405-line television system

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The 405-line monochrome analogue television broadcasting system was the first 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, and was also used between 1961 and 1982 in Ireland as well as from 1957 for the Rediffusion Television cable television service in 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 system was the first to use interlacing; the 405 scanning lines were broadcast in two complementary fields, 50 times per second, creating 25 frames per second. Of the 405 lines, 377 were used for the image. By today's standards, it could be called 377i.

Though articles at the time of its introduction referred to the 405-line system as "high definition", that is in relation to the crude mechanical television systems that preceded it; it should not be confused with modern-day HDTV.

Contents

[edit] History

A mock-up of a 1930s EMI Emitron 405-line television camera, constructed for the 1986 BBC drama Fools on the Hill.
A mock-up of a 1930s EMI Emitron 405-line television camera, constructed for the 1986 BBC drama Fools on the Hill.

The system was used by the BBC Television Service 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 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 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 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.

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 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 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.

[edit] 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] Hong Kong

Main article: Rediffusion Television

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.

[edit] 405-line video recordings

[edit] Original

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] Modern

405 line programming may be recorded and played on an unmodified VHS or Betamax video recorder, as long as the input to the recorder is baseband rather than RF. Thus various modern video recordings of 405 line programming also exist.

[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 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

Because an interlaced system requires accurate positioning of scanning lines it is important to make sure that the horizontal and vertical timebases are in a precise ratio. This is accomplished by passing the one through a series of electronic divider circuits to produce the other. Each division is by a prime number. Therefore 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 1930s 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 × 405 ÷ 2)

  • 2 × 3 × 3 × 5 Gives 90 (Non Interlaced)
  • 2 × 2 × 2 × 2 × 2 × 3 Gives 96 (Non Interlaced)
  • 2 × 2 × 3 × 3 × 5 Gives 180 (Non Interlaced)
  • 2 × 2 × 2 × 2 × 3 × 5 Gives 240 (Used for the experimental Baird transmissions in Britain)
  • 3 × 3 × 3 × 3 × 3 Gives 243
  • 7 × 7 × 7 Gives 343 (Early North American system also used in Poland before WW2)
  • 3 × 5 × 5 × 5 Gives 375
  • 3 × 3 × 3 × 3 × 5 Gives 405 (Used in Britain, Ireland and Hong Kong before 1985)
  • 2 × 2 × 2 × 5 × 11 Gives 440 (Non Interlaced)
  • 3 × 3 × 7 × 7 Gives 441 (Used by RCA in North America before the 525 NTSC standard was adopted and widely used before WW2 in Continental Europe with different frame rates)
  • 2 × 3 × 3 × 5 × 5 Gives 450 (Non Interlaced)
  • 5 × 7 × 13 Gives 455 (Used in France before WW2)
  • 3 × 5 × 5 × 7 Gives 525 (A compromise between the RCA and Philco systems Still used today in most of the Americas and parts of Asia)
  • 3 × 3 × 3 × 3 × 7 produces 567 (used for a while after WW2 in the Netherlands)
  • 5 × 11 × 11 Gives 605 (Proposed by Philco in North America before the 525 standard was adopted)
  • 5 × 5 × 5 × 5 Gives 625 (Still used today in most parts of the world)
  • 3 × 3 × 7 × 13 Gives 819 (Used in France in the 1950’s)
  • 3 × 7 × 7 × 7 Gives 1029 (Proposed but never adopted around 1948 in France)
  • 3 × 5 × 5 × 5 x 3 Gives 1125 (Used for 1080i and 1080p)

[edit] 405 compared with later standards

[edit] Bandwidth

When used with vestigial sideband filtering the total bandwidth of a 405-line TV channel is 5 MHz, significantly less than the 8 MHz required by the 625-line system I, which replaced it in Britain. Systems in other countries used anything between six and fourteen megahertz of bandwidth per channel.

[edit] Coverage

The use of VHF frequencies combined with the narrow vision bandwidth (AM signals are less affected by noise as bandwidth is reduced) meant that 405-line signals could be received well even under marginal conditions and consequently it was possible to cover virtually all of the country with a relatively small number of transmitting stations.

[edit] Susceptibility to impulse interference

The use of AM (rather than FM) for sound and the use of positive (rather than negative) video modulation made 405-line signals very susceptible to impulse interference, such as that generated by the ignition systems of vehicles. Such interference manifested itself as a loud popping on sound and large bright spots on the picture which viewers found a lot more noticeable than the dark spots encountered when such interference is encountered on a signal using negative video modulation.

[edit] Whistle due to lineoutput transformer magnetostriction

The 405-line system produced a noticeable 10,125 Hz whistle in many sets, equal to the number of lines per second. This high-pitched whistle is caused by magnetostriction in the line-timebase transformer, that is, the transformer used to power the beam 10,125 times from left to right and back again changes dimensions in tune with that frequency.[2][3] This is a common effect of sets that use a cathode ray tube, and while all CRT-based television systems produce such a noise, the higher number of lines per second in later standards produce frequencies (PAL's 15,625 Hz and NTSC's 15,734 Hz) that are at the upper end of the audible spectrum, and which not all people are able to hear; more modern sets also tend to be less susceptible to this effect.

[edit] Interlace/timing issues

The lack of equalising pulses in the vertical blanking period of the 405-line system often resulted in imperfect interlacing, which manifested itself as increased flicker and poor resolution when the two fields per frame failed to line up correctly.

[edit] Spot Wobble

On larger TV screen sizes, the scanned lines were not fat enough to give 100% coverage of the CRT. The result was a lined picture with darkness between each horizontal scanned line. This doesn't look good, and reduces picture brightness and contrast.

Larger screen sets often used a spot wobble oscillator that wobbled the scanning spot slightly at high frequency to avoid this line separation effect.

[edit] Experimental colour transmissions

During the late 1950s and early-mid 1960's 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. Tests with PAL SECAM and other NTSC subcarrier frequencies were also attempted.[4]

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

  1. ^ Pawley, Edward. BBC Engineering 1922 - 1972, ISBN 0-563-12127-0, p 366.
  2. ^ Television Repair and Restoration. Retrieved on 2007-04-03.
  3. ^ 405 Alive - Welcome. Retrieved on 2007-04-03.
  4. ^ World Analogue Television Standards and Waveforms

Robson, Neil, 'Living Pictures Out of Space: The Forlorn Hopes for Television in Pre-1939 London', Historical Journal of Film, Radio and Television, vol. 24, no. 2 (June 2004), pp. 223-32.

[edit] See also

[edit] External links