405-line television system
The 405-line monochrome analogue television broadcasting system was the first fully electronic television system to be used in regular broadcasting. The number of television lines influences the image resolution, or quality of the picture.
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. 405-line was approved as System A in the CCIR assignment of broadcast systems.
Sometimes called the Marconi-EMI system, it was developed in 1934 by the EMI Research Team led by Isaac Shoenberg. The figure of 405 lines had been chosen following discussions over Sunday lunch at the home of Alan Blumlein. The system used interlacing; 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 376 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 "376i".
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.
In the United States, the FCC had briefly approved a 405-line color television standard in October 1950, which was developed by CBS. The CBS system was incompatible with existing black-and-white receivers. It used a rotating color wheel, reduced the number of scan lines from 525 to 405, and increased the field rate from 60 to 144, but had an effective frame rate of only 24 frames per second.
History
United Kingdom
Development
In 1934, the British government set up a committee to advise on the future of TV broadcasting. The committee recommended that a "high definition" service should be run and established by the BBC. 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.Initial broadcasts
Broadcasting of the resulting BBC Television Service from its Alexandra Palace site began in November 1936, at first time-sharing broadcasts with the 240-line Baird system; however, in January 1937, after three months of trials, 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 US, 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 16mm movie film. This is now considered to be the only surviving example of pre-war live British television. The images recorded included two of the original three BBC announcers, Jasmine Bligh and Elizabeth Cowell, 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, for the outbreak of World War II 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.
The BBC lost its monopoly of the British television market in 1954, and the following year the commercial network ITV, comprising a consortium of regional companies, was launched.
Experimental colour transmissions
During the late 1950s and early-to-mid 1960s, some experimental colour broadcasts were made in the UK with the 405-line system using NTSC colour encoding. The subcarrier frequency was 2.6578125 MHz with an "I" signal bandwidth of 500 kHz and a "Q" signal bandwidth of 300 kHz. Tests with PAL, SECAM and other NTSC subcarrier frequencies were also attempted.Some of these broadcasts were on UHF, while others were carried over the regular VHF network outside of normal broadcasting hours.
Co-existence with 625-line broadcasts
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 with 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 with BBC2 in colour. Over time in 1968 and 1969, the different ITV regional channels and BBC1 switched over to broadcasting on the 625-line as well as the 405-line, a process which once completed meant that only 625-lines were necessary to receive all channels, with dual standard receivers no longer necessary.In November 1969, BBC1 and ITV started broadcasting in 625-line PAL colour on UHF. Their programming was now entirely produced using the new 625-line standard, and thus the 405-line broadcasts served only as a rebroadcast in monochrome for people who did not have the newer receivers and who could only receive BBC1 and ITV. This situation continued up to 3–4 January 1985, with 405-line VHF receivers only being able to pick up BBC1 and ITV regionals, and in monochrome only, while 625-line UHF receivers could also receive BBC2 and Channel 4/S4C in addition to BBC1 and ITV, and in either colour or monochrome.
Switchover
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 4 January 1985 in Scotland; they had been officially 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; they are now used for other purposes, including DAB and trunked PMR commercial two-way radio systems.
Ireland
Ireland's use of the 405-line system began in 1961, with the launch of Telefís Éireann, but only extended to two main transmitters and their five relays, serving the east and north of the country. This was because many people in these areas already had 405-line sets for receiving UK broadcasts from Wales or Northern Ireland. Telefís Éireann's primary standard was 625-line; it began using this in the summer of 1962, more than two years before the UK had any 625-line channels.The last 405 line transmitters in the UK and Ireland were shut down in 1985.
The last 405-line relays, in County Donegal, were turned off in 1982; the main transmitters had been shut down in 1978 to free up frequencies for RTÉ 2, and after then the relays had been fed by standards converters from the local 625-line transmitter.
For the last five years of RTÉ's 405-line simulcasting, a simple orthicon converter was used, essentially a 405-line camera pointed at a 625-line monitor, as the more expensive system converters that RTÉ had previously used were now inoperable.
Hong Kong
The 405-line system was used in the Rediffusion Television cable television service in Hong Kong, established in 1957, making it both the first British colony and the first predominantly Chinese city to have television. The service of 405-line system ended in 1973, replaced by 625-line PAL system free-to-air broadcast.Europe
For a brief time in 1939 there were experimental 405-line transmissions from stations in Montrouge in France and Eindhoven in the Netherlands, Czechoslovakia and Switzerland.United States (field-sequential color system)
After the U.S. adopted the NTSC 525-line monochrome standard for commercial broadcasting in 1941, subsequent efforts were made to upgrade the standard so that it could also accommodate a "compatible" colour broadcasting system. Eventually these efforts would prove successful, but because repeated attempts had consistently produced unsatisfactory results, in 1950 the United States Federal Communications Commission officially approved for commercial broadcast an alternate 405-line broadcasting system which the Columbia Broadcasting System had developed over the past decade.This system was a field-sequential colour system which electronically transmitted a 405 lines, 144 fields per second monochrome picture. Colour was provided mechanically by means of a synchronized rotating transparent Red-Green-Blue disk, which was placed in front of the receiver screen.
Regular broadcast channels were used to transmit the 405-line system signals, but the millions of existing NTSC 525-line television receivers could only correctly process the audio portion of these transmissions, so unless these sets were modified they would only display a jumbled picture.
CBS aired a variety show special entitled Premiere on 25 June 1951 to officially launch commercial 405-line colour broadcasting, but just four months later CBS ended its colour broadcasts. CBS's efforts were hindered from the beginning by a widespread lack of acceptance, and the ultimate setback came at the end of the year when the U.S. government temporarily banned the manufacture of colour televisions, ostensibly to conserve resources during the Korean War.
In 1953, the FCC rescinded its approval of the CBS 405-line colour system. In its place it approved a newly improved and now satisfactory second NTSC 525-line standard which had been developed by RCA. It provided for colour broadcasting yet remained compatible with existing 525-line monochrome sets.