405-line
Encyclopedia
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, and 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
. 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. Of the 405 lines, 377 were used for the image. By today's DVB nomenclature standards, it could be called 377i except that 405 line has never been broadcast or recorded digitally.
Though at the time of its introduction the 405-line system was referred to as "high definition", that was in relation to the crude mechanical television systems that preceded it; this should not be confused with modern-day high-definition television
.
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 5000 km (3,106.9 mi) 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 September 1, 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 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. PAL
colour was introduced in 1967.
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.
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.
) was also simulcast on 625-line from the summer of 1962 onwards, two years before the BBC had any 625 channels.
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.
, France and Eindhoven, Netherlands # Czechoslovakia and Switzerland http://www.bvws.org.uk/405alive/faq/405_hist.html
s still survive. However, the majority of surviving 405-line programmes are in the form of black and white film telerecordings, usually with optical soundtracks.
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 3 April 1950 when it changed to the more common 4:3 format.
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.
) because studio lighting generally uses alternating current lamps and if these were not synchronized 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. However, the main problem was the susceptibility of the electron beam in the CRT being deflected by stray magnetic fields from nearby transformers or motors. If the picture was locked to the mains frequency, this interference would at least be static on the screen and thus relatively unnoticeable.
.
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), with 405 being derived from (3 × 3 × 3 × 3 × 5).
in the line output transformer
.
This is a common artifact in sets that use a cathode ray tube
. 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
. Modern sets using plasma, LCD or even LED displays are completely free of this effect as there is no requirement to generate the scanning signal.
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.
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.
Monochrome
Monochrome describes paintings, drawings, design, or photographs in one color or shades of one color. A monochromatic object or image has colors in shades of limited colors or hues. Images using only shades of grey are called grayscale or black-and-white...
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
BBC One
BBC One is the flagship television channel of the British Broadcasting Corporation in the United Kingdom. It was launched on 2 November 1936 as the BBC Television Service, and was the world's first regular television service with a high level of image resolution...
in 1936, suspended for the duration of World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
, and remained in operation in the UK
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
until 1985, and was also used between 1961 and 1982 in Ireland
Ireland
Ireland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...
as well as from 1957 to 1973 for the Rediffusion Television
Rediffusion Television
Rediffusion Television was the first television station in Hong Kong, established on May 29, 1957, making it both the first British colony and the first predominantly Chinese city to have television...
cable
Cable television
Cable television is a system of providing television programs to consumers via radio frequency signals transmitted to televisions through coaxial cables or digital light pulses through fixed optical fibers located on the subscriber's property, much like the over-the-air method used in traditional...
service in Hong Kong
Hong Kong
Hong Kong is one of two Special Administrative Regions of the People's Republic of China , the other being Macau. A city-state situated on China's south coast and enclosed by the Pearl River Delta and South China Sea, it is renowned for its expansive skyline and deep natural harbour...
.
Sometimes called the Marconi-EMI system, it was developed in 1934 by the EMI
EMI
The EMI Group, also known as EMI Music or simply EMI, is a multinational music company headquartered in London, United Kingdom. It is the fourth-largest business group and family of record labels in the recording industry and one of the "big four" record companies. EMI Group also has a major...
Research Team led by Sir Isaac Shoenberg
Isaac Shoenberg
Sir Isaac Shoenberg was an electronic engineer born in Russia who was best known for his role in history of television....
. The figure of 405 lines had been chosen following discussions over Sunday lunch at the home of Alan Blumlein
Alan Blumlein
Alan Dower Blumlein was a British electronics engineer, notable for his many inventions in telecommunications, sound recording, stereo, television and radar...
. 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. Of the 405 lines, 377 were used for the image. By today's DVB nomenclature standards, it could be called 377i except that 405 line has never been broadcast or recorded digitally.
Though at the time of its introduction the 405-line system was referred to as "high definition", that was in relation to the crude mechanical television systems that preceded it; this should not be confused with modern-day high-definition television
High-definition television
High-definition television is video that has resolution substantially higher than that of traditional television systems . HDTV has one or two million pixels per frame, roughly five times that of SD...
.
History
Broadcasting of the resulting BBC Television Service from their Alexandra Palace
Alexandra Palace
Alexandra Palace is a building in North London, England. It stands in Alexandra Park, in an area between Hornsey, Muswell Hill and Wood Green...
site began in November 1936
1936 in television
The year 1936 in television involved some significant events.Below is a list of television-related events in 1936.-Events:*July 7 - At David Sarnoff's request for an experiment of RCA's electronic television technology, NBC's first attempt at actual programming is a 30-minute variety show featuring...
, at first time-sharing broadcasts with the 240-line Baird
John Logie Baird
John Logie Baird FRSE was a Scottish engineer and inventor of the world's first practical, publicly demonstrated television system, and also the world's first fully electronic colour television tube...
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
RCA
RCA Corporation, founded as the Radio Corporation of America, was an American electronics company in existence from 1919 to 1986. The RCA trademark is currently owned by the French conglomerate Technicolor SA through RCA Trademark Management S.A., a company owned by Technicolor...
Research Station, Riverhead, Long Island, New York
New York
New York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...
, in the USA, were able to receive the BBC signal 5000 km (3,106.9 mi) 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
A film, also called a movie or motion picture, is a series of still or moving images. It is produced by recording photographic images with cameras, or by creating images using animation techniques or visual effects...
film
Photographic film
Photographic film is a sheet of plastic coated with an emulsion containing light-sensitive silver halide salts with variable crystal sizes that determine the sensitivity, contrast and resolution of the film...
. This is now considered to be the only surviving example of pre-war, live British television. The images recorded included Jasmine Bligh
Jasmine Bligh
Jasmine Lydia Bligh was one of the first three BBC Television Service presenters in the 1930s, along with Leslie Mitchell and Elizabeth Cowell, providing continuity announcements and introducing programmes in-vision....
and a brief shot of Elizabeth Cowell
Elizabeth Cowell
Elizabeth Cowell was a British broadcaster.She was one of the first three BBC Television Service presenters, along with Jasmine Bligh and Leslie Mitchell. She began announcing when the Television Service started in 1936, and returned in 1946 after its nearly seven-year hiatus due to the Second...
, two of the original three BBC announcers, an excerpt from an unknown period costume drama, and the BBC's station identification
Station identification
Station identification is the practice of radio or television stations or networks identifying themselves on air, typically by means of a call sign or brand name...
transmitted at the beginning and end of the day's programmes.
The BBC temporarily ceased transmissions on September 1, 1939 the day of the German invasion of Poland
Invasion of Poland (1939)
The Invasion of Poland, also known as the September Campaign or 1939 Defensive War in Poland and the Poland Campaign in Germany, was an invasion of Poland by Germany, the Soviet Union, and a small Slovak contingent that marked the start of World War II in Europe...
, as war
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
was imminent. After the BBC Television Service recommenced in 1946, distant reception reports were received from various parts of the world, including Italy
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...
, South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
, India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
, the Middle East
Middle East
The Middle East is a region that encompasses Western Asia and Northern Africa. It is often used as a synonym for Near East, in opposition to Far East...
, North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
and the Caribbean
Caribbean
The Caribbean is a crescent-shaped group of islands more than 2,000 miles long separating the Gulf of Mexico and the Caribbean Sea, to the west and south, from the Atlantic Ocean, to the east and north...
.
In 1954 the BBC
BBC
The British Broadcasting Corporation is a British public service broadcaster. Its headquarters is at Broadcasting House in the City of Westminster, London. It is the largest broadcaster in the world, with about 23,000 staff...
lost its monopoly of the British
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
market, and the following year the commercial
Commercial broadcasting
Commercial broadcasting is the broadcasting of television programs and radio programming by privately owned corporate media, as opposed to state sponsorship...
network ITV
ITV
ITV is the major commercial public service TV network in the United Kingdom. Launched in 1955 under the auspices of the Independent Television Authority to provide competition to the BBC, it is also the oldest commercial network in the UK...
, comprising a consortium of regional companies, was launched.
Some ITV companies, notably Lew Grade
Lew Grade
Lew Grade, Baron Grade , born Lev Winogradsky, was an influential Russian-born English impresario and media mogul.-Early years:...
's ATV
Associated TeleVision
Associated Television, often referred to as ATV, was a British television company, holder of various licences to broadcast on the ITV network from 24 September 1955 until 00:34 on 1 January 1982...
, proposed broadcasting in colour using a 405-line variation on the NTSC
NTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
system, but after some trials in 1955-1956 on its Alexandra Palace
Alexandra Palace
Alexandra Palace is a building in North London, England. It stands in Alexandra Park, in an area between Hornsey, Muswell Hill and Wood Green...
then on its Crystal Palace
Crystal Palace
- United Kingdom :* The Crystal Palace, an 1851 building in south London destroyed by fire in 1936** The Great Exhibition, the event the building was built for, sometimes also known as Crystal Palace...
405 lines transmitters the BBC
BBC
The British Broadcasting Corporation is a British public service broadcaster. Its headquarters is at Broadcasting House in the City of Westminster, London. It is the largest broadcaster in the world, with about 23,000 staff...
persuaded the Government that colour should await the introduction of a higher-definition system.
In 1964 the BBC
BBC
The British Broadcasting Corporation is a British public service broadcaster. Its headquarters is at Broadcasting House in the City of Westminster, London. It is the largest broadcaster in the world, with about 23,000 staff...
launched its BBC2 service on UHF
Ultra high frequency
Ultra-High Frequency designates the ITU Radio frequency range of electromagnetic waves between 300 MHz and 3 GHz , also known as the decimetre band or decimetre wave as the wavelengths range from one to ten decimetres...
using only a 625-line system, which older sets could not receive. PAL
PAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
colour was introduced in 1967.
In November 1969 BBC1 and ITV
ITV
ITV is the major commercial public service TV network in the United Kingdom. Launched in 1955 under the auspices of the Independent Television Authority to provide competition to the BBC, it is also the oldest commercial network in the UK...
also started broadcasting in 625-line PAL
PAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
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.
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
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
, 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
Digital audio broadcasting
Digital Audio Broadcasting is a digital radio technology for broadcasting radio stations, used in several countries, particularly in Europe. As of 2006, approximately 1,000 stations worldwide broadcast in the DAB format....
and trunked
Trunked radio system
A trunked radio system is a complex type of computer-controlled radio system. Trunked systems use a few channels , and can have virtually unlimited talkgroups. The control channel computer sends packets of data to enable one talkgroup to talk together, regardless of frequency...
PMR
Professional Mobile Radio
Professional mobile radio are field radio communications systems which use portable, mobile, base station, and dispatch console radios...
commercial two way radio systems.
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 OneRTÉ One
RTÉ One is the flagship television channel of Raidió Teilifís Éireann , and it is the most popular and most watched television channel in Ireland. It was launched as Telefís Éireann on 31 December 1961, it was renamed RTÉ Television in 1966, and it was renamed as RTÉ One upon the launch of RTÉ...
) 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 RTE2RTÉ TwoRTÉ Two is a free-to-air general entertainment channel operated by Irish state broadcaster Raidió Teilifís Éireann. RTÉ Two is available throughout the island of Ireland through digital terrestrial service Saorview, VHF and UHF bands, and is also available via satellite to Irish subscribers of...
; 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.
Hong Kong
The 405-line system was used in the Rediffusion Television cable televisionCable television
Cable television is a system of providing television programs to consumers via radio frequency signals transmitted to televisions through coaxial cables or digital light pulses through fixed optical fibers located on the subscriber's property, much like the over-the-air method used in traditional...
service in Hong Kong
Hong Kong
Hong Kong is one of two Special Administrative Regions of the People's Republic of China , the other being Macau. A city-state situated on China's south coast and enclosed by the Pearl River Delta and South China Sea, it is renowned for its expansive skyline and deep natural harbour...
, 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
PAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
system free-to-air
Free-to-air
Free-to-air describes television and radio services broadcast in clear form, allowing any person with the appropriate receiving equipment to receive the signal and view or listen to the content without requiring a subscription or one-off fee...
broadcast.
Other countries
In 1939 there was also (briefly) experimental 405 line transmissions from stations in MontrougeMontrouge
Montrouge is a commune in the southern Parisian suburbs, located from the center of Paris, France. It is one of the most densely populated municipalities in Europe...
, France and Eindhoven, Netherlands # Czechoslovakia and Switzerland http://www.bvws.org.uk/405alive/faq/405_hist.html
Original
A few 405-line videotapeVideotape
A videotape is a recording of images and sounds on to magnetic tape as opposed to film stock or random access digital media. Videotapes are also used for storing scientific or medical data, such as the data produced by an electrocardiogram...
s still survive. However, the majority of surviving 405-line programmes are in the form of black and white film telerecordings, usually with optical soundtracks.
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.System A
405-line is system A in the CCIRITU-R
The ITU Radiocommunication Sector is one of the three sectors of the International Telecommunication Union and is responsible for radio communication....
assignment of broadcast systems
Broadcast television system
Broadcast television systems are encoding or formatting standards for the transmission and reception of terrestrial television signals. There are three main analog television systems in current use around the world: NTSC, PAL, and SECAM...
. The audio uses Amplitude Modulation
Amplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
rather than the Frequency Modulation
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
in use on modern analogue systems. In addition, the system was broadcast in an aspect ratio
Aspect ratio
The aspect ratio of a shape is the ratio of its longer dimension to its shorter dimension. It may be applied to two characteristic dimensions of a three-dimensional shape, such as the ratio of the longest and shortest axis, or for symmetrical objects that are described by just two measurements,...
of 5:4 until 3 April 1950 when it changed to the more common 4:3 format.
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 | Lines | Frame rate | Channel bandwidth (in MHz) | Visual bandwidth (in MHz) | Sound offset | Vestigial sideband | Vision mod. | Sound mod | Aspect ratio | Effective resolution (4:3). |
|---|---|---|---|---|---|---|---|---|---|---|
| System A | 405 | 25 | 5 | 3 | -3.5 | 0.75 | Pos. | AM | 4:3 (5:4 before 1950) | 503 x 377 (theoretical) |
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 thereof) 50 Hz in most countries (60 Hz in the AmericasAmericas
The Americas, or America , are lands in the Western hemisphere, also known as the New World. In English, the plural form the Americas is often used to refer to the landmasses of North America and South America with their associated islands and regions, while the singular form America is primarily...
) because studio lighting generally uses alternating current lamps and if these were not synchronized 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. However, the main problem was the susceptibility of the electron beam in the CRT being deflected by stray magnetic fields from nearby transformers or motors. If the picture was locked to the mains frequency, this interference would at least be static on the screen and thus relatively unnoticeable.
Why 405 lines
Because an interlaced system requires accurate positioning of scanning lines, it is important to make sure that the horizontal and vertical timebase 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 numberPrime number
A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself. A natural number greater than 1 that is not a prime number is called a composite number. For example 5 is prime, as only 1 and 5 divide it, whereas 6 is composite, since it has the divisors 2...
.
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
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
(Standard AC mains supply frequency in Britain) and a horizontal one of 10,125Hz (50 × 405 ÷ 2), with 405 being derived from (3 × 3 × 3 × 3 × 5).
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.Coverage
The use of VHF frequencies combined with the narrow vision bandwidth (AM signals [at VHF low band frequencies] are less affected by noise as bandwidth is reduced) meant that 405-line signals could be received well even under marginal conditions. Therefore it was possible to cover virtually all of the UK with a relatively small number of transmitting stations.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. Further, the amplitude of any interference would quickly match the amplitude of the sync pulses rendering them indistinguishable from the noise. With negative modulation, the amplitude of the interference would have to be much larger to match the sync pulses (which were represented by peak carrier power). If they were able to do this, the picture had already become unwatchable.Poor AGC control
The AGC circuit had to detect the average value of the transmitted signal (In the positively modulated carrier, peak power represented peak white - not guaranteed to be present in any picture). Thus for a completely black picture, the AGC circuit increased the RF gain to restore the average carrier amplitude. The result was a screen that was not black but mid grey. In fact the total light output of early TV sets was practically constant regardless of the picture content. The introduction of negative modulation in later systems solved this problem because peak carrier power represented sync pulses, something guaranteed to be present. It was only necessary to gate the AGC circuit so that it only detected the amplitude of the sync pulses.Whistle due to line output 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 magnetostrictionMagnetostriction
Magnetostriction is a property of ferromagnetic materials that causes them to change their shape or dimensions during the process of magnetization. The variation of material's magnetization due to the applied magnetic field changes the magnetostrictive strain until reaching its saturation value, λ...
in the line output transformer
Flyback transformer
A flyback transformer , also called a line output transformer , is a special transformer, which is used for conversion of energy in electronic circuits. It was initially designed to generate high current sawtooth signals at a relatively high frequency...
.
This is a common artifact in sets that use a cathode ray tube
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets 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
Hearing (sense)
Hearing is the ability to perceive sound by detecting vibrations through an organ such as the ear. It is one of the traditional five senses...
. Modern sets using plasma, LCD or even LED displays are completely free of this effect as there is no requirement to generate the scanning signal.
Equalizing pulses
The absence of equalizing pulses to facilitate interlace was defended at the start of the BBC service on the grounds that it only caused a lack of interlace with field synchronizing separators of the integrator type, and that there were, even at that time, numerous other circuits which gave completely accurate interlace without equalizing pulses. The question was raised again from time to time, but a series of tests, conducted during 1952 in cooperation with the British Radio Equipment Manufacturers' Association, confirmed that there was no general need for equalizing pulses.Spot wobble
On some 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, reducing picture brightness and contrast. Larger screen sets often used a spot wobble oscillator that slightly elongated the scanning spot vertically at high frequency to avoid this line separation effect without reducing horizontal sharpness.Experimental colour transmissions
During the late 1950s and early-mid 1960s some experimental colour broadcasts were made in the UK using the 405-line system using NTSCNTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
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
PAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
, SECAM
SECAM
SECAM, also written SÉCAM , is an analog color television system first used in France....
and other NTSC subcarrier frequencies were also attempted.
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.