Monday, November 27, 2006

Electromechanical 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, and the invention of a scanning disk by Paul Nipkow in 1884. The two inventions 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 were being transmitted by telegraph and telephone lines as a newspaper service.

The German student Paul Nipkow proposed and patented the first electromechanical television system in 1884. 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.

However, it wasn't until 1907 that developments in amplification tube technology made the design practical. The first demonstration of the instantaneous transmission of still 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.[1]

In 1911, Boris Rosing and his student Vladimir Kosma 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) 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. But if television is defined as the transmission of live, moving, half-tone (grayscale) images, and not silhouette or still images, Baird achieved this privately on October 2, 1925.[2] 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 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 he developed a video disk recording system dubbed "Phonovision"; a number of the Phonovision[1] recordings, dating back to 1927, still exist. In 1929 he became involved in the first experimental electromechanical television service in Germany. 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.

In the U.S., 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, 16 pictures per second. AT&T's Bell Telephone Laboratories transmitted half-tone images of transparencies in May 1925. But Bell Labs gave the most dramatic demonstration of television yet on April 7, 1927, when it 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, which included Secretary of Commerce Herbert Hoover, were illuminated by a flying spot beam and scanned by a 50-aperture disk at 16 pictures per second.

Saturday, November 11, 2006

History TV

The origins of what would become today's television system can be traced back as far as the discovery of the photoconductivity of the element selenium by Willoughby Smith in 1873 and the invention of the scanning disk by Paul Nipkow in 1884. All practical television systems use the fundamental idea of scanning an image to produce a time series signal representation. That representation is then transmitted to a device to reverse the scanning process. The final device, the television (or T.V. set), relies on the human eye to integrate the result into a coherent image.
Electromechanical techniques were developed in the 1920s and 1930s, most notably by John Logie Baird. Baird gave the world's first public demonstration of a working television system that transmitted moving images with tone graduation (grayscale) on 26 January 1926 at his laboratory in London, and built a complete experimental broadcast system around his technology. Baird further demonstrated the world's first color television transmission on 3 July 1928. Other prominent developers of mechanical television included Charles Francis Jenkins, who demonstrated a primitive television system in 1923, Frank Conrad who demonstrated a movie-film-to-television converter at Westinghouse in 1928, and Frank Gray and Herbert E. Ives at Bell Labs who demonstrated wired long-distance television in 1927 and two-way television in 1930.
Completely electronic television systems relied on the inventions of Philo Taylor Farnsworth, Vladimir Zworykin and others to produce a system suitable for mass distribution of television programming. Farnsworth gave the world's first public demonstration of an all-electronic television system at the Franklin Institute in Philadelphia on 25 August 1934.
Regular broadcast programming occurred in the United States,[1] the United Kingdom,[2] Germany,[3] France,[4] and the Soviet Union[5] before World War II. The first regular television broadcasts with a modern level of definition (240 or more lines) were made in England in 1936, soon upgraded to the so-called "System A" with 405 lines. Large scale network broadcasting began in the United States in 1946, and television became common in American homes by the middle 1950s. While North American over-the-air broadcasting was originally free of direct marginal cost to the consumer (i.e., cost in excess of acquisition and upkeep of the hardware) and broadcasters were compensated primarily by receipt of advertising revenue, increasingly United States television consumers obtain their programming by subscription to cable television systems or direct-to-home satellite transmissions. In the United Kingdom, on the other hand, the owner of each television must pay a licence fee annually which is used to support the British Broadcasting Corporation.

Channel models

A channel can be physically modelled by trying to calculate the physical processes which modify the transmitted signal. For example in wireless communications the channel can be modelled by calculating the reflection off every object in the environment. A sequence of random numbers might also be added in to simulate external interference and/or electronic noise in the receiver.
Statistically a communication channel is usually modelled as a triple consisting of an input alphabet, an output alphabet, and for each pair (i, o) of input and output elements a transition probability p(i, o). Semantically, the transition probability is the probability that the symbol o is received given that i was transmitted over the channel.
Statistical and physical modelling can be combined. For example in wireless communications the channel is often modelled by a random attenuation (known as fading) of the transmitted signal, followed by additive noise. The attenuation term is a simplification of the underlying physical processes and captures the change in signal power over the course of the transmission. The noise in the model captures external interference and/or electronic noise in the receiver. If the attenuation term is complex it also describes the relative time a signal takes to get through the channel (technically called a phase shift). The statistics of the random attenuation are decided by previous measurements or physical simulations.
Channel models may be continuous channel models in that there is no limit to how precisely their values may be defined.
Communication channels are also studied in a discrete-alphabet setting. This corresponds to abstracting a real world communication system in which the analog->digital and digital->analog blocks are out of the control of the designer. The mathematical model consists of a transition probability that specifies an output distribution for each possible sequence of channel inputs. In information theory, it is common to start with memoryless channels in which the output probability distribution only depends on the current channel input.