a complex of mechanical, optical, and electronic devices used to transmit images of stationary, two-dimensional objects through telecommunication channels or to receive such images; reception includes the reproduction of the original image as a copy. Facsimile apparatus are classified as transmitters, receivers, and transmitter-receivers.
A facsimile transmitter, or the transmitter section of a receiver-transmitter, consists of an analyzing system, which converts the image of the subject copy (the original) into a video signal, and an electronic modulator, which converts the video signal into a form that can be conveniently transmitted through communications channels. The analyzing system has an optical lighting device that produces a narrow light beam; the beam forms a fine spot of light on the surface of the original. The system also includes a scanning device, which directs the light beam successively and in a given sequence to all elemental areas on the original. As a result, the light flux is modulated in intensity to correspond with the reflectivity of the elemental areas.
A photoelectric converter converts the reflected light flux into an electric current (the video signal), which is proportional to the light flux. A video signal conversion unit modulates the electrical oscillations of the signal. This modulation may be of several types: in positive amplitude modulation, the maximum level of the carrier-frequency oscillations corresponds to the black field of the transmitted image; in negative amplitude modulation, the maximum level of oscillations corresponds to the white field of the image; and in negative frequency modulation the higher frequency corresponds to the black field.
A facsimile receiver, or the receiving section of a transmitter-receiver, contains an electronic unit for discrimination of the video signal, designed to demodulate the received oscillations, and a synthesizing system, consisting of a scanning device and a recording device, which produces a copy of the transmitted image. The carriers currently in use include photographic paper and film, electrographic, electrochemical, electrothermal and ordinary writing paper, and ferromagnetic materials.
The scanning devices of facsimile transmitters and receivers are often similar; they may be of either mechanical or electronic design. The most widely used types of facsimile apparatus feature mechanical scanners of the drum, flat-bed, or arc types, driven by synchronous motors.
In drum scanners, the subject copy or the record carrier is fastened to a cylinder. Scanning may be accomplished as the cylinder is rotated and shifted along its axis, with the scanning element held stationary, or as a result of the rotation of the cylinder and the simultaneous shifting of the scanning element (the light spot) along the cylinder generatrix. In flat-bed scanning, the original is fastened between drawing rollers. Each line is scanned by the scanning element, which is moved across the original with the aid of an oscillating mirror. Vertical scanning—the transfer of the scanning element to the next line—is achieved by shifting the subject copy. In arc scanning, the original or the record carrier is located within a cylindrical chamber. Scanning takes place as the optical system is rotated and the chamber is shifted by one increment for each rotation.
Synchronization of the scanning devices of the transmitter and receiver may be independent, in which case the electric motors of the scanning devices are independently powered by frequency-stable tuning-fork or quartz oscillators; in other cases, frequency-synthesizing signals are fed from the transmitter to the receiver, or synchronous motors fed by a common power supply may be used. Receiver scanning may be kept in phase automatically, semiautomatically, or manually.
The basic parameters of facsimile apparatus include the scanning speed N, that is, the number of lines analyzed per minute; the index of cooperation, equal to Dm for drum-type equipment and Lm/π for flat-bed equipment, where D is the diameter of the drum, L is the length of the line, and m is the number of lines scanned per millimeter; and the transmission time, which is equal to hm/N, where h is the height or length of copy.
REFERENCESMel’nik, S. O., and M. I. Oksman. Fototelegrafnye apparaty. Moscow, 1966.
Fototelegrafnye apparaty i detsentralizovannoe pechatanie tsentral’nykh gazet. Moscow, 1971.
Kopnichev, L. N., and V. S. Kogan. Telegrafnye apparaty i apparatura peredachi dannykh. Moscow, 1975.
S. O. MEL’NIK