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Mechanical television

From Wikipedia, the free encyclopedia

Watching a homemade mechanical-scan television receiver in 1928.  The "televisor" (right) which produces the picture uses a spinning metal disk with a series of holes in it, called a Nipkow disk, in front of a neon lamp.  Each hole in the disk passing in front of the lamp produces a scan line which makes up the image. The video signal from the television receiver unit (left) is applied to the neon lamp, causing its brightness to vary with the brightness of the image at each point.  This system produced a dim orange image 1.5 inches square, with 48 scan lines, at a frame rate of 7.5 frames per second.
Watching a homemade mechanical-scan television receiver in 1928. The "televisor" (right) which produces the picture uses a spinning metal disk with a series of holes in it, called a Nipkow disk, in front of a neon lamp. Each hole in the disk passing in front of the lamp produces a scan line which makes up the image. The video signal from the television receiver unit (left) is applied to the neon lamp, causing its brightness to vary with the brightness of the image at each point. This system produced a dim orange image 1.5 inches square, with 48 scan lines, at a frame rate of 7.5 frames per second.

Mechanical television or mechanical scan television is a television system that relies on a mechanical scanning device, such as a rotating disk with holes in it or a rotating mirror, to scan the scene and generate the video signal, and a similar mechanical device at the receiver to display the picture. This contrasts with modern television technology, which uses electronic scanning methods, for example electron beams in cathode ray tube (CRT) televisions, and liquid-crystal displays (LCD), to create and display the picture.

Mechanical-scanning methods were used in the earliest experimental television systems in the 1920s and 1930s. One of the first experimental wireless television transmissions was by John Logie Baird on November 25, 1925, in London. By 1928 many radio stations were broadcasting experimental television programs using mechanical systems. However the technology never produced images of sufficient quality to become popular with the public. Mechanical-scan systems were largely superseded by electronic-scan technology in the mid-1930s, which was used in the first commercially successful television broadcasts which began in the late 1930s in Great Britain.

A mechanical television receiver is also called a televisor in some countries.

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In my last video, we explored how analog television works. You can check out the whole video either through this card or through the link in the description, but here’s a brief overview. At its core, analog television is just an amplitude modulated radio transmission where the strength of the signal dictates brightness of a light source, with a strong signal producing a bright light, and a weak signal producing little to no light. The television set uses an electromagnet to deflect the source of the light, an electron beam, in a pattern called a raster, which is really just a ton of horizontal lines. This has the effect of producing a glowing series of lines on the face of a picture tube. The electronics of the TV set line up the incoming signal with the movement of the beam to create an image, with each part of the imaging being drawn brightly or darkly along with the signal’s instantaneous strength. With everything in alignment, you get an image. By far the most complicated part of making an image appear on the screen is making that raster pattern. The electronic components and other crucial parts such as the picture tube and deflection yoke are primitive by today’s standards, but still pretty complicated. We really just need a way make a repeating pattern of lines from a light source, there’s got to be a simpler way to do it! Enter: Mechanical Television. The earliest televisions actually got some of their inspiration from fax machines, really?, and relied on a couple of important developments. So first a bit about the fax machines. Facsimile transmission actually predates the telephone? (what?), with images such as signatures being commercially reproduced over telegraph wires as far back as 1865, and the earliest fax-like device being invented by Scottish inventor Alexander Bain in 1846. Now, I’m simplifying a great deal here, but the theory was that if you could synchronize the movement of a scanning device with a drawing device, you could replicate an image. If you scanned a piece of paper line by line and sent a signal over a wire to match the darkness of the ink, you could reproduce the image by syncing up a drawing mechanism with the scanning one. These early fax machines worked, but they were very slow. Nevertheless, they showed us that you could, via electro-mechanical means, reproduce an image. Fast forward to 1884, and 23-year old Paul Julius Gottlieb Nipkow created the Nipkow disc. This is the core of most mechanical television systems. Nipkow realized that a spinning disc could methodically scan an image line-by-line simply by placing evenly-spaced holes in a spiral pattern. This is a home-made Nipkow disc. I took a really awful vinyl record that I’d be happy to destroy, and marked 32 divisions around the circumference, like 32 very skinny pie slices. Then I methodically drilled a hole along these lines, with each hole being drilled a 32nd of an inch (roughly .8 milimeters) closer to the center than the last. The result is a spiral pattern of holes. This might not seem like much, but it’s actually extremely clever. If you put a square-ish shaped mask in front of the holes, its height being slightly less than the distance between the holes, you’ve made a device which mechanically creates a raster scan using these physical holes. John Logie Baird realized that with this Nipkow disc, you could in theory focus an image with a lens onto the disc, and you could use a light sensor to give an instantaneous reading of how bright each part of the image was, with the holes in the nipkow disc serving as a way to divide the image into transmittable pieces. Back in 1873, Willoughby Smith discovered the photoconductivity of selenium, and with this knowledge Baird used some selenium to create the light sensor for his televisor. I’ve mounted this Nipkow disc to an AC motor which will spin it at 1,800 RPM, giving a complete revolution 30 times per second. Before I turn it on, look through the mask. I’ve put an extremely bright LED behind the disc so you can see the holes. As I slowly turn the disc, you’ll see that only one hole is visible at a time, and each hole gets closer to the left than the next one. When I switch the motor on, the holes blend into a moving line, and as it gets faster, the line seems to widen into a square. This square is very uneven because my homemade Nipkow disc was made hastily and with poor precision. But here’s the key. Only one of the holes is actually visible through the mask at any given moment. It’s just moving too fast to see. Baird used the selenium light sensor to create a signal from an image being scanned by the disc, and on the receiving end, another identical disc would spin at precisely the same speed, and a light source such as a neon lamp would vary its brightness along with the signal strength presented by the light sensor, and thus, you’d get an image. I shall now attempt to show you how this worked. Now before you get too excited, I’ll admit that my mechanical television doesn’t work as well as I had hoped. And that’s all on my insistence in using crap I had laying around, rather than going through the process to make a proper LED driver. However, I hope you’ll get an understanding of what’s going on. This 10W LED chip is what we’ll use as a light source. It’s really bright and, importantly, it can react very quickly to changes in the voltage it receives. First, I’ll simply power the LED continuously. As the disc spins up, the lines start to blend into each other, and eventually the whole “screen” is illuminated. Now, I’m going to switch the LED on and off at a higher and higher frequency. First, 5 hz. The screen appears to just be flashing, nothing too extraordinary, but you might be able to see some odd stuff happening as the light switches states. Now I’ll switch it on and off at 60 hz. Something odd starts to be visible here. See, the disc makes a complete revolution 30 times per second, and with the light flashing at twice that frequency, only some of the holes are lit up as the disc passes over the LED. Now let’s move to 1,800 hz. Frequencies that are a multiple of 30 will appear stable as an even number of pulses fit within each revolution. If you mess with that, though, things get weird. Bumping the frequency up just a tad makes the pattern move in relation to the disc. The holes in the disc are directly responsible for creating the patterns you see. Without the disc, the LED appears to just be continuously illuminated. But, it’s not. It’s flashing really quickly. The disc allows for that flashing to be visible because it physically obscures different parts of the light source over time. This is just like the electron beam in the CRT television, except instead of electromagnets moving a beam across the surface of a picture tube, the light source is physically moved via the location of these holes. It’s a pretty crafty way of producing a raster scan, and it actually works. This is the best imagery I could get my televisor to produce. This pattern was generated through manipulating audio samples in Audacity. To give you an idea of how poorly this mechanical TV works, well the image I intended to make was not a map of the world as this vaguely suggests, but that of a circle. Here’s a look at true video. What you’re seeing here is a very low contrast, very low resolution image of Seth Meyers. I mean obviously, how could you not recognize him? Yeah OK, it’s garbage, but you can see that there is certainly something there and it’s moving slightly. Like a talk show host’s head might when said talk show host is talking. On his show. To make this image, I simply placed my phone behind the televisor with the screen brightness all the way up, and I placed this solar panel with an audio cord patched into it into one of my trusty Tascam DR-05 audio recorders, which I use all the time. In fact there’s one in my pocket right now. And yes, that’s directly from a solar garden light. The solar panel would produce a high current whenever it saw bright light, and it would produce low current with less light. Duh. As the disc spun, it would only allow the tiniest bit of the image through to the solar panel at any given time. This would produce a quickly varying signal with amplitude corresponding to image brightness. The TASCAM would just encode these relative brightnesses as sound samples, at a sample rate of 48 kilohertz, and then because I’m really lazy, I just hooked my LED into an audio amplifier and played that sound back. The LED would become brighter with a stronger signal from the amplifier, though as it’s a diode it would filter out any AC components of the signal. Quite honestly I’m amazed it produces anything at all. I opened the file in Audacity just to see what it looked like, and it’s pretty intriguing. Here’s what it sounds like, for those interested. Now, in case it’s not obvious, let’s go over the reasons mechanical television didn’t catch on. First, up until now, I’ve not let you hear what this sounds like. Here’s what a 12 inch vinyl disc sounds like at 1,800 RPM. I’m sure that would never get old. But aside from that, there are just so many practical concerns with mechanical TV. First of all, the image is tiny. And it’s a horribly low resolution--only 32 lines. That’s the only reason a signal can be recorded as an audio file. Not a lot of bandwidth is needed. Because the disc obscures almost all of the light source, hardly any light gets through. This LED is fricken bright, it’s painful to look at directly, and yet through the Nipkow disc, nearly all of the light is blocked, and it produces a dull image. When these devices were first in development, the light source would often be a neon lamp, like the orange light in a powerstrip’s switch. Imagine how dark the image would be with only that for a light source. One of the biggest troubles with mechanical television is image synchronization. Because we’re using a big spinning thing to divide the light into chunks, the disc has to be in precisely the right place if you want the image to land where it should. If we take the mask away, you can see that the image just repeats itself over and over. But each adjacent image is actually shifted one line up or down. The most critical part of synchronization was ensuring the disc is spinning at the exact same speed as the scanning disc of a camera, but it would also be necessary to slow down or speed up the disc in slight increments to get the image aligned with the viewing mask, and with the top and bottom in the right place. But the most damning problem is that of geometry. Imagine we wanted to make a display with the resolution and size of this small CRT television. Well, the face of the tube is about 15 cm wide. With 480 lines of resolution, there would need to be 480 holes in the nipkow disc. Remember, only one hole can be seen through the mask at once for this to work, so the holes have to be at a minimum 15 centimeters apart. So the disc’s circumference would have to be 72 meters, with a diameter of roughly 23 meters, or about 75 feet. I live in a building that’s 6 stories tall. A mechanical television to rival this TV would be taller than my building! And, it would have to spin at 1,800 RPM just like this one to make 30 frames per second possible. This things scares me spinning this fast. I’m pretty sure a 75 foot disc would just explode. In fact, let’s do the math. A 72 meter circumference means that the edge of the disc would travel 2.160 kilometers per second, or well above mach 6. Yeah. If the disc were rolling, it would make it from New York to Los Angeles in about 35 minutes--not in a straight line, mind you, but by traveling along actual roads. So, the Baird television system didn’t get too far. It was certainly genious and is an important part of the history of television. But is was far too limited, clunky, and, to be honest, it had crappy image quality. I’ve added some links in the description to videos of mechanical televisions that actually work, and I think you’ll agree that’s they’re pretty cool, but it’s a damn good thing they didn’t become mainstream. As always, thank you so much for watching. If you liked this video, a thumbs up would be most appreciated. I’m absolutely thrilled that this channel has over 21,000 subscribers now! I never thought that would be possibl. If you’re not one of the people in that number, and you liked this video, I humbly ask that you become one of them by pressing that subscribe button. I’m doing my best to keep videos like this headed your way, and I’ll see you next time!



Early research

The first mechanical raster scanning techniques were developed in the 19th century for facsimile, the transmission of still images by wire. Alexander Bain introduced the facsimile machine in 1843 to 1846. Frederick Bakewell demonstrated a working laboratory version in 1851. The first practical facsimile system, working on telegraph lines, was developed and put into service by Giovanni Caselli from 1856 onward.[1][2][3]

Willoughby Smith discovered the photoconductivity of the element selenium in 1873, laying the groundwork for the selenium cell phototube which was used as a pickup in most mechanical scan systems.

Ernst Ruhmer demonstrating his experimental television system, which was capable of transmitting images of simple shapes over telephone lines, using a 25-element selenium cell receiver (1909)[4]
Ernst Ruhmer demonstrating his experimental television system, which was capable of transmitting images of simple shapes over telephone lines, using a 25-element selenium cell receiver (1909)[4]

The first demonstration of the instantaneous transmission of images was made by a German physicist, Ernst Ruhmer, who arranged 25 selenium cells as the picture elements for a television receiver. In late 1909 he successfully demonstrated in Belgium the transmission of simple images over a telephone wire from the Palace of Justice at Brussels to the city of Liege, a distance of 115 kilometers (72 miles). This demonstration was described at the time as "the world's first working model of television apparatus".[5] The limited number of elements meant his device was only capable of representing simple geometric shapes, and the cost was very high; at a price of £15 (US$45) per selenium cell, he estimated that a 4,000 cell system would cost £60,000 (US$180,000), and a 10,000 cell mechanism capable of reproducing "a scene or event requiring the background of a landscape" would cost £150,000 (US$450,000). Ruhmer expressed the hope that the 1910 Brussels Exposition Universelle et Internationale would sponsor the construction of an advanced device with significantly more cells, as a showcase for the exposition. However, the estimated expense of £250,000 (US$750,000) proved to be too high.[6]

The publicity generated by Ruhmer's demonstration spurred two French scientists, Georges Rignoux and A. Fournier in Paris, to announce similar research that they had been conducting.[7] A matrix of 64 selenium cells, individually wired to a mechanical commutator, served as an electronic retina. In the receiver, a type of Kerr cell modulated the light and a series of variously angled mirrors attached to the edge of a rotating disc scanned the modulated beam onto the display screen. A separate circuit regulated synchronization. The 8x8 pixel resolution in this proof-of-concept demonstration was just sufficient to clearly transmit individual letters of the alphabet.[8] An updated image was transmitted "several times" each second.[9]

In 1911, Boris Rosing and his student Vladimir Zworykin created a system that used a mechanical mirror-drum scanner to transmit, in Zworykin's words, "very crude images" over wires to the "Braun tube" (cathode ray tube or "CRT") in the receiver. Moving images were not possible because, in the scanner, "the sensitivity was not enough and the selenium cell was very laggy".[10]

Television demonstrations

The Nipkow disk. This schematic shows the circular paths traced by the holes, that may also be square for greater precision. The area of the disk outlined in black shows the region scanned.
The Nipkow disk. This schematic shows the circular paths traced by the holes, that may also be square for greater precision. The area of the disk outlined in black shows the region scanned.

As a 23-year-old German university student, Paul Julius Gottlieb Nipkow proposed and patented the Nipkow disk in 1884.[11] This was a spinning disk with a spiral pattern of holes in it, so each hole scanned a line of the image. Although he never built a working model of the system, Nipkow's spinning-disk "image rasterizer" was the key mechanism used in most mechanical scan systems, in both the transmitter and receiver.[12]

Constantin Perskyi had coined the word television in a paper read to the International Electricity Congress at the International World Fair in Paris on August 24, 1900. Perskyi's paper reviewed the existing electromechanical technologies, mentioning the work of Nipkow and others.[13] However, it was the 1907 invention of the first amplifying vacuum tube, the triode, by Lee de Forest, that made the design practical.[14]

Baird in 1925 with his transmitter equipment and dummies "James" and "Stooky Bill" (right).
Baird and his television receiver

Scottish inventor John Logie Baird in 1925 built some of the first prototype video systems, which employed the Nipkow disk. On March 25, 1925, Baird gave the first public demonstration of televised silhouette images in motion, at Selfridge's Department Store in London.[15] Since human faces had inadequate contrast to show up on his primitive system, he televised a ventriloquist's dummy named "Stooky Bill" talking and moving, whose painted face had higher contrast. By January 26, 1926, he demonstrated the transmission of image of a face in motion by radio. This is widely regarded as first television demonstration. Baird's system used the Nipkow disk for both scanning the image and displaying it. A bright light shining through a spinning Nipkow disk set with lenses projected a bright spot of light which swept across the subject. A selenium photoelectric tube detected the light reflected from the subject and converted it into a proportional electrical signal. This was transmitted by AM radio waves to a receiver unit, where the video signal was applied to a neon light behind a second Nipkow disk rotating synchronized with the first. The brightness of the neon lamp was varied in proportion to the brightness of each spot on the image. As each hole in the disk passed by, one scan line of the image was reproduced. Baird's disk had 30 holes, producing an image with only 30 scan lines, just enough to recognize a human face. In 1927, Baird transmitted a signal over 438 miles (705 km) of telephone line between London and Glasgow. In 1928, Baird's company (Baird Television Development Company/Cinema Television) broadcast the first transatlantic television signal, between London and New York, and the first shore-to-ship transmission. In 1929, he became involved in the first experimental mechanical television service in Germany. In November of the same year, Baird and Bernard Natan of Pathé established France's first television company, Télévision-Baird-Natan. In 1931, he made the first outdoor remote broadcast, of The Derby.[16] In 1932, he demonstrated ultra-short wave television. Baird's mechanical system reached a peak of 240-lines of resolution on BBC television broadcasts in 1936 though the mechanical system did not scan the televised scene directly. Instead a 17.5mm film was shot, rapidly developed and then scanned while the film was still wet.

An American inventor, Charles Francis Jenkins also pioneered the television. He published an article on "Motion Pictures by Wireless" in 1913, but it was not until December 1923 that he transmitted moving silhouette images for witnesses, and it was on June 13, 1925, that he publicly demonstrated synchronized transmission of silhouette pictures. In 1925 Jenkins used Nipkow disk and transmitted the silhouette image of a toy windmill in motion, over a distance of five miles from a naval radio station in Maryland to his laboratory in Washington, D.C., using a lensed disk scanner with a 48-line resolution.[17][18] He was granted the U.S. patent No. 1,544,156 (Transmitting Pictures over Wireless) on June 30, 1925 (filed March 13, 1922).

On December 25, 1925, Kenjiro Takayanagi demonstrated a television system with a 40-line resolution that employed a Nipkow disk scanner and CRT display at Hamamatsu Industrial High School in Japan. This prototype is still on display at the Takayanagi Memorial Museum in Shizuoka University, Hamamatsu Campus.[19] By 1927, he improved the resolution to 100 lines, which was unrivaled until 1931.[20] By 1928, he was the first to transmit human faces in half-tones. His work had an influence on the later work of Vladimir K. Zworykin.[21] By 1935, Takayanagi had invented the first all-electronic television.[22] His research in creating a production model was halted by the US after Japan lost World War II.[19]

Herbert E. Ives and Frank Gray of Bell Telephone Laboratories gave a dramatic demonstration of mechanical television on April 7, 1927. The reflected-light television system included both small and large viewing screens. The small receiver had a two-inch-wide by 2.5-inch-high screen. The large receiver had a screen 24 inches wide by 30 inches high. Both sets were capable of reproducing reasonably accurate, monochromatic moving images. Along with the pictures, the sets also received synchronized sound. The system transmitted images over two paths: first, a copper wire link from Washington to New York City, then a radio link from Whippany, New Jersey. Comparing the two transmission methods, viewers noted no difference in quality. Subjects of the telecast included Secretary of Commerce Herbert Hoover. A flying-spot scanner beam illuminated these subjects. The scanner that produced the beam had a 50-aperture disk. The disc revolved at a rate of 18 frames per second, capturing one frame about every 56 milliseconds. (Today's systems typically transmit 30 or 60 frames per second, or one frame every 33.3 or 16.7 milliseconds respectively.) Television historian Albert Abramson underscored the significance of the Bell Labs demonstration: "It was in fact the best demonstration of a mechanical television system ever made to this time. It would be several years before any other system could even begin to compare with it in picture quality."[23]

In 1928, General Electric launched their own experimental television station W2XB, broadcasting from the GE plant in Schenectady, New York. The station was popularly known as "WGY Television", named after the GE owned radio station WGY. The station eventually converted to an all-electronic system in the 1930s and in 1942, received a commercial license as WRGB. The station is still operating today.

Meanwhile, in the Soviet Union, Léon Theremin had been developing a mirror drum-based television, starting with 16 lines resolution in 1925, then 32 lines and eventually 64 using interlacing in 1926, and as part of his thesis on May 7, 1926, he electrically transmitted and then projected near-simultaneous moving images on a five-foot square screen.[18] By 1927 he achieved an image of 100 lines, a resolution that was not surpassed until 1931 by RCA, with 120 lines.[citation needed]

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.[24] 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, Quebec, Canada.[25][26]

Color television

A color televisor. A test card (the famous test card F) can just be seen through the lens on the right.
A color televisor. A test card (the famous test card F) can just be seen through the lens on the right.

John Baird's 1928 color television experiments had inspired Goldmark's more advanced field-sequential color system.[27] The CBS color television system of Peter Goldmark used such technology in 1940.[28] 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.[29] 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.


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. 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. The last mechanical television broadcasts ended in 1939 at stations run by a handful of public universities in the United States.

Modern applications of mechanical scanning

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 used electronic systems utilising the P7 CRT until the 1980s and PCs thereafter. There are three known mechanical monitor forms.[citation needed] Two 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 a 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 feature that was common on 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 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.

Television Machine with 4 LED – Strips
Television Machine with 4 LED – Strips

Technical aspects

Flying spot scanners

Flying spot scanner in a television studio in 1931. This type was used for "head shots" of performers speaking, singing or playing instruments. A bright spot of light projected from the lens at center scanned the subject's face, and the light reflected at each point was picked up by the 8 phototubes in the dish-shaped mirrors.
Flying spot scanner in a television studio in 1931. This type was used for "head shots" of performers speaking, singing or playing instruments. A bright spot of light projected from the lens at center scanned the subject's face, and the light reflected at each point was picked up by the 8 phototubes in the dish-shaped mirrors.

The most common method for creating the video signal was the "flying spot scanner", developed as a remedy for the low sensitivity that photoelectric cells had at the time. Instead of a television camera that took pictures, a flying spot scanner projected a bright spot of light that scanned rapidly across the subject scene in a raster pattern, in a darkened studio. The light reflected from the subject was picked up by banks of photoelectric cells and amplified to become the video signal.

In the scanner the narrow light beam was produced by an arc lamp shining through the holes in a spinning Nipkow disk. Each sweep of the spot across the scene produced a "scan line" of the picture. A single "frame" of the picture was typically made up of 24, 48, or 60 scan lines. The scene was typically scanned 15 or 20 times per second, producing 15 or 20 video frames per second. The varying brightness of the point where the spot fell reflected varying amounts of light, which was converted to a proportionally varying electronic signal by the photoelectric cells. To achieve adequate sensitivity, instead of a single cell, a number of photoelectric cells were used. 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:

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

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 scene being televised by flying spot scanner in a television studio in 1931. The Nipkow disk in the flying spot scanner (bottom) projects a spot of light that scans the subject in a raster pattern in the darkened studio. Nearby photocell pickup units convert the reflected light to a signal proportional to the brightness of the reflected area, which goes through the control board to the transmitter.
A scene being televised by flying spot scanner in a television studio in 1931. The Nipkow disk in the flying spot scanner (bottom) projects a spot of light that scans the subject in a raster pattern in the darkened studio. Nearby photocell pickup units convert the reflected light to a signal proportional to the brightness of the reflected area, which goes through the control board to the transmitter.

Larger videos

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.[30]

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).

Aspect ratios

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 and close in proportion to a typical doorway.

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[31] and Ulises Armand Sanabria.[32]) 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.


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.[33]

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 television video recordings of a human.[34]


  • Beyer, Rick, The Greatest Stories Never Told : 100 tales from history to astonish, bewilder, & stupefy, A&E Television Networks, 2003, ISBN 0-06-001401-6
  • Cavendish, Marshall (Corp), Inventors and Inventions, Marshall Cavendish, 2007, ISBN 0-7614-7763-2
  • Huurdeman, Anton A., The worldwide history of telecommunications, Wiley-IEEE, 2003, ISBN 0-471-20505-2
  • Sarkar, Tapan K. et al., History of wireless, John Wiley and Sons, 2006, ISBN 0-471-71814-9

See also


  1. ^ Huurdeman, p. 149 The first telefax machine to be used in practical operation was invented by an Italian priest and professor of physics, Giovanni Caselli (1815–1891).
  2. ^ Beyer, p. 100 The telegraph was the hot new technology of the moment, and Caselli wondered if it was possible to send pictures over telegraph wires. He went to work in 1855, and over the course of six years perfected what he called the "pantelegraph." It was the world's first practical fax machine.
  3. ^ "Giovanni Caselli and the Pantelegraph". Archived from the original on 2016-01-15.
  4. ^ "Another Electric Distance-Seer", Literary Digest, September 11, 1909, page 384.
  5. ^ "Seeing by Wire", Industrial World, January 31, 1910, pp. viii-x (reprinted from the London Mail).
  6. ^ Ibid.
  7. ^ "Television on the Way", Kansas City Star, January 30, 1910, p. 20C. (Reprinted in American Broadcasting, edited by Lawrence W. Lichty and Malachi C. Topping, 1976, pp. 45-46.)
  8. ^ "Television 'In Sight'", The Literary Digest, January 2, 1910, pp. 138-139.
  9. ^ Henry de Varigny, "La vision à distance", L'Illustration, Paris, December 11, 1909, p. 451.
  10. ^ R. W. Burns, Television: An International History of the Formative Years, IET, 1998, p. 119. ISBN 0-85296-914-7.
  11. ^ Shiers, George and May (1997), Early Television: A Bibliographic Guide to 1940. Taylor & Francis, pp. 13, 22. ISBN 978-0-8240-7782-2.
  12. ^ Shiers & Shiers, p. 13, 22.
  13. ^ "Télévision au moyen de l'électricité", Congrès Inographs by Telegraph", The New York Times, Sunday Magazine, September 20, 1907, p. 7.
  14. ^ "Sending Photographs by Telegraph", The New York Times, Sunday Magazine, September 20, 1907, p. 7.
  15. ^ "Current Topics and Events", Nature, vol. 115, April 4, 1925, p. 505–506, doi:10.1038/115504a0.
  16. ^ J. L. Baird, "Television in 1932", BBC Annual Report, 1933.
  17. ^ "Radio Shows Far Away Objects in Motion", The New York Times, June 14, 1925, p. 1.
  18. ^ a b Glinsky, Albert (2000). Theremin: Ether Music and Espionage. Urbana, Illinois: University of Illinois Press. pp. 41–45. ISBN 0-252-02582-2.
  19. ^ a b Kenjiro Takayanagi: The Father of Japanese Television, NHK (Japan Broadcasting Corporation), 2002, retrieved 2009-05-23.
  20. ^ High Above: The untold story of Astra, Europe's leading satellite company, page 220, Springer Science+Business Media
  21. ^ Albert Abramson, Zworykin, Pioneer of Television, University of Illinois Press, 1995, p. 231. ISBN 0-252-02104-5.
  22. ^ Popular Photography, November 1990, page 5
  23. ^ Abramson, Albert, The History of Television, 1880 to 1941, McFarland & Co., Inc., 1987, p. 101. ISBN 978-0-89950-284-7.
  24. ^ Donald F. McLean, Restoring Baird's Image (London: IEEE, 2000), p. 184.
  25. ^ "VE9AK entry at". Retrieved 2010-03-02.
  26. ^ "Peck Television Corporation  Console Receiver and Camera". Early Television Museum. Retrieved 18 February 2012.
  27. ^ The Smith, Kline & French Medical Color TV Unit.
  28. ^ CBS Field Sequential Color System Archived 2010-01-05 at the Wayback Machine.
  29. ^ Hawes Mechanical Television Archive, How Col-R-Tel Works.
  30. ^ "Ulises Armand Sanabria".
  31. ^ Media quotations.
  32. ^ "Ulises Armand Sanabria at Early Television website". Retrieved 2010-03-02.
  33. ^ The World's Earliest Television Recordings.
  34. ^ Phonovision: The Recovered Images.

External links

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