Mechanical television
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Mechanical television was a television system that used mechanical or electromechanical devices to capture and display images. However, the images themselves were usually transmitted electronically and via radio waves. The reason for this dual nature of mechanical television lies in the history of technology. Mechanical television electronics came from Nineteenth Century inventors. Twentieth Century inventors added electronics.
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[edit] Mechanical television in history
The mechanical part usually consists of a Nipkow disk, which has a series of holes in a spiral pattern. In the camera, the disk has a light-detecting device, usually a photoelectric cell, behind it. In the reproducer (the display), a modulated light source, usually a neon tube, replaces the light detector. As each hole flies by, it produces a scan line. An AM radio wave or closed circuit carries the scan line to the reproducer.
Because only a limited number of holes could be made in the disks, image resolution on mechanical television broadcasts was typically very low, ranging from about 30 lines up to 120 or so. A few systems ranging into the 200-line region also went on the air. One of these was the 180-line system that Peck Television employed at Canadian station VE9AK.
Actually, mechanical technology is quite capable of producing pictures with a thousand or more lines. The existence of high-resolution laser printers and scanners proves this point. Mechanical television technology made these printers and scanners possible. Yet in the 1930s, few people could see the way to high-resolution, mechanical pictures.
There are several other technologies that can be used instead of a Nipkow disk. Other arrangements often made use of a rotating drum, either with holes or with a series of mirrors mounted on it. Another scanning method was the "flying spot," developed as a remedy for the low sensitivity that individual photoelectric cells had at the time. The television subject, standing in a darkened studio, would be illuminated by a bright, narrow beam of light, shining through the holes of a Nipkow disk. 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, whose combined signals gave a stronger picture. The flying spot method was used by the BBC television service as late as 1935 and in Germany as late as 1938. The disadvantages of the flying spot method were that the camera's subjects had to perform in near darkness, and the method could not be used outdoors in daylight.
A few of these systems were able to produce images several feet wide and of comparable quality to the cathode ray tube (CRT) televisions that were to follow. CRT technology at that time was limited to small, low-brightness screens. Perhaps the best mechanical televisions ever devised used the Scophony system, which could produce images of more than 400 lines and display them on screens at least 9×12 feet (2.8×3.7 m) in size (at least a few models of this type were actually produced). The Scophony system used multiple drums rotating at fairly high speed to create the images. One using a 441-line American standard of the day had a small drum rotating at 39,690 rpm (a second slower drum moved at just a few hundred rpm).
Some mechanical equipment scanned lines vertically rather than horizontally as in modern TVs. An example of this method is the Baird 30-line system. Baird's British system created a picture in the shape of a vertical rectangle. This shape created a portrait image, instead of the landscape orientation that is common today. The orientation of the scanning lines was determined simply by where the framing mask was placed along the Nipkow disk — placement of the framing mask at the left or right side of the disk gave vertictal scan lines, placement at the top or bottom of the disk gave horizontal scan lines. Because of the low definition of Baird's earliest television images, enough to show only one person clearly, a vertical portrait image made more sense than a horizontal landscape image. When images increased to 60 lines or more, and several people could be photographed at one time, the mask was switched to a horizontal image, and has stayed there to the present day.
The advancement of all-electronic television (including image dissectors and other camera tubes and cathode ray tubes for the reproducer) marked the beginning of the end for mechanical systems as the dominant form of television. Mechanical TV usually only produced small images. It was the main type of TV until the 1930s. All-electronic television, first demonstrated publicly by Philo Farnsworth in 1934, and first used for broadcasting in 1936, was quickly advancing past this point, reaching 400 to more than 600 lines with fast field scan rates in the next few decades. The last mechanical television broadcasts ended in 1939 at stations run by a handful of public universities in the United States.
[edit] Mechanical Television Recording
In the days of commercial mechanical television transmissions, a system of recording images (but not sound) was developed, using a modified gramaphone recorder. Marketed as "Phonovision", this system, which was never fully perfected, proved to be complicated to use as well as quite expensive, yet managed to preserve a number of early broadcast images that would normally have been lost. Scottish computer engineer Donald F. McLean has painstakingly reconstructed the analogue playback technology required to view these recordings, and has given lectures and presentations on his collection of mechanical television recordings made between 1925 and 1933. [1]
Among the discs in Dr. McLean's collection are a number of test recordings made by television pioneer John Logie Baird himself. One disc, dated "28th March 1928" and marked with the title "Miss Poundsford", shows several minutes of a woman's face in what appeares to be very animated conversation. The woman was identified in 1993 to be Mabel Poundsford, and her brief appearance on the disc is one of the earliest known video recordings of a human being. [2]
[edit] Recent uses of mechanical television
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 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.
[edit] The re-emergence of mechanical TV techniques
Today, a mechanical system of a sort has seen moderate popularity. DLP (Digital Light Processing) 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.
Another place where high-quality imagery is produced by opto-mechanics is the laser printer, where a small rotating mirror is used to deflect a modulated laser beam in one axis while the motion of the photoconductor provides the motion in the other axis. A modification of such a system using high power lasers is used in laser video projectors, with resolutions as high as 1024 lines and each line containing >1500 points. Such systems produce, arguably, the best quality video images. They are used, for instance, in planetariums.
