facsimile

/fak sim"euh lee/, n., v., facsimiled, facsimileing, adj.

n.

1. an exact copy, as of a book, painting, or manuscript.

2. Also called fax. Telecommunications.

a. a method or device for transmitting documents, drawings, photographs, or the like, by means of radio or telephone for exact reproduction elsewhere.

b. an image transmitted by such a method.

3. dropout (def. 5).

v.t.

4. to reproduce in facsimile; make a facsimile of.

adj.

5. Also, fax. Telecommunications.

a. (of an image) copied by means of facsimile: facsimile mail.

b. (of a method or device) used to produce a facsimile: facsimile transmission.

[1655-65; earlier fac simile make the like, equiv. to L fac (impv. of facere) + simile, n. use of neut. of similis like; see SIMILE]

Syn. 1. replica, likeness. 1, 4. duplicate.

* * *

▪ communications

Introduction

also called  Fax, or Telefax,  

      in telecommunications, the transmission and reproduction of documents by wire or radio wave. Common fax machines are designed to scan printed textual and graphic material and then transmit the information through the telephone network to similar machines, where the documents are reproduced in close to their original form. Such machines, because of their low cost, reliability, speed, and simplicity of operation, have revolutionized business and personal correspondence. They have virtually replaced telegraphic services, and they also present an alternative to government-run postal services and private courier services.

Standard fax transmission.

      Most office and home fax machines conform to the Group 3 standard, which was adopted in 1980 in order to ensure the compatibility of digital machines operating through public telephone systems worldwide. As a standard letter-size sheet is fed through a machine, it is scanned repeatedly across its width by a strip of 1,728 photosensors. Each photosensor in turn generates a low or a high variation in voltage, depending on whether the scanned spot is black or white. Since there normally are 100 scan lines per inch (4 lines per mm), the scanning of a single sheet can generate almost 2 million variations in voltage. The high/low variations are converted to a stream of binary digits, or bits (bit), and the bit stream is subjected to a source encoder, which reduces the number of bits required to represent long runs of white or black spots. The encoded bit stream can then be modulated onto an analog carrier wave by a voiceband modem and transmitted through the telephone network. With source encoding, the number of bits required to represent a typewritten sheet can be reduced from 2 million to less than 400,000. At standard fax modem speeds of 4,800, 9,600, or 14,400 bits per second, this reduction results in transmission rates of a minute and a half to as little as half a minute per page.

      Communication between a transmitting and a receiving fax machine opens with the dialing of the telephone number of the receiving machine. This begins a process known as the handshake, in which the two machines exchange signals that establish compatible features such as modem speed, source code, and printing resolution. The page information is then transmitted, followed by a signal that indicates no more pages to be sent. The called machine signals receipt of the message, and the calling machine signals to disconnect the line.

      At the receiving machine, the signal is demodulated, decoded, and stored for timed release to the printer. The document may be reproduced on special thermally sensitive paper, using a print head that has a row of fine wires corresponding to the photosensors in the scanning strip. More commonly, it is reproduced on plain paper by a xerographic process, in which a minutely focused beam of light from a semiconductor laser or a light-emitting diode, modulated by the incoming data stream, is swept across a rotating, electrostatically charged drum. The drum picks up toner powder in charged spots corresponding to black spots on the original document and transfers the toner to the paper.

      Group 3 facsimile transmission can be conducted through all telephone channels, whether they be copper wire, optical fibre, microwave radio, or cellular radio. Using the proper hardware and software, computer files can be sent to fax machines without first being printed and scanned. Facsimile transmission can also take place directly between computers, the document being reproduced on a desktop printer.

Other applications of fax.

      In addition to the conventional office use of fax as described above, there are several other applications worth noting. The first is in point-to-point transmission of newspapers (newspaper) for remote printing. A number of newspapers are available simultaneously in several editions nationwide or even worldwide. Although the paper is usually composed at only one location, a special high-resolution (800–1,800 lines per inch) fax machine is often used to transmit an exact replica of each page to remote printing sites, where the received fax is printed for local distribution.

      Another application of fax is in the distribution of weather maps (weather map) and charts via high-frequency radio. The maps are generally intended for use by maritime vessels and are usually transmitted by governmental bodies at a rate of 120 lines per minute.

      Yet another application of fax technology is in the transmission of weather-satellite (weather satellite) photographs of the Earth. A scanning radiometer is employed in the satellite to take either visible or infrared images of the Earth—the former for use in daylight with minimal cloud cover and the latter for all other conditions. The weather images are transmitted at a rate of 240 lines per minute to Earth stations, using a facsimile signal modulated onto either a very-high frequency or an ultrahigh frequency carrier. Ground stations, often associated with television stations and newspapers, then use special weather fax receivers and displays to receive the images.

* * *