Television is a field of science, technology and culture associated with the transmission of visual information (moving and still images) over a distance by radio-electronic means, as well as the method of such transmission itself. Along with radio broadcasting, television is one of the most popular means of distributing information (political, cultural, scientific, educational, etc.). It is one of the main means of communication used for scientific, organizational, technical, and other applications (e.g., industrial and transportation command and control systems, space and nuclear research, military, surveillance systems, etc.).
The human eye serves as the final link in television transmission, so television systems are built with eyesight in mind. The real world is perceived by a person visually in colors, objects – in relief, located in the volume of some space, and events – in dynamics, movement, therefore, an ideal television system should provide an opportunity to reproduce these properties of the material world as fully as possible. In modern television, the tasks of transmitting motion and color have been successfully solved both technically and practically.
The first experiments in the transmission of a still image were made in the era of the electric telegraph. In 1856.
J. Caselli built and demonstrated a prototype of an electromechanical system for transmitting images at a distance. He called this device “Pantelegraph” as a derivative of the words “pantograph” (pantograph) – the main instrument of copying and “telegraph” – the method of transmission at a distance.
The sender of the message wrote it on a pewter sheet of ink that did not conduct electricity. Then the sheet was attached to a curved metal plate and scanned with a pendulum pen (resolution – three lines per mm). On the receiving side the image was reproduced with special inks that reacted chemically with the paper impregnated with potassium iron cyanide
In order to synchronize the transmission and reproduction of the image, the pendulums were controlled by high-precision clockwork mechanisms. The basic principle of converting a two-dimensional image into a one-dimensional electrical signal has not changed to this day.
D. Caselli’s invention soon became widespread not only in France, but all over the world. It was patented in Europe in 1861 (European Patent No. 2532) and in the USA in 1863 (US Patent No. 37,563).
J. Xelli’s invention was based on the principle of alternating transmission of image elements. It was proposed anew in 1878 by A. Paiva. This principle is the basis of all modern television systems.
However, the transmission of moving images required a faster method of converting the image into an electrical signal. The principle possibility of realizing high-speed transmission appeared thanks to the discovery of the external and internal photoelectric effect, which could be used to convert light into electric current.
In 1907, B. L. Rosing developed a system of “cathode telescopy” (in which an electron-beam tube was used to reproduce images). In July 1907, he filed an application for a “Method of electric transmission of images at a distance.” This application was followed by patent No. 18076 issued to him on October 30, 1910. In 1908-1909 the discovery of a new method of receiving images in television was confirmed by patents issued in Great Britain and Germany. In 1911, an improved television device was patented in Great Britain, Germany and the United States.
Б. On May 9, 1911, B.L. Rozing in his laboratory managed to ensure the reception of the simplest images of figures with the device he had designed. This was the world’s first television transmission, marking the beginning of the television era. However, many complicated problems had to be solved in order to bring television to the stage of practical application, so electronic television broadcasting did not appear until a quarter of a century later.
The possibility of element-by-element transmission of images over a communication channel is based on the property of human vision to perceive pulsating light as continuous, if the frequency of pulsations exceeds the critical frequency, which depends on the brightness of the source and is a few tens of hertz. The process of successive transformation of image elements into electrical signals during transmission and the reverse process during reception are called image scanning. The processes of image analysis and synthesis must be synchronous (equal in frequency) and in-phase (equal in phase). The sweep law is determined by the purpose of the television system.
One of the first devices for transmitting image elements, based on the use of a rotating disk with holes, was proposed by P. Nipkov. In 1884 he received a patent for the optical-mechanical device (“electronic telescope”) for decomposition of the image into elements for transmission and reception of TV signals. It was used in the early, still imperfect electromechanical television systems.