405-line
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The 405-line monochrome analogue television broadcasting system was the first standard-definition electronic television system to be used in regular broadcasting. It was introduced with the BBC Television Service in 1936, and remained in operation in the UK until 1985, and was also used for some time in Ireland and Hong Kong. Sometimes called the Marconi-EMI system, it was invented in 1934 by Alan Dower Blumlein of the EMI Research Team led by Sir Isaac Shoenberg. The 405 scanning lines, of which 377 were used for the image, are roughly equivalent in modern terminology to a resolution of 480x368 pixels, and the format is sometimes (though rarely in practice) referred to as 368i, in keeping with terminology of more recent standards.
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[edit] History
The system was used by the BBC from their Alexandra Palace site beginning 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.
In 1955 the BBC lost its de facto 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.
[edit] The phasing out of the 405-line system (UK)
In 1964 the BBC launched its BBC Two service on UHF using only a 625-line (576i) system, which older sets could not receive. PAL colour was introduced in 1967. In November 1969 BBC One and ITV also started broadcasting on 625-line on UHF in colour. 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.
The last 405-line transmissions were seen on January 3, 1985, in Scotland, having been shut down one day earlier in the rest of the UK. This left only the UHF PAL system (using NICAM audio) in operation in the UK. The frequencies used by the 405-line system were initally left empty, but were later sold off, used now for other purposes including DAB.
[edit] The phasing out of the 405-line system (Ireland)
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 converters from the local 625-line transmitter.
- For the last five years of RTÉ 405-line programming relays a simple orthicon converter, essentially a 405-line camera pointed at a 625-line monitor, was used as the more expensive system converters that RTÉ previously used had broken down.
[edit] The phasing out of the 405-line system elsewhere
Many British colonies used the 405-line system until they became independent. After that, many newly independent countries from the British Empire still used this system, until they switched to other TV broadcast systems, generally PAL-B/G/I.
[edit] 405-line video recordings
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.
[edit] 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 an aspect ratio of 5:4 until 3rd April 1950 when it changed to the more common 4:3 format.[1]
All System A transmitters used vestigial sideband transmission, with the single exception of Alexandra Palace in London, which closed down in 1957 when it was replaced by Crystal Palace.
| 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 | -3.5 | 0.75 | Positive | AM |
[edit] Why 50 fields per second
Since the mid-1930s this frequency it has been standard practice to use a field frequency equal to the AC mains electric supply frequency (or a submultiple therof) 50 Hz in most countries (60 Hz in others) because studio lighting generally uses alternating current lamps and if these were not synchronised with the field frequency, an unwelcome strobe effect could appear on TV pictures. Secondly the smoothing (filtering) of power supply circuits in early TV receivers was rather poor and ripple superimposed on the DC could cause visual interference. If the picture was locked to the mains frequency, this interference would at least be static on the screen and thus less obtrusive.
[edit] Why 405 lines
For technical reasons, there has to be a straightforward mathematical relationship between the line and field frequencies, the latter being derived by dividing down from the former. Technology constraints of the 1930's meant that this division process could only be done using small integers, preferably no greater than 7, for good stability. The number of lines was odd because of 2:1 interlace. The 405 line system used a vertical frequency of 50 Hz (Standard AC mains supply frequency in Britain) and a horizontal one of 10,125Hz (50 x 3 x 3 x 3 x 3 x 5 divided by 2)
- 3 x 3 x 7 x 7 gives 441 (a line standard used in several countries before WW2)
- 3 x 3 x 3 x 3 x 7 produces 567 (used for a while after WW2 in the Netherlands) and
- 5 x 5 x 5 x 5 gives 625
The 405-line system produced a noticeable 10,125 Hz whistle in many sets, equal to the number of lines per second.[2][3] This is a common effect of the cathode ray tube, and while all CRT-based television systems produce a noise, the higher number of lines per second produce frequencies (such as PAL's 15,625 Hz), which not all people are able to hear.
[edit] Experimental colour transmissions
During the late 1950s some experimental colour broadcasts were made in the UK using the 405-line system using NTSC colour encoding. The subcarrier frequency was 2.6578125 MHz (525/2 times line frequency) with an "I" signal bandwidth of 500 kHz and a "Q" signal bandwidth of 300 KHz.
Some of these broadcasts were on UHF (also an experimental technology at the time), while others were carried over the regular VHF network outside of normal broadcasting hours.
[edit] Notes and References
- ^ Pawley, Edward. BBC Engineering 1922 - 1972, ISBN 0-563-12127-0, p 366.
- ^ http://www.vintageampdoctor.co.uk/television_repairs_and_restoration.htm
- ^ http://www.bvws.org.uk/405alive/
[edit] See also
[edit] 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
- The last 405-line BBC1 closedown (requires real player)
