Facsimile

facsimile (făksĭm`əlē) or 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 modems and transmit at data rates up to 9600 bits per second. Images are produced with a resolution of 200 dots per inch. Personal computers 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.

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See fax.

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Facsimile

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.

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(communications)

facsimile - ("fax") A process by which fixed graphic material including pictures, text, or images is scanned and the information converted into electrical signals which are transmitted via telephone to produce a paper copy of the graphics on the receiving fax machine. Some modems can be used to send and receive fax data. V.27 ter and V.29 protocols are used.