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Television Channel History
Before getting into the Television Channel History here is the basic concept behind this. Technically speaking, the television set is a telecommunication system of transmitting and receiving sound and images using electronic signals. These signals are transmitted through coaxial cables, optical fiber cables or electromagnetic radiation. Generally speaking, the television set is a source of entertainment and information.
Mechanical Television History
Paul Gottlieb Nipkow, a German inventor developed a rotating-disc technology to transmit pictures over wire in 1884 called the Nipkow disk. This was the very first electromechanical television scanning system. John Logie Baird is remembered as being an inventor of mechanical television. Mechanical television was developed in late 1920s in England. During World War II, Baird developed the first color picture tube - Shadow Mask Tube. Charles Francis Jenkins was an American who invented a mechanical television system called radio vision and claimed to have transmitted the earliest moving silhouette images on June 14, 1923.
Electronic Television History
Electronic television is based on the development of the cathode ray tube, which is the picture tube found in modern television sets. Karl Braun, a German scientist invented the cathode ray tube oscilloscope (CRT) in 1897. Vladimir Kosma Zworykin invented the cathode-ray tube called the kinescope in 1929, a tube needed for television transmission. Zworykin also invented the iconoscope, an early television camera. Philo T. Farnsworth was a farm boy who conceived the basic operating principles of electronic television at the age of just 13 years.
Transmission Standards
As Different countries use different types of broadcast systems they will have different Transmission Standards. Television receivers require a source of "field timing reference signals". These are signals that tell the television receiver to be ready to receive the next picture in the stream of images. Early set designers decided to use the Mains power supply frequency as the source. The first reason was that if the mains supply and power source were not at exactly the same frequency, rolling hum bars would appear on the picture. The second reason was that the television studios would have had enormous problems with flicker on their cameras when making programs.
There are two Mains power frequencies widely used around the world, 50Hz and 60Hz, dividing the worlds TV systems into two sects, the 25 frames per second sect (50Hz) and the 30 frames per second sect (60Hz). The 60Hz camp made a small adjustment and changed the field rate to 59.94Hz when they added color to the signals. The biggest compatibility problems between TV standards are related to the field rate; these are also the hardest problems to solve. Color broadcasting further created sub-divisions within the camps. The majority of 60Hz based countries use a technique known as NTSC. A committee called the National Television Standards Committee originally developed NTSC in the United States. NTSC works perfectly in a video or closed circuit environment but not in a broadcast environment. Some still call the NTSC jokingly as 'Never The Same Color' owing to NTSC's inherent color instability arising out of propagation delays.
PAL (Phase Alternate Lines) is a modified version of NTSC. The hue change problem in NTSC was caused by shifts in the color sub-carrier phase of the signal. In PAL, the sub-carrier phase was reversed on each second line. Modern TV receivers employ Delay lines and SAW filters in addition to COMB filter chips to correct and enhance color reproduction. PAL has been adopted by a few 60Hz countries, most notably Brazil. PAL has been the most widely adopted system in the 50Hz countries.
SECAM (SEquential Couleur Avec Memoire) was a system designed by the French, primarily for political reasons to protect their domestic manufacturing companies. SECAM was widely adopted in former Eastern Block countries to encourage incompatibility with Western transmissions, again a political motive.
In Europe, a few Direct Satellite Broadcasting services use a system called D-MAC. It is not wide-spread at present and it is transcoded to PAL or SECAM to permit video recording of it's signals. There are other MAC-based standards in use around the world including B-MAC in Australia and B-MAC60 on some private networks in the USA.
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