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Thu February 13, 2014
Making Pictures Fly Through The Air
Television as we now know it, an electronically scanned and reproduced image, was a development of the 1930s. I’ll spend more time discussing electronic TV at a later date. For now though, let’s take a look at what came first, something called electro-mechanical TV. Although it is not my intent to write this series as a chronological history of radio, we have started out by exploring the beginnings of the art, and many readers may find it surprising to learn that the concept behind television came about long before radio itself.
Alexander Graham Bell invented the telephone in 1876. Despite the limited technology of the day, there were those who believed that if it was possible to send sound over wires, why not pictures?
The photoconductive properties of the element selenium had been discovered in 1873 by Willoughby Smith (*1), an English electrical engineer. Mr. Smith had already achieved wealth and success derived from his development of high quality wire for the telegraph industry. Selenium's electrical conductivity varies with the amount of light that it receives, making it ideal for converting varying amounts of light into an electrical signal. The more light that it receives, the more electricity it conducts. Mr. Smith thought that he could use this characteristic to send images over wires. He attempted to focus an image on a number of selenium cells, each cell connected to a corresponding light bulb. The bulbs would in theory reproduce the image that fell on the selenium cells. This is comparable in concept to each cell and bulb representing a single pixel of a modern digital screen, only much bigger. In theory this should work, except that the cells and the bulbs of the day were far too large to produce any resolution. It would have also required at least one wire for each cell and bulb, requiring an impractical number of wires to send such an image over any appreciable distance. Mr. Smith eventually discontinued his attempts to develop the system.
In 1884 Paul Nipkow(*2), a German college student, designed (on paper) and received a patent for an electromechanical television system. It is not known if he actually built a working model, but he is credited as being the first person to utilize a scanning principle that broke the image down into a series of individual lines. His patent application drawing depicted a rotating disc, perforated with holes in a spiral pattern, each hole focusing a single line of a scanned image on a selenium cell. At the receiving end, the intensity of a light source placed behind a similar rotating disc would vary due to the changing electrical current from the selenium cell in the sending unit. The resulting “sweep lines” would be focused on a screen, thus reproducing the image. This system only required one selenium cell, one bulb, and two or three wires to connect them together, the same number of wires already used by the phone companies of the day to deliver audio over wires.
Nipkow’s original design only produced 18 sweep lines, not much when compared to the 525 lines in a modern analog TV. At 18 lines, resolution would not have been very good. Long before radio, Mr. Nipkow’s patent described sending his images over wires. Although not known for certain, it is likely that Mr. Nipkow envisioned the primary application for his invention to be the ability to hold video conversations over the phone, as we now can do with smart phones and tablets using programs such as Skype. Technological limits of the day made it difficult to synchronize the sending and receiving wheels, and other problems made actual implementation of the invention impractical. The whole idea was all but forgotten for thirty nine years.
Note: a simple explanation of how mechanical TV works can be found at: http://www.pbs.org/opb/crashcourse/tv_grows_up/mechanicaltv.html
In a repeat of history, not long after sending audio over radio waves became popular, there were those who envisioned sending pictures over the airwaves. The nearly forgotten “Nipkow Wheel” was brought out of the closet. Reports of crude video images being transmitted by amateur radio operators using Nipkow devices were noted as early as 1923.
John Logie Baird, (*3) a Scottish scientist is generally credited as sending the first reliable moving pictures using Nipkow technology over radio in 1925.
By 1926 there were a variety of commercial scanning disk TVs available, but sales were low due to the poor image and lack of programming. Another problem was lack of standardization. Each design used a different number of scan lines, a different number of frames scanned per second, and different methods of synchronizing the sending and receiving discs. Most were purchased by experimenters and amateur operators and few examples remain today. The electro mechanical receiving units were connected to conventional radio receivers via the speaker or headphone jack. The screens were small, many were no larger than a square inch. Some commercial units like the General Electric "Octagon" pictured here were equipped with a magnifying lens to make the screen appear to be about three inches wide.
Experimentation and even some broadcast shows (a bit more popular in Great Britain) using Nipkow technology continued until the early 1930s. Most sets offered pictures with 60 lines or less, but near the end a few produced fairly viewable pictures with nearly 100 sweep lines. Mechanical television met its demise with the invention of the cathode ray camera and the picture tube in the 1930s, with the first public demonstration of that technology occurring at the 1936 World’s Fair. These devices evolved into analog television that held sway until it was replaced by digital TV in the last decade. Nipkow technology was not completely dead and forgotten though. It received one last hurrah in the 1960s and 70s when its spinning disk (and later a similar spinning mirror) was used to sweep a beam of light over a moving document in what was to become the popular FAX machine. That too, has been replaced by digital technology.
*1 historyinfomation.com, http://en.wikipedia.org/wiki/Willoughby_Smith, Selenium: Its Electrical Qualities and the Effect of Light Thereon published by Hayman Bros. and Lilly, Printers, 1877
*2 Paul Nipkow and his invention: www.earlytelevision.org en.wikipedia.org/wiki/Paul_Gottlieb_Nipkow, http://inventors.about.com/od/germaninventors/a/Nipkow.htm, http://www.bairdtelevision.com/nipkow.html
*3 Baird’s contribution to mechanical TV: http://www.bairdtelevision.com, en.wikipedia.org/wiki/John_Logie_Baird, http://www.bbc.co.uk/history/historic_figures/baird_logie.shtml