405-line
Encyclopedia : 4 : 40 : 405 : 405-line
405 line is the name of a monochrome analogue television broadcasting system in operation in the UK between 1936 and 1985, and also used for some time in Ireland and Hong Kong. The system was invented in 1934 by Alan Dower Blumlein of the EMI Research Team led by Sir Isaac Shoenberg. The 405 scanning lines are roughly equivalent in modern terminology to a vertical resolution of 360 pixels, although the lines on black and white sets were continuous rather than being divided into discrete pixels as on colour sets.
History
Initially the system was used by the BBC from their Alexandra Palace site, 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.
In 1955 the BBC lost its monopoly of the British Television market, when 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 the BBC persuaded the Government that colour should await the introduction of a higher-definition system.
In 1964 the BBC launched its BBC Two service on UHF only using a 625-line (576i) system, introducing PAL colour in 1967. In November 1969 BBC One and ITV also started broadcasting on 625-line on UHF in colour. Thereafter the 405-line broadcasts served only as a rebroadcast in monochrome for people who did not have the newer receivers.
Finally in January 1985 broadcasts ceased, leaving only the UHF PAL system in operation. The frequencies left empty were sold off, used now for other purposes including DAB.
Ireland's use of the 405-line system began only in 1961, with the launch of Telefís Éireann, but extended solely to two transmitters and five relays of them, serving the east and north of the country where many people had sets for receiving broadcasts from Wales or Northern Ireland. Telefís Éireann (later to become RTÉ One) was also simulcast on 625-line from the summer of 1962 onwards, two years before the BBC had any 625 channels. The last 405 line relays, in County Donegal were turned off in 1982, with the main transmitters having been disabled in 1978 to free up frequency for RTÉ Two; with the relays being fed with standards convertors from the local 625 line transmitter.
A few 405-line videotapes still survive. However, the majority of surviving 405-line programmes are in the form of black and white film telerecordings, usually with optical soundtracks.
Many British colonies used the 405-line system until they became independent. After that, many newly independent countries from the British Empire switched to other TV broadcast systems, e.g. NTSC or PAL-B/G/I.
System A
405-line is system A in the CCIR assignment of broadcast systems. The audio uses Amplitude Modulation rather than the Frequency Modulation in use on modern analogue systems. In addition, the system was broadcast in a ratio of 5:4 until 3rd April 1950 when it changed to the more common 4:3 format.[1]
| System | No of Lines per frame | Bands Used | Channel Width (MHz) | Vision Bandwidth (MHz) | Main Sound-Vision Spacing (MHz)(M/M,S/M,N) | Vestigial Side-band (MHz) | Vision Modulation Sense | Main Sound Carrier Mode |
| A | 405 | vhf | 5 | 3 | ||||
| 0.75 | Positive | AM |
Other analogue TV standards
Other analogue TV standards have included 30, 90, 120, 180, 240, 243, 343, 441, 455, 525, 605, 625, 819 and 1125 lines but only the 525, 625 and 1125 line standards are still in use today.
Notes and References
1. Pawley, Edward. BBC Engineering 1922 - 1972, ISBN 0563121270, p 366.External links
- ["405 Alive" enthusiast website]
- ["405 lines in Ireland"]
- ["UK 405 line network"]
- [Information on the experimental NTSC 405 line system tests in the 1950s/60s]
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