With the introduction of the telegraph, enthusiasts began practical research to transmit and receive images using telegraph lines. In 1840, Scottish watchmaker and inventor Alexander Bain manufactured and patented in 1843, a device for electrical transmission of images with their electrochemical reproduction. It consisted in changing the color of paper impregnated with chemicals that are sensitive to electric current. A movable contact was used, which slid on the surface of the paper.
Satisfactory quality of the transmitted images could not be achieved due to imperfect synchronization. However, the transmission speed of the graphic image (Morse code) was increased five to seven times. In 1847, the English physicist Frederick Bakewell improved O. Bain’s device by improving synchronization and for the first time carried out wire image transmission over a distance of about 50 miles. The image quality was low, so it did not go further than laboratory research.
During 1856 -1861, the Italian priest, physicist-enthusiast Giovanni Caselli developed and manufactured a device that found commercial application for the transmission of images by telegraph wires, called by the author “pantelephone”. It was a cast iron device about 2 meters high in the shape of the letter “A”. A pendulum weighing about 8 kg was attached to the top. Synchronization of pendulum movements in the transmitting and receiving apparatus was provided by high-precision clockwork. The image to be transmitted was applied with special non-conductive ink on a conductive metal base.
A platinum contact attached to the transmitter pendulum scanned the surface of the base in parallel lines. At the same time, depending on whether the contact with the base was present or absent when moving along the non-conductive ink image, electrical impulses were created or absent, which were sent to the receiver through the line. An iron contact was connected to the receiver pendulum. There was a similar, synchronized with the scanning in the transmitting apparatus, scanning of the surface of the paper impregnated with potassium ferrocyanide, which changed its color when the current passed. Thus, the electrochemical reproduction of the transmitted image was carried out. The resolution of the pantelegram was 3 lines per mm, the size of the image (white on a blue background) could be up to 10 cm by 15 cm.
A common disadvantage of these devices was the use of contact scanning. Further development of TV was caused by the discovery in 1873 of the internal photoelectric effect by American physicist Willaby Smith[en] and in 1887 of the external photoelectric effect by German scientist Heinrich Hertz. This created the conditions for the transmission of real images without their preliminary preparation. In 1881, the English physicist Shelford Bidwill used a selenium photocell in a transmitting device, which made it possible for the first time to non-contact scan an image formed by an optical lens system.
The creation of these devices showed the ways of further development of methods and practical implementation of means for the transmission of moving images. This is a line scanning of the image, with its simultaneous conversion into an electrical signal in the transmitting means (analysis, scanning, rasterization) and synchronized with the process of line scanning, line reproduction of the image in the receiving means (synthesis, vectorization, rendering).
The first electromechanical TV systems
In 1884, Paul Gottlieb Nipkow, a university student in Germany, patented an electromechanical television system consisting of two moving disks with electric power. The opaque disk rotating around its axis had holes located at equal angular distances along the Archimedean spiral. Subsequently, this disk was called the “Nipkov disk”. On a small segmental area at the top or side of the disk, the image intended for transmission was focused. A photocell was located behind the disk, opposite this area.
When the disk rotated, its holes (because they were arranged spirally) scanned the image one after another in almost horizontal parallel rows from top to bottom. The number of rows corresponded to the number of holes on the disk. Depending on the amount of light that penetrated through the moving holes, the voltage of the photocell changed. Thus, the image was scanned and simultaneously converted into an electrical video signal.
To reproduce the image, a disk with the same number of holes was also used, which rotated synchronously with the transmission disk. Moving holes were illuminated by a light source, the brightness of which varied (modulated) in accordance with the voltage of the video signal. For this purpose, the patent used a prototype of an electro-optical modulator consisting of an analyzer, a gas device for changing the polarization angle under the influence of a control voltage and a normalizer. Later, a neon lamp with a high frequency of light brightness change (up to tens of kilohertz) was used as a modulated light source.
