Mechanical television

From Wikipedia, the free encyclopedia
Jump to: navigation, search
"Televisor" redirects here. It is not to be confused with a television set, called a "televisor" in several languages.
This schematic shows the circular paths traced by the holes in a Nipkow disk.

Mechanical television is a television system that relied on a mechanical scanning device when generating or displaying the video signal. The signal itself was purely electronic and could be transmitted by radio or over a wire.

The reason for the dual nature of mechanical television lay in the history of its technology. The basic principles of television originated in the late 19th century, when only rudimentary electronic elements such as simple selenium cells were available. 20th century inventors added electronic amplification and other improvements as they were developed. Partly mechanical systems were used in television broadcasting as late as 1939, overlapping the all-electronic television era by several years.

A mechanical television receiver is sometimes called a televisor because the Baird Televisor was the most commercially successful model.


Main article: History of television

The essential mechanical component was usually a rapidly spinning Nipkow disk, which has a series of holes in a spiral pattern. In the camera, the disk worked in conjunction with a light source and one or more photoelectric cells to scan the subject and convert the point-to-point variations in its brightness into a corresponding electrical signal. An AM radio wave or closed circuit transmitted the signal to the reproducer (the display), in which a similar disk rotated synchronously. Behind the disk was a light source, usually a neon glow lamp, the brightness of which was modulated by the video signal. As each hole in the disk passed by, one scan line of the transmitted image was therefore reproduced.

Block diagram of General Electric mechanical scan television system, Radio News (April 1928)

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.[1] In the 1920s, the Japanese electrical scientist Yasujiro Niwa invented a simple device for phototelegraphic transmission through cable and later via radio.[citation needed]

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 station WCFL, the first mechanical television station to go on the air in Chicago on June 12, 1928.[2] By sending the sound signal to station WIBO[3] and the video signal on station 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.)


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.

Flying spot scanners[edit]

The most common scanning method for creating the video signal was the "flying spot", developed as a remedy for the low sensitivity that photoelectric cells had at the time. A bright, narrow beam of light would shine through the holes of a Nipkow disk. This light would then illuminate the television subject, standing in a darkened studio.

Whipping back and forth and up and down, the spot of light would complete sixteen or more scans per second. The light would reflect back to not one, but a bank of photoelectric cells. The combined signals of these cells gave a strong picture. Like mechanical television itself, flying spot technology grew out of phototelegraphy (facsimile). This scanning method began in the 19th century.


  • Actors must perform in near darkness;
  • Flying spot cameras tend to work unreliably outdoors in daylight.





Mechanical television with large pictures[edit]




Aspect ratios for different purposes[edit]






Rise of electronic television[edit]



Color mechanical television[edit]

Color wheel in a modern single-chip Digital Light Processing projector.

Mechanical television returned to the United States as a method of painting colors over a monochrome CRT. The CBS color television system of Peter Goldmark used such technology in 1940.[10] John Baird's 1928 color television experiments had inspired Goldmark's more advanced field-sequential color system.[11] In Goldmark's system, stations transmit color saturation values electronically. Yet mechanical methods also come into play. At the transmitting camera, a mechanical disc filters hues (colors) from reflected studio lighting. At the receiver, a synchronized disc paints the same hues over the CRT. As the viewer watches pictures through the color disc, the pictures appear in full color.



Today, some Digital Light Processing (DLP) projectors still use color filter wheels.




Modern uses of mechanical scanning[edit]

Since the 1970s, some amateur radio enthusiasts have experimented with mechanical systems. The early light source of a neon lamp has now been replaced with super-bright LEDs. There is some interest in creating these systems for narrow-bandwidth television, which would allow a small or large moving image to fit into a channel less than 40 kHz wide (modern TV systems usually have a channel about 6 MHz wide, 150 times larger). Also associated with this is slow-scan TV, although that typically uses electronic systems using the P7 CRT up till the 1980s and PC there after, but there are 3 known mechanical monitor forms 2 Fax printer like monitors made in the 1970s and in 2013 a small drum monitor with a coating of glow paint where the image is painted on the rotating drum with an uv laser .

Digital Light Processing (DLP) projectors use an array of tiny (16 μm²) electrostatically-actuated mirrors selectively reflecting a light source to create an image. Many low-end DLP systems also use a color wheel to provide a sequential color image, a common feature of many early color television systems before the shadow mask CRT provided a practical method for producing a simultaneous color image.



Laser lighting display techniques are combined with computer emulation in the LaserMAME[dead link] project. It is a vector-based system, unlike the raster displays thus-far described. Laser light reflected from computer-controlled mirrors traces out images generated by classic arcade software which is executed by a specially modified version of the MAME emulation software.

Television Machine with 4 LED - Strips


See also[edit]

External links[edit]