Saturday, August 22, 2009

What is a FAX?

The transmission of photographs, drawings, maps, and written or printed words by electric signals. Light waves reflected from an image are converted into electric signals, transmitted by wire or radio to a distant receiver, and reconstituted on paper or film into a copy of the original.
Facsimile is used by news services to send news and photos to newspapers and television stations, by banks, airlines, and railroads to transmit the content of documents, and by many other businesses as an aid in data handling and record keeping.
Facsimile systems involve optical scanning, signal encoding, modulation, signal transmission, demodulation, decoding, and copy making.
Scanning is done in a manner similar to that used in television. An original, a photo for example, is illuminated and systematically examined in small adjacent areas called pixels (picture elements). Light reflected from each pixel is converted into electric current by an electronic device, a photocell, photodiode, or charge-coupled device (CCD).
A single such device may be used to cover one pixel after another in a row, row after row from top to bottom until the entire image has been translated into electric impulses. This is rectilinear scanning. Scanning may also be done a row at a time by a battery of devices; this is array scanning.
In multispot scanning, a vertical array of photodevices moves across the image, examining the pixels column by column. As the array passes down the copy, it produces a set of current pulses from each photodevice. The separate currents, however produced, are then transmitted successively over a single circuit to the distant receiver.
To secure fine detail in the reproduced image it is necessary to use very small pixels. In one standard, Group 3 of the International Telegraph and Telephone Consultative Committee ( CCITT ), each pixel is a rectangle 0.12 by 0.13 mm ( 1 inch=25.4 mm ). On this standard, subject copy measuring 8 by 11 inches ( 20 x 28 cm ) is divided into 3.6 million pixels.
This compares with about 200,000 pixels for televised images. The pixels used in high-resolution facsimile systems have dimensions one-fifth those of the CCITT standard mentioned above, whereas in low-definition systems the dimensions may be twice as great.
The image may be illuminated as in rectilinear scanning, or a relatively large area of the image may be illuminated, the photodevice viewing the image through a lens aperture that restricts its field to a single pixel at a time.
In a commonly used facsimile scanning system ( invented by Frederick Bakewell in 1848 and based on Alexander Bain's work of 1842 ) the subject copy is wrapped around a drum. A finely focused spot of light falls on the copy and the light reflected from that pixel is picked up by the photodevice. The drum is rotated so that the light spot traces a line across the copy, examining each pixel in turn.
As the drum rotates, the light source is moved slowly on a carriage parallel to the drum axis, tracing out a spiral of adjacent lines until the entire area of the copy has been scanned. At least once in each rotation of the drum a signal transmitted to the recorder keeps the scanner and the recorder in step.
In drum scanning, the copy may also be illuminated broadly and examined by a photodevice fitted with a lens aperture.
Copy cannot always be conveniently wrapped around a drum. In such cases, flat copy may be scanned by a spot of light directed across its surface by a moving mirror. Mirror scanning may also be used when the copy is wrapped on a drum, or while it is being pulled from a roller. Laser light produces a very fine beam that travels across the copy, row by row, as the copy moves vertically.
In one arrangement the mirror is rocked back and forth, moving the beam across the copy. In another, a rotating polygonal mirror is used. This mirror typically has 18 flat mirror surfaces on its periphery, each capable of scanning a row of pixels.
Very fast scanning can be achieved by rapid rotation of the mirror and corresponding vertical motion of the copy. The beam is reflected from each pixel into a photodevice that converts successive light values into corresponding currents. Electronic scanning of flat copy may also be done by arrays of photodiodes or charge-coupled devices.
For scanning rates higher than about 6 rows per second laser beams with polygonal mirrors and arrays of photodevices are favored.

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