great number of individual channels may be obtained with only one electrical circuit.

Microwave Transmission

The use of microwave radio transmission for long-distance telegraphic communication all over the world grew to be of major importance after World War II ended in 1945. The first commercial microwave radio link in telegraphy began operation between Philadelphia and New York City in 1947. It was fol-lowed in 1948 by a three-way network linking New York City, Washington, D.C., and Pittsburgh. The system then spread rapidly across the United States through the use of microwave relay antenna towers.

Microwave telegraphy is capable of carrying vocal, printed, graphic, photo-graphic, and video communication almost instantaneously and in large quan-tities. It operates in the 4000-megahertz range of the commercial communi-cations band. In this range, 40 voice bands are available in either direction, providing about 800 telegraph channels. The radio signals originating at the broadcast source are relayed to their destination by a series of parabolic reflector antennas mounted at the top of tall masts. In order to overcome weakening of the signal by distance and the curvature of the earth, these microwave relay antennas are placed at line-of-sight intervals about 48 km (about 30 mi) apart. For intercontinental communication, artificial geosynchro-nous satellites are used as relay antennas for voice, data, graphic, and video signals between ground-based stations.

Modern Telegraph Services

In the 1950s and 1960s a variety of public and private telegraphic services became available from various carriers.

Telex

In 1958 a system of direct-dial teleprinter exchange, called Telex, was in-troduced, and within ten years it had more than 25,000 subscribers. The Telex system enabled subscribers to send messages and data directly to other sub-scribers in North America and, through the facilities of international carriers, in many other parts of the world. In some areas Telex subscribers could also send messages to non-subscribers by dialing special communications cen-ters that delivered the messages as telegrams.

Broadband Exchange Service

This service, introduced in 1964, provided subscribers with a choice of high-quality radio channels for the rapid transmission of data in various forms, for facsimile and other record communications, and for voice communication. Improvements to the system made it possible to achieve high-speed transmission between computers and business machines.

Private Wire Systems

These services, used for high-speed exchange of data, are leased by busi-nesses or government agencies that have branches in many parts of the world. They operate through digital computer centers by means of punched cards, perforated tape, and magnetic tape. The largest and most advanced of these systems is the Automatic Digital Data Network (AUTODIN), which serves the U.S.Department of Defense. The Advanced Record System (ARS) serves the General Services Administration of the federal government. Other private wire systems serve large brokerage firms and banks.

Computer Centers

In response to the needs of subscribers for a variety of communication and information services, "computer-library" centers have been established to provide exchange of data and collection of information of every possible type. The computer centers are available to subscribers through the Telex system and through normal telephone lines.

Comprehension Check Up

1. Who invented the first electrical instruments for telegraphic transmissi-on? 2. How were messages transmitted? 3. How did the original Morse receiver work? 3. What do you know about duplex, quadruplex and multiplex telegraphy? 4. What are the basic systems of modern telegraphic communication? 5. How may the electrical impulses be carried? 6. When did the commercial microwave radio begin operation in telegraphy? 7. How does microwave telegraphy work? 8. When was microwave transmission established? 9. What can you say about the Telex system? 10. What did Broadband Exchange service provide subscribers with? 11. What were private wire systems used for? 12. Why were computer centers established?

III. Choose the best answer.

1. The word telegraph refers to …

a) radio-relay system; b) system of communication; c) pulse-modulation system; d) control system.

2. Duplex telegraphy implies that…

a) four messages can be transmitted simultaneously in each direction bet-ween 2 stations; b) three messages can be transmitted simultaneously in each direction between 2 stations; c) two messages can be transmitted simul-taneously in each direction between 2 stations; d) one message can be trans-mitted simultaneously in each direction between 2 stations.

3. The word receiver defines …

a) equipment that generates and sends electro-magnetic impulses; b) equip-ment that creates signals; c) equipment that receives signals; d) equipment that converts signals.

4. To transmit messages implies that one should …

a) send information to smb.; b) deliver messages; c) send a letter to smb.; d) dial smb.’s number.

5. Telegraphic communication consists of …

a) two basic systems: the teleprinting system (teletype) and the facsimile; b) teletypewriter switching system; c) television relay system; d) time-division multiplex system.

6. The word transmitter refers to …

a) equipment that transforms signals; b) equipment that reflects signals; c) equipment that sends out signals; d) equipment that modulates signals.

7. The word communication means …

a) the process by which people exchange information; b) data collection; c) the process of making a telephone call; d) the process of mastering new technologies.

8. The best explanation of Telex might be …

a) a triplex system; b) a system of sending messages from one place to another; c) a system of direct-dial teleprinter exchange; d) an optical commu-nication system.

9. Microwave telegraphy deals with

a) scanning process; b) carrying vocal, printed, graphic, photographic and video communication almost instantaneously; c) developing process; d) litho-graphy process.

10. The best explanation of the Broadband Exchange Service might be …

a) service which provides subscribers with a choice of high-quality radio

channels for the rapid transmission of data in various forms; b) long distance service; c) broadcasting service; d) public radio communication system.

11. The word teleprinting means that …

a) the message is received in the form of symbols; b) the message is recei-ved in the form of codes; c) the message is received in the form of typed words on a paper form; d) the message is received in the form of digits.

12. The best definition of the Facsimile telegraph systems might be …

a) systems which disseminate information from public information supplies into home or office; b) paging systems; c) systems which display still frames of information from a remote data base on a home TV set screen; d) systems which send and receive images and texts.

IV. Use the right verb from those given below.

to lease, to send, to deliver, to relay, to operate, to make up, to receive, to print, to record, to set up, to mount.

1. In teleprinting the message is … in the form of typed words on a paper form. 2. The teleprinter may … the message on tape. 3. The receiver is basi-cally a typewriter without a keyboard that … the message on a tape or a pa-per form. 4. Signal correction devices had to be … at regular points along the line. 5. Facsimile telegraph systems … and … images and text. 6. The elec-trical impulses … telegraph messages. 7. Microwave telegraphy … in the 4000-megahertz range of the commercial communication band. 8. The radio signals are … to their destination by parabolic reflector antennas. 9. Parabolic reflector antennas are … at the top of tall masts. 10. In some areas Telex sub-scribers … messages to non-subscribers by dialing special communication centers that … the messages as telegrams. 11. Private wire systems are … by business agencies that have branches in many parts of the world.

V. Are the following statements True or False?

1. The first electrical instruments for telegraphic transmission were inven-ted in Russia. 2. Morse’s apparatus, which sent the first public telegram in 1844, resembled a simple electric switch. 3. In quadruples telegraphy, inven-ted in 1874 by the American engineer Thomas Edison, four messages were transmitted in each direction simultaneously. 4. The first commercial micro-wave radio link in telegraphy began operation between Philadelphia and New York City in 1947. 5. In 1958 a system of direct dial teleprinter exchange, called Telex, was introduced.

VI. Express the main idea of each paragraph in one sentence.

VII. Summarize the text and be ready to retell it.

VIII. Tell us about the advantages and disadvantages of using telegraph.