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The 405-line monochrome analogue television broadcasting system was the first fully electronic television system to be used in regular broadcasting. It was introduced with the BBC Television Service in 1936, suspended for the duration of World War II, and remained in operation in the UK until 1985, it was also used between 1961 and 1982 in Ireland as well as from 1957 to 1973 for the Rediffusion Television cable service in Hong Kong. Sometimes called the Marconi-EMI system, it was developed in 1934 by the EMI Research Team led by Sir Isaac Shoenberg. The figure of 405 lines had been chosen following discussions over Sunday lunch at the home of Alan Blumlein.〔Alexander, Robert Charles (1999). ''The Inventor of Stereo: The Life and Works of Alan Dower Blumlein'', p. 160. Oxford, Focal Press. ISBN 0-240-51628-1.〕 The system was the first broadcast system in Britain to use interlacing, though EMI had been experimenting with a 243 line all-electronic interlaced system since 1933. In the 405 system the scanning lines were broadcast in two complementary fields, 50 times per second, creating 25 frames per second. The actual image was 377 lines high and interlaced, with additional unused lines making the frame up to 405 lines to give the slow circuitry time to prepare for the next frame; in modern terms it would be described as 377i. At the time of its introduction the 405-line system was referred to as "high definition", which it was compared to earlier systems, although of lower definition than 625-line and later standards. == History == In 1934 the British government set up a committee (the "Television Committee") to advise on the future of TV broadcasting. The committee recommended that a "high definition" service (defined by them as being a system of 240 lines or more) to be run by the BBC be established. The recommendation was accepted and tenders were sought from industry. Two tenders were received: one from the Baird company offering a 240 line mechanical system, and the other from EMI offering a 405-line all-electronic one. The Television Committee advised that they were unable to choose between the two systems and that both tenders should be accepted, the two systems to be run together for an experimental period. Broadcasting of the resulting BBC Television Service from their Alexandra Palace site began in November 1936, at first time-sharing broadcasts with the 240-line Baird system; however, after three months of trials (in January 1937) the Baird system was abandoned in favour of exclusive broadcasting with the 405-line Marconi-EMI system on VHF. This became the standard for all British TV broadcasts until the 1960s. It soon became apparent that television reception was also possible well outside the original intended service area. In February 1938, engineers at the RCA Research Station, Riverhead, Long Island, New York, in the USA, were able to receive the BBC signal away, due to the signal being "bounced" back to earth from the ionosphere. A few minutes of programming were recorded on 16 mm movie film. This is now considered to be the only surviving example of pre-war, live British television. The images recorded included Jasmine Bligh and a brief shot of Elizabeth Cowell, two of the original three BBC announcers, an excerpt from an unknown period costume drama, and the BBC's station identification transmitted at the beginning and end of the day's programmes. The BBC temporarily ceased transmissions on 1 September 1939, the day of the German invasion of Poland, as war was imminent. After the BBC Television Service recommenced in 1946, distant reception reports were received from various parts of the world, including Italy, South Africa, India, the Middle East, North America and the Caribbean. In 1954 the BBC lost its monopoly of the British television market, and the following year the commercial network ITV, comprising a consortium of regional companies, was launched. Some ITV companies, notably Lew Grade's ATV, proposed broadcasting in colour using a 405-line variation on the NTSC system, but after some trials in 1955-1956 〔(【引用サイトリンク】 work=Chronomedia )〕 on its Alexandra Palace then on its Crystal Palace 405 lines transmitters, the BBC persuaded the Government that colour should await the introduction of a higher-definition system. In 1964 the BBC launched its BBC2 service on UHF using only a 625-line system, which older sets could not receive. For several years BBC1 and ITV transmitted using the 405-line and BBC2 the 625-line standard; the only way to receive them all was to use a complex "dual-standard" 405- and 625-line, VHF and UHF, receiver. The introduction of colour on BBC2 in 1967 necessitated an even more complex dual-standard set to receive all three channels. In November 1969 BBC1 and ITV also started broadcasting in 625-line PAL colour on UHF. As their programming was now entirely produced using the new standard, the 405-line broadcasts served only as a rebroadcast in monochrome for people who did not have the newer receivers. Thereafter, receivers were of a simpler single standard design which could not receive the legacy 405 line transmissions. One reason for the long switchover period was the difficulty in matching the coverage level of the new UHF 625 line service with the very high level of geographic coverage achieved with the 405-line VHF service. The last 405-line transmissions were seen on 3 January 1985, in Scotland, having been shut down one day earlier in the rest of the UK. This left only the UHF PAL system in operation in the UK. The frequencies used by the 405-line system were initially left empty, but were later sold off, used now for other purposes including DAB and trunked PMR commercial two way radio systems. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「405-line television system」の詳細全文を読む スポンサード リンク
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