Facsimile (in another way - phototelegraph) -this is the transmission to a distant distance of a still image (text, illustration, photograph) and then reproducing it at the received point. Historically it arose as a result of the development of telegraph communication, but is able to transmit more types of information and is more resistant to interference.
Facsimile is used for transmissionphototelegrams, with decentralized printing of periodicals (transfer of newspaper stripes and illustrations), at large enterprises for the exchange of production information, for obtaining data from spacecraft and meteorological stations (exchange of hydrometeocards), as well as in many other cases.
The transmission process is performed using the apparatusfacsimile communication (fax). The principle of the fax is as follows: the transmitter of the device makes a division of the surface of the transmitted image (text, picture) into many small parts (so-called elementary areas), which differ from each other by a certain feature, most often by optical density). Then all the elements are subsequently converted from the graphic image into a stream of electrical pulses. The information about the transferred object is stored according to the selected characteristics.
Then a fax message in the form of a serieselectric signals is transmitted through the communication line, which serves as an ordinary telephone line. In the receiving device, an inverse transformation takes place, preserving the sequence of elements, resulting in a copy of the image we need.
The principle of facsimile communication was established in 1855the Italian physicist D. Caselli. The device designed by him transmitted an image printed on lead foil with a special varnish with an extremely low degree of electrical conductivity (almost zero). Thus, the image consisted of alternating elements of high (foil) and low electrical conductivity. The contact pin, sliding on the image, transmitted alternately current and non-current impulse signals. The received image was electrochemically recorded on the foil.
These prototypes of modern fax were usedfor the transmission of messages on the Paris-Marseille and Moscow-St. Petersburg telegraph lines, but gradually came to naught due to their imperfection and the laboriousness of the transfer process.
In 1868, B. Meyer invented a method of receiving an image and recording it on plain paper with a rotating spiral covered with paint. The spiral, snuggling up to the paper at the right moments, left a series of strokes, from which an image formed. This method, having improved, is still used today.
In the twentieth century, facsimile communicationa qualitatively different level due to the appearance of a wide network of communication lines, the invention of electron tubes, the discovery of a photoelectric effect. In the 1930s, the first phototelegraph apparatus appeared in our country. In the future, the use of photographic materials and methods has found application around the world.
Modern facsimile communication is carried out attransmitter, receiver and communication link. In the transmitter, a spot light spot line bypass the entire surface of the original, breaking it into elementary areas (areas) with different reflective powers. Reflecting from them, a light flux of variable intensity falls on the photoelectric converter and turns into a video signal, then modulation of the oscillations (amplitude or frequency) is performed to convert the video signal into a convenient form for transmission through the communication line. Communication lines are ordinary wired or radiotelephone channels. In the case of the transmission of a large amount of information (newspaper pages, for example), multi-channel communication with broadband channels is used.
The receiver of the fax machine isdemodulation of the received signal (selection) and transformation of the latter into a copy, which is recorded on the medium (i.e., its convolution) in the same sequence as on the original. Ways of recording the received signal:
- Photographic (on photo paper or film,which is placed in a light-tight cassette). It is impossible to verify the quality of the received message until the end of the photochemical processing of such a carrier.
- Electrochemical (the image is applied to a special paper, which turns black when passing current through it).
- Inky - on ordinary paper with a blurred roller or pen.