Computer communications:Telex,Teletex,Facsimile and Videophones and videoconferencing

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Telex

Telex is a popular way of transmitting text over the tele­phone network, though for reasons which are explained below its use is declining, being replaced by facsimile.

Telex is transmitted and received by teletypewriters, which are a kind of communicating typewriter with a paper tape punch and a paper tape reader. Contact between two teletypes is established by dialling in the usual way, and the text may then be typed in to the transmitting teletype by the telex operator. However, in order to save on telephone line time, the message will normally be typed in and recorded on paper tape before the call takes place, and then read rapidly from the tape after dial-up.

Many major organizations are telex users, and until recently the service boasted over a million subscribers world-wide. Two factors contributed to this success:

• All telex machines observe common communications protocols, i.e. they are all compatible.

• There is a comprehensive directory of telex users.

However, facsimile and other text communication services are superseding telex (see below). The reason is that the transmission rate of telex is low (50 bps, i.e. about a word per second), which means that the length of each call, and therefore the phone charge, is relatively high.

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Teletex

Teletex is an enhanced telex system designed to take advan­tage of modern digital packet-switched telecommunications systems. The transmission rate is 2400 bps, almost 50 times faster than telex. Although this system is expected eventu­ally to take over from telex, its current usage is low because teletex standards have not been precisely specified, so there is some incompatibility between different manufacturers' machines.

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Facsimile

Facsimile, or fax for short, was developed at the beginning of this century to transmit images, such as newspaper photographs, by telephone line. A fax machine contains a photoelectric cell which scans the image and converts the blacks, greys, and whites into electrical signals, which mod­ ulate the telephone carrier wave. The same device will also act as a receiver, able to decode incoming signals and print them as image on special paper. Nowadays, the 'images' that are transmitted by fax are usually pages of text.

Fax machines are grouped according to quality of scan­ning and reproduction. Group 1 machines give high quality results, with good differentiation between the various shades of grey. The amount of information that has to be transmit­ ted in this case is high, and so the transmission time for an A4 document is a lengthy six minutes. Group 2 machines give lower quality results but cut the transmission time in half.

Of most interest so far as text transmission is concerned are the Group 3 and 4 machines, which give black and white results without grey tones. Unlike Group 1 and 2, these are digital machines, converting a black (or dark grey) dot on paper to a binary 1, and a white (or light grey) dot to a binary 0. They are well suited to digital communications, and the transmission time - and therefore the cost of each call - is very low. Current models are about the size of a small dot of matrix printer, and they can transmit an A4 page in just a few seconds.

In the past, the use of fax was inhibited by:

• The lack of a directory of users.

• The fact that fax machines differed widely in their transmission rates, so that one manufacturer's models could not communicate with those of another.

Today, however, there is a directory of users, and the digital Group 3 and 4 machines have standardized transmission rates. This, and the high transmission speeds, have brought about the boom in fax usage.

Videophones and videoconferencing

Nowadays, it is possible to see as well as hear a telephone caller by using a videophone, a device consisting of both a video system and a phone. The video system consists of a black-and-white video camera and a monitor.

An ordinary black-and-white video picture occupies the same bandwidth as 600 telephone conversations. To avoid prohibitively expensive calls, videophone systems transmit pictures intermittently, to give a series of still images rather than a continuously moving one. Also, they remove redun­dant information from the picture and use compression techniques to reduce the information that has to be sent.

The system can be used, for example, to send pictures of components for fault diagnosis, pictures of damaged parts of a patient's body in the health care field, or pictures of products in the field of retailing.

Video conferencing systems are also available, which allow several callers to hold meetings without needing to travel long distances. British Telecom's Confravision system provides this sort of service, linking major cities by a network of videoconferencing studios for remote meetings.

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