Mechanical television (also called televisor) was a broadcast television system that used mechanical or electromechanical devices to capture and display video images. However, the images themselves were usually transmitted electronically and via radio waves. The reason for the dual nature of mechanical television lay in the history of technology.
The earliest mechanical television components originated with 19th century inventors, with 20th century inventors later adding electronic components as they were created. Mechanical systems were used in television broadcasting from 1925 to 1939, overlapping the all-electronic television era by three years.
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.[1] 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.[2] By sending the sound signal to station WIBO[3] 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.[4] 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 started in 1935 at station VE9AK in Montreal.[5]
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.
Another scanning method was the "flying spot." 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.
The BBC television service used the flying spot method until 1935. German television used flying spot methods as late as 1938. This year was by far not the end of flying spot scanner technology. The German inventor Manfred von Ardenne designed a flying spot scanner with a CRT as the light source. In the 1950s, DuMont marketed Vitascan, an entire flying-spot color studio system. Today, graphic scanners still use this scanning method. The flying spot method has two disadvantages:
A note about outdoor telecasts with a flying spot scanner: In 1928, Ray Kell from the United States' General Electric proved that flying spot scanners could work outdoors. The scanning light source must be brighter than other incident illumination.
Kell was the engineer who ran a 24-line camera that telecast pictures of New York governor Al Smith. Smith was accepting the Democratic nomination for presidency. As Smith stood outside the capital in Albany, Kell managed to send usable pictures to his associate Bedford at station WGY, which was broadcasting Smith's speech. The rehearsal went well, but then the real event began. The newsreel cameramen switched on their floodlights.
Unfortunately for Kell, his scanner only had a 1 kW lamp inside it. The floodlights threw much more light on Governor Smith. These floods simply overwhelmed Kell's imaging photocells. In fact, the floods made the unscanned part of the image as bright as the scanned part. Kell's photocells couldn't discriminate reflections off Smith (from the AC scanning beam) from the flat, DC light from the floodlamps.
The effect is very similar to extreme overexposure in a still camera: The scene disappears, and the camera records a flat, bright light. Use the camera in favorable conditions, though, and the picture comes out fine. Similarly, Kell proved that outdoors in favorable conditions, his scanner worked fine.
A few mechanical TV systems could 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. One such system was developed by Ulises Armand Sanabria in Chicago. By 1934, Sanabria demonstrated a projection system which had a 30-foot image.[6]
Perhaps the best mechanical televisions of the 1930s 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). Today, DLP mechanical TV technology from Texas Instruments far outstrips the capabilities of the Scophony system.
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 very narrow, vertical rectangle.
This shape created a portrait image, instead of the landscape orientation that is common today. The position of a framing mask before the Nipkow disk determines the scan line orientation. Placement of the framing mask at the left or right side of the disk gives vertical scan lines. Placement at the top or bottom of the disk gives horizontal scan lines.
Baird's earliest television images had very low definition. These images could only show one person clearly. For this reason, a vertical, portrait image made more sense to Baird than a horizontal, landscape image. Baird chose a shape three units wide by seven high. Actually this shape is only about half as wide as a traditional portrait. You can imagine this shape this way: A typical doorway also has the proportions three by seven.
Instead of entertainment television, Baird might have had point-to-point communication in mind. Another television system followed that reasoning. The 1927 system developed by Herbert E. Ives at AT&T's Bell Laboratories was a large-screen television system and the most advanced television of its day. The Ives 50-line system also produced a vertical "portrait" picture. Since AT&T intended to use television for telephony, the vertical shape was logical: phone calls are usually conversations between just two people. A picturephone system would depict one person on each side of the line.
Meanwhile, in the US, Germany and elsewhere, other inventors planned to use television for entertainment purposes. These inventors began with square or landscape pictures. (For example, consider the television systems of these men: Ernst Alexanderson, Frank Conrad, Charles Francis Jenkins, William Peck[7] and Ulises Armand Sanabria.[8]) These inventors realized that television is about relationships between people. From the very beginning, these inventors allowed picture space for two-shots. Soon, images increased to 60 lines or more. The camera could easily photograph several people at once. Then even Baird switched his picture mask to a horizontal image. Baird's "zone television" is an early example of rethinking his extremely narrow screen format. For entertainment and most other purposes, even today, landscape remains the more practical shape.
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 in September 1927 in San Francisco by Philo Farnsworth, and then publicly by Farnsworth at the Franklin Institute in Philadelphia in 1934, was rapidly overtaking mechanical television. Farnsworth's system was first used for broadcasting in 1936, reaching 400 to more than 600 lines with fast field scan rates, along with competing systems by Philco and DuMont Laboratories. The last mechanical television broadcasts ended in 1939 at stations run by a handful of public universities in the United States. In 1939, RCA paid Farnsworth $1 million for his patents after ten years of litigation, and RCA began demonstrating all-electronic television at the 1939 World's Fair in New York City.
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.[9] John Baird's 1928 color television experiments had inspired Goldmark's more advanced field-sequential color system.[10] 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.
Of course, simultaneous color systems superseded the CBS-Goldmark system. Yet mechanical color methods continued to find uses. Early color sets were very expensive, over $1,000 in the money of the time. Inexpensive adapters allowed owners of black-and-white, NTSC television sets to receive color telecasts. The most prominent of these adapters is Col-R-Tel, a 1955 NTSC to field-sequential converter.[11] This system operates at NTSC scanning rates, but uses a disc like the obsolete CBS system had. The disc converts the black-and-white set to a field-sequential set. Meanwhile, Col-R-Tel electronics recover NTSC color signals and sequence them for disc reproduction. The electronics also synchronize the disc to the NTSC system. In Col-R-Tel, the electronics provide the saturation values (chroma). These electronics cause chroma values to superimpose over brightness (luminance)changes of the picture. The disc paints the hues (color) over the picture.
A few years after Col-R-Tel, Apollo moon missions also adopted field-sequential techniques. The lunar color cameras all had color wheels. These Westinghouse and later RCA cameras sent field-sequential color television pictures to earth. The earth receiving stations included mechanical equipment that converted these pictures to standard television formats.
Today, some Digital Light Processing (DLP) projectors still use color filter wheels.
In the days of commercial mechanical television transmissions, a system of recording images (but not sound) was developed, using a modified gramophone 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 otherwise 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.[12]
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 Pounsford", shows several minutes of a woman's face in what appears to be very animated conversation. In 1993, the woman was identified by relatives as Mabel Pounsford, and her brief appearance on the disc is one of the earliest known video recordings of a human being.[13]
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.
Today, a mechanical system of a sort has seen moderate popularity. 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.
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.
The closest modern systems to the original mechanical scan camera is the long wave infrared cameras used in military applications such as giving fighter pilots night vision. These cameras use a high sensitivity infrared photo receptor (usually cooled to increase sensitivity), but instead of disks of lenses, these systems use rotating prisms to provide a 525 or 625 line standard video output. The optical parts are made from germanium, because glass is opaque at the wavelengths involved. These cameras have found a new role in sporting events where they are able to show (for example) where a ball has struck a bat.
Laser lighting display techniques are combined with computer emulation in the LaserMAME 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.
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||