Facsimile communication (in other words, a photo telegraph) is a long-distance transmission of a still image (text, illustration, photograph) with its subsequent reproduction at the received point. Historically arose as a result of the development of telegraph communication, but is capable of transmitting more types of information and is more resistant to interference.
Facsimile communications are used in the transmission of photo telegrams, in the decentralized printing of periodicals (transmission of newspaper strip materials and illustrations), in large enterprises for the exchange of production information, for receiving data from spacecraft and meteorological stations (exchange of hydrometeorological maps), as well as in many other cases.
The transmission process is performed using the facsimile machine (fax). The principle of the fax is as follows: the device’s transmitter splits the surface of the transmitted image (text, image) into many small parts (so-called elementary areas) that differ from each other in a certain way, most often in optical density). Then all the elements are sequentially converted from a graphic image into a stream of electrical pulses. Information about the transmitted object is stored according to the selected characteristics.
Then a facsimile message in the form of a series of electrical signals is transmitted over the communication line, which is a regular telephone line. In the receiving device, the inverse transformation takes place while preserving the sequence of elements, as a result of which we obtain a copy of the image we need.
The principle of fax communication was established in 1855 by the Italian physicist D. Caselli. The apparatus he designed transmitted an image deposited 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 along the image, alternately transmitted current and non-current impulses-signals. The received image was electrochemically recorded on foil.
These prototypes of modern fax machines were used to send messages on the telegraph lines Paris - Marseille and Moscow - St. Petersburg, but gradually came to naught due to their imperfection and the complexity of the transmission process.
In 1868, B. Meyer invented a method for receiving an image and recording it on ordinary paper using a rotating spiral coated with paint. The spiral, clinging to the paper at the right moments, left a series of strokes, from which the image formed. This method, having improved, is used to this day.
In the twentieth century, facsimile communication reached a qualitatively different level due to the emergence of a wide network of communication lines, the invention of electronic tubes, and the discovery of the photoelectric effect. In the 30s, the first photo telegraph devices 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 using a transmitter, receiver and communication line. In the transmitter, a point light spot line-by-line runs around the entire surface of the original, breaking it into elementary sections (platforms) with different light reflectivity. Reflecting from them, a variable-intensity light stream enters the photoelectric converter and turns into a video signal, then oscillation modulation (amplitude or frequency) is performed to convert the video signal into a form convenient for transmission via a communication line. Communication lines - ordinary wired or radiotelephone channels. In the case of the transmission of a large amount of information (newspaper strips, for example), multichannel communication with broadband channels is used.
In the receiver of the facsimile apparatus, the received signal is demodulated (highlighted) and the latter is converted into a copy, which is recorded on the medium (i.e., its convolution) in the same sequence as on the original. Ways to record a received signal:
- Photographic (on photo paper or film that fits in an opaque cassette). However, it is impossible to control 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 blackens when current is passed through it).
- Ink - on plain paper with a greased roller or pen.