Mechanical Television - Mechanical Television in History

Mechanical Television in History

The essential mechanical component usually consisted of a Nipkow disk, which has a series of holes in a spiral pattern. In the camera, the disk had a light-detecting device, usually a photoelectric cell, behind it. In the reproducer (the display), a modulated light source, usually used a neon tube, replacing the light detector. As each hole flew by, it produced a scan line. An AM radio wave or closed circuit then carried the scan line to the TV reproducer.

Facsimile transmission of still photographs employed some of the principles of mechanical television as early as the 19th century. For instance, Shelford Bidwell demonstrated such a system in 1881. For decades, earlier systems had pioneered scanning in the transmission of type and line art. Photographic transmission was a greater challenge due to the selenium in early photoelectric cells having very low sensitivities. Scanning a photograph at a resolution suitable for newspaper reproduction could take several minutes. With silhouette or duotone still images, instantaneous transmission was possible by 1909.

American inventor Charles Francis Jenkins developed mechanical television systems in the 1920s and early 1930s. In 1923, Jenkins transmitted the first moving silhouette images, and on June 13, 1925 publicly demonstrated synchronized transmission of images and sound. Over 400 patents were issued to Jenkins, including 75 devoted to mechanical television alone. In the 1920s, the Japanese electrical scientist Yasujiro Niwa invented a simple device for phototelegraphic transmission through cable and later via radio.

Mechanical television transmitting a live, moving image in tone gradations (grayscale images) was demonstrated by British inventor John Logie Baird on January 26, 1926, at his laboratory in London. Unlike later electronic systems with several hundred lines of resolution, Baird's vertically scanned image, using a scanning disk embedded with a double spiral of lenses, had only 30 lines, just enough to reproduce a recognizable human face.

Ulises Armand Sanabria was the builder and engineer of WCFL, the first mechanical television station to go on the air in Chicago on June 12, 1928. By sending the sound signal to station WIBO and the video signal on WCFL, he was the first to transmit sound and picture simultaneously on the same wave band on May 19, 1929. Several US universities established and maintained mechanical television stations from 1930 to 1939. (See External Links below for list of such stations US and Canada 1928-1939.)

Because only a limited number of holes could be made in the disks, and disks beyond a certain diameter became impractical, image resolution on mechanical television broadcasts was relatively low, ranging from about 30 lines up to 120 or so. Nevertheless, the image quality of 30-line transmissions steadily improved with technical advances, and by 1933 the UK broadcasts using the Baird system were remarkably clear. A few systems ranging into the 200-line region also went on the air. Two of these were the 180-line system that Compagnie des Compteurs (CDC) installed in Paris in 1935, and the 180-line system that Peck Television Corp. started in 1935 at station VE9AK in Montreal.

Instead of a Nipkow disk, mechanical television could also use several other technologies. Other arrangements often made use of a rotating drum, either with holes or with a series of mirrors on it.