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fax,in communications, system for transmitting pictures or other graphic matter by wire or radio. Facsimile is used to transmit such materials as documents, telegrams, drawings, pictures taken from satellites, and even entire newspapers. The surface of the material to be sent is traversed by a light-beam and a photodiode that translates the light and dark areas of the material thus scanned into electric signals for transmission. A receiving station reproduces the transmitted material by a variety of means. Newspapers and television stations have long transmitted and recorded news photographs using a process in which the received electric signals activate a variable lamp that is used to scan a photographic film.
A modern office fax machine scans a page to make an electronic representation of its text or graphics, compresses the data to save transmission time, and transmits it to another fax machine (or computer emulating a fax machine). The receiving machine decrypts the signal and uses a printer (usually built in) to make a facsimile of the original page. Because of the adoption of Group 3 digital standards in 1980 by the CCITT (International Telegraph and Telephone Consultative Committee), facsimile devices have become extremely prevalent in offices. These machines work over the public telephone network; they use digital modemsmodem
[modulator/demodulator], an external device or internal electronic circuitry used to transmit and receive digital data over a communications line normally used for analog signals.
..... Click the link for more information. and transmit at data rates up to 9600 bits per second. Images are produced with a resolution of 200 dots per inch. Personal computerscomputer,
device capable of performing a series of arithmetic or logical operations. A computer is distinguished from a calculating machine, such as an electronic calculator, by being able to store a computer program (so that it can repeat its operations and make logical
..... Click the link for more information. can emulate Group 3 facsimile machines if they are equipped with a fax modem, printer, and appropriate software. Facsimile machines that produce higher-resolution images or color and gray-scale images are also available.
a type of telecommunication in which two-dimensional, stationary images—line drawings, illustrations, or alphanumeric characters—are transmitted and reproduced. The images may be transmitted by wire or radio in the form of electric signals. Historically, facsimile has been considered a part of telegraphy; however, it is capable of transmitting a greater variety of document information and is less susceptible to interference than telegraphy.
Facsimile is used to transmit photographic telegrams and printed newspaper pages when the newspaper is not printed at a central location. It is also used for the rapid transmission of illustrations for printed periodicals, visual information from a spacecraft, engineering data in large intraplant communications systems, and hydrometeorological maps between meteorological stations.
In facsimile transmission, the transmitter divides the surface of the subject copy into a large number of rather small elements (elemental areas), which can be distingished by some specific physical property, such as optical density. The image of the subject copy is then converted sequentially, element by element, into a series of electrical impulses that carry information describing the subject copy with respect to the specific property. The signals are then transmitted through the communications channel to a receiver, where they are reconverted and recorded in the original sequence to obtain a copy of the original image.
History. The first transmission of a stationary image over a distance was achieved in 1855 by the Italian physicist J. Caselli. His electromechanical apparatus was capable of transmitting the image of a text, outline, or sketch that had been previously applied on lead foil with a special insulating varnish. The original image was thus reproduced as an aggregate of alternating elements of high electrical conductivity (the foil) and negligibly small conductivity (the varnish). The transmitting device “read” the image elements through a contact pin that slid over the image and fed current or no-current signals into the communications line. The received image was recorded electrochemically on moistened paper impregnated with a solution of potassium ferricyanide. Casein’s apparatus was used in communications lines between Moscow and St. Petersburg (1866–68), Paris and Marseille, and Paris and Lyon. However, its application was limited by imperfections in the apparatus and, particularly, by the necessity of transferring the image to be transmitted to foil.
In 1868 the German inventor B. Meyer proposed a method for recording the received image by means of a single-turn spiral covered with a layer of printer’s ink. A piece of plain paper was pressed against the rotating spiral at specific instants, and this contact produced small dashes on the paper that composed the image. An improved version of this method is still used.
New facsimile techniques were developed during the 1920’s in the wake of the discovery of the photoelectric effect, the invention of the electron tube and electrical amplifiers, and the construction of vast networks of suitable communications lines and channels. Phototelegraphic apparatus, such as the models ZFT-A4, FT-37, and FT-38, were developed and widely used in the USSR during the 1930’s; they used photographic techniques and materials for recording the transmitted images. Similar apparatus were called Bildtelegraph in Germany and telefax and telautograph in the USA. During the 1950’s and 1960’s facsimile came into use for transmitting not only photographic telegrams but also cartographic materials and newspaper pages. Other techniques of image recording were developed in addition to the photographic method. For this reason, the International Consultative Committee for Telephone and Telegraph (CCITT) recommended in 1953 that the term “phototelegraphic communication” be replaced by the more general term “facsimile communication.”
Technology, equipment, and methods. A facsimile communications channel includes a transmitter, a communications line, and a receiver.
In the transmitter, the subject copy (the original) is analyzed by a fine spot of light, the scanning element, which sweeps the surface of the original, line by line, and breaks it up into elemental areas that present differing reflectivities to the incident light flux. The light flux is reflected from the surface of the original and modulated in intensity to correspond with the reflectivities of the elemental areas. It then strikes a photoelectric converter and is converted into electrical current (the video signal) proportional to the light flux. The photoelectric converter of a facsimile apparatus is usually a photomultiplier or, less frequently, a photoelectric cell. The transmitter modulates the high-frequency oscillations with the video signal in order to convert the signal into a form that can be conveniently transmitted. Facsimile usually uses amplitude modulation, but frequency modulation may also be used.
Communications channels used for facsimile may be standard land-line telephone channels or radiotelephone channels, with a passband of 0.3–3.4 kilohertz (kHz). This frequency range is inadequate for the rapid transmission of large quantities of facsimile information, such as newspaper pages. In this case, it becomes necessary to use channels with wider passbands, such as a primary channel with a passband of 48 kHz or a secondary channel with a passband of 240 kHz (seeMULTICHANNEL COMMUNICATION).
In a facsimile receiver, the line signal is demodulated and the video signal discriminated. The latter is then converted into an image (copy), which is recorded on the carrier. The image is synthesized in the receiver from all the elemental areas, which are applied to the carrier in the same sequence as they appear on the original. This operation is called roll, or translation.
Several methods of recording received images have been used in facsimile. In photographic recording, photographic paper or film serves as the carrier and the recording is made by a point-shaped light source. The brightness of the light source varies over time according to variations in the video signal. Electrochemical recording uses a special paper that is blackened by the passage of current. The paper is placed between two pointed electrodes, which serve as the recording element, and the recording is made directly by the video signal, which is amplified to the required magnitude. In yet another method, the carrier is ordinary paper and the recording element is a roller covered with a special dye or an inked pen driven by an electromagnet. In another version of this method the recording is made through a sheet of carbon paper.
Photographic recording uses enclosed processing: the photographic paper or film is located in a lightproof cassette, and the quality of the reproduced copy cannot be examined until the reception and subsequent photochemical treatment of the carrier are completed. The other recording methods mentioned do not have this drawback, since they use open processing. Some other techniques, such as electrothermal and electrostatic recording, are not widely used.
In all recording methods, the recording element travels along a line across the carrier and is then transferred to the next line. The scanning element of the transmitter also travels line by line. In order to provide complete correspondence between the reproduced copy and the original, the operations of the transmitter and the receiver must by synchronized and in phase. This means that the scanning element of the transmitter and the recording element of the receiver must travel at the same speed and must start at the same instant at the beginning of each line. Failure to do so causes geometric distortions in the received image or a total loss of image. Synchronization and phasing in the facsimile apparatus may be achieved manually or automatically with the aid of special devices that control the motion of the scanning and recording elements.
Facsimile apparatus and the images they transmit may be divided into two groups: those that reproduce only two levels of optical density—black and white—and those that reproduce several density gradations, such as black, dark gray, gray, light gray, and white. The former images include manuscripts, drawings, maps, images of newspaper pages, and typewritten texts. Photographs are a good example of the halftone group. In order to obtain high-quality reproductions of photographs, no less than 8–12 gradations of optical density must be transmitted. Black-and-white images can be recorded in the receiver by any of the methods described above. Satisfactory reproduction of halftone materials requires the use of photographic methods.
Physical parameters. Facsimile operations are characterized by several physical parameters.
(1) Dimensions of the transmitted image. The standard image format is 220 × 290 mm; newspaper pages are 422 × 600 mm.
(2) Speed of transmission, measured by the number of lines transmitted per minute. Transmission by means of telephone and radiotelephone channels is standardized at 60, 120, and 250 lines/min. Newspaper pages are transmitted at speeds of 178,1,500, or 2,250 lines/min.
(3) The transmission time, which depends on the speed of transmission. It ranges from 6 to 25 min for a format of 220 × 290 mm and from 2.8 to 50 min for newspaper pages.
(4) Resolution, which characterizes the quality of reproduction of fine details. This ability is measured by the maximum number of lines per millimeter of row length that can be reproduced by the receiver without any merging of lines. Conventional facsimile equipment has a resolution of 5 lines/mm; equipment used to transmit newspaper pages has a resolution of 13–16 lines/mm.
(5) The number of gradations of optical density that can be reproduced. This applies only to halftone equipment.
REFERENCESPeredacha discretnoi informatsii i telegrafiia, 2nd ed. Moscow, 1974.
Kopnichev, L. N., and V. S. Kogan. Telegrafnye apparaty i apparaturaperedachi dannykh. Moscow, 1975.
L. N. KOPNICHEV
The process by which a document is scanned and converted into electrical signals which are transmitted over a communications channel and recorded on a printed page or displayed on a computer screen. The scanner may be compared with a camcorder, and the recorder is similar to an office copier or a computer printer. As an alternative to scanning, a document stored in computer memory can be transmitted. As an alternative to recording, a text facsimile (fax) image can be captured in computer memory and converted into computer-processable text by optical character recognition (OCR) software. Telephone lines or satellites provide the communication channel.
Most facsimile units communicate over the Public Switched Telephone Network, alternatively called the General Switched Telephone Network. A built-in high-speed digital modem automatically selects the highest modem speed (28,800–2400 bits/s) common to both facsimile units. If the telephone-line quality is not good enough for this transmission speed, a lower speed is negotiated during initialization. See Modem, Telephone service
In the scanning process, an image of the original page is formed by a lens in a way similar to that of an ordinary camera. A charge-coupled-device linear array of small photodiodes is substituted in the facsimile scanner for the camera film. The portion of the image falling on the linear diode array is a thin line, 0.005 in. (0.13 mm) high, across the top of the page being transmitted. Typically, 1728 diodes are used to view this line for a page 8½ in. (216 mm) wide. The photodiode corresponding to the left edge of the page is first checked to determine whether the very small portion of the image it detects is white (the paper background) or black (a mark). The spot detected by a single photodiode is called a picture element (a pel for short if it is recorded as either black or white, or a pixel if a gray scale is used). Each of the 1728 diodes is checked in sequence, to read across the page. Then the original page is stepped the height of this thin line, and the next line is read. The step-and-read process repeats until the whole page has been scanned. See Charge-coupled devices
Another class of flatbed scanner uses a contact image sensor linear array of photodiodes whose width is the same as the scanned width. One version has a linear array of fiber-optics rod lenses between the page being scanned and the sensor array. Light from a fluorescent lamp or a linear light-emitting-diode array illuminates the document beneath the rod lenses. The reflected light picked up by the sensor generates a signal that is proportional to the brightness of the spot being scanned. A second version has a hole in the center of each square pixel sensor element. Light from a light-emitting diode passes through this hole to illuminate the area of the document page at this pixel. No lenses or other optical parts are used.
In drum-type scanning, the original sheet of paper is mounted on a drum that rotates while the scan head with a photosensor moves sideways the width of one scanning line for each turn of the drum. Drum-type scanners are used mainly for remote publishing facsimiles and for color scanning in graphic arts systems.
In the recording process, facsimile signals are converted into a copy of the original. Facsimile receivers commonly print pages as they are received, but in an alternative arrangement pages may be stored and viewed on a computer screen.
Some modems can be used to send and receive fax data. V.27 ter and V.29 protocols are used.