Facsimile - from invention to the present day

Facsimile (in another way - phototelegraph) is the transmission to a distant distance of a still image (text, illustration, photograph) with the subsequent reproduction of it in 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 communication is used for the transmission of phototegrams, with decentralized printing of periodicals (transfer of newspaper stripes and illustrations), in large enterprises for the exchange of production information, for obtaining data from spacecraft and meteorological stations (exchange of hydrometeocards), and in many other cases.

The transmission process is performed using a facsimile (fax) machine. 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 facsimile message in the form of a series of electrical signals is transmitted over a communication line, which is the usual telephone line. In the receiving device, an inverse transformation takes place, preserving the sequence of elements, as a result of which we get a copy of the image we need.

The principle of facsimile communication was established in 1855 by the 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 used to transmit 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, pressing against the paper at the right moments, left a series of strokes, from which an image was formed. This method, having improved, is still used today.

In the twentieth century, facsimile communication reached a qualitatively new level due to the emergence of a wide network of communication lines, the invention of electronic tubes, the discovery of a photoelectric effect. In the 1930s, the first phototelegraphic apparatus appeared in our country. In the future, the use of photographic materials and methods has found application worldwide.

Modern facsimile communication is carried out with the help of a transmitter, a receiver and a communication line. In the transmitter, a spot light spot line-by-line runs around the entire surface of the original, breaking it into elementary areas (areas) with different light reflectivity. Reflecting from them, the light flux of variable intensity falls on the photoelectric converter and turns into a video signal, then the 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 demodulates the received signal (selection) and converts the signal into a copy, which is recorded on the medium (i.e., its convolution) in the same sequence as the original. Ways of recording the received signal:

- Photographic (on photo paper or film, which is placed in a lightproof cassette). It is impossible to verify the quality of the received message until the end of the process of photochemical processing of such a carrier.

- Electrochemical (the image is applied to special paper, which turns black when passing current through it).

- Ink - on plain paper with a blurred ink or pen.