Electromechanical Television

Main article: Mechanical television

The origins of what would become today's television system can be traced back to the discovery of the photoconductivity of the element selenium by Willoughby Smith in 1873, the invention of a scanning disk by Paul Gottlieb Nipkow in 1884, John Logie Baird's demonstration of televised moving images in 1926 and Philo Farnsworth's Image dissector in 1927.

The 20 year old German university student Nipkow proposed and patented the first electromechanical television system in 1884,[1] although he never built a working model of the system. Nipkow's spinning disk design is credited with being the first television image rasterizer. 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 25, 1900. Perskyi's paper reviewed the existing electromechanical technologies, mentioning the work of Nipkow and others. The photoconductivity of selenium and Nipkow's scanning disk were first joined for practical use in the electronic transmission of still pictures and photographs, and by the first decade of the 20th century halftone photographs, composed of equally spaced dots of varying size, were being transmitted by facsimile over telegraph and telephone lines as a newspaper service.

However, it wasn't until 1907 that developments in amplification tube technology, by Lee DeForest and Arthur Korn among others, made the design practical.[2] The first demonstration of the instantaneous transmission of still silhouette images was by Georges Rignoux and A. Fournier in Paris in 1909, using a rotating mirror-drum as the scanner, and a matrix of 64 selenium cells as the receiver.[3]

In 1911, Boris Rosing and his student Vladimir Kozmich Zworykin created a television system that used a mechanical mirror-drum scanner to transmit, in Zworykin's words, "very crude images" over wires to the electronic 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".

On March 25, 1925, Scottish inventor John Logie Baird gave a demonstration of televised silhouette images in motion at Selfridge's Department Store in London. AT&T's Bell Telephone Laboratories transmitted halftone still images of transparencies in May 1925. Charles Francis Jenkins was able to demonstrate on June 13, 1925, the transmission of the silhouette image of a toy windmill in motion from a naval radio station to his laboratory in Washington, using a lensed disk scanner with 48 lines per picture,[4] 16 pictures per second.

However, if television is defined as the live transmission of moving images with continuous tonal variation, Baird first achieved this privately on October 2, 1925. But strictly speaking, Baird had not yet achieved moving images on October 2. His scanner worked at only five images per second, below the threshold required to give the illusion of motion, usually defined as at least 12 images per second. By January, he had improved the scan rate to 12.5 images per second. Then he gave the world's first public demonstration of a working television system to members of the Royal Institution and a newspaper reporter 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.

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. He also demonstrated an electromechanical color, infrared (dubbed "Noctovision"), and stereoscopic television, using additional lenses, disks and filters. In parallel, Baird developed a video disk recording system dubbed "Phonovision"; a number of the Phonovision recordings, dating back to 1927, still exist.[5] In 1929, he became involved in the first experimental electromechanical television service in Germany. In November 1929, Baird and Bernard Natan of Pathe established France's first television company, Télévision-Baird-Natan. In 1931, he made the first live transmission, of the Epsom Derby. In 1932, he demonstrated ultra-short wave television. Baird's electromechanical system reached a peak of 240 lines of resolution on BBC television broadcasts in 1936, before being discontinued in favor of a 405-line all-electronic system developed by Marconi-EMI.

In parallel, Herbert E. Ives of Bell Labs gave another dramatic demonstration of low-frame-rate television on April 7, 1927, when he field tested reflected-light television systems using small-scale (2 by 2.5 inches) and large-scale (24 by 30 inches) viewing screens over a wire link from Washington to New York City, and over-the-air broadcast from Whippany, New Jersey. The subjects, who included Secretary of Commerce Herbert Hoover, were illuminated by a flying-spot scanner beam that was scanned by a 50-aperture disk at a rate of 16 pictures per minute (about one picture every 4 seconds).

Meanwhile in Soviet Russia, 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.[4] By 1927 he achieved an image of 100 lines, a resolution that was not surpassed until 1931 by RCA, with 120 lines.

On December 25, 1926, 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 protype is still on display at the Takayanagi Memorial Museum in Shizuoka University, Hamamatsu Campus. His research in creating a production model were halted by the US after Japan lost World War II.

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Padhang Bulan