volume 2
january 2000

Lines, frames and frequencies

A brief history of television line standards

by Andrew Emmerson

Fifties cartoon, mocking the British decision to continue using the 405-lines system

Up until 1985 Great Britain was using another line-system to transmit their television programmes than for instance in Holland and Belgium. It was called the "405-lines system". Why did this system use 405 lines — and not 404 or 403 or even 400? Here Andrew Emmerson, editor of the magazine 405 Alive, tries to answer this question.

Stephen Hawking's book A Brief History of Time is a weighty tome; heaven knows how large a complete history of time would be. Producing a brief history of television line standards is also a near impossibility, simply because the subject is far more complex than might appear. So, where do we start? The best place is with the product, the television picture. Setting aside newfangled wide-screen experiments, most TV pictures have had a 4:3 aspect ratio (width to height), the same as most old motion pictures. Psychologically this format represents a convenient prospect to view without too much head swivelling and ensures that all visual action is contained within a reasonable field of view.

Philips television factory at Croydon (1961)

The aspect ratio may be important, but it will tells us nothing at all about the number of lines. To get to that number we need an equation. The picture repetition frequency, or vertical frequency, is the first important factor in that equation. Just as in motion pictures there is no real motion in television. What we see on the screen is a sequence of still pictures, presented at a rate of about 16 or more per second. All these pictures vary slightly, thereby creating the illusion of motion. Early silent films used a picture (or frame) rate of 16 and 18 per second. When sound was introduced this rate was increased to 24 frames per second. This was primarily necessary to meet the quality needs of the added sound track. Unlike film television has frame rates of 25 and 30 per second depending on the country. If we want to explain these deviant rates, we first have to double them to 50 and 60.

Here the lines become important. Each complete television picture is typically divided into two interwoven segments of lines. The odd-numbered lines are scanned first and then the even-numbered lines are interleaved in-between. Together both segments create a complete picture. This is done to reduce flicker and brightness variations. This principle of so-called interlaced scanning was introduced in the 1930s. Each of these odd and even segments is called a field. So the complete picture or frame always consists of two fields. The picture frequency generally is termed the frame rate. As each frame has two fields, the corresponding field rate — or refresh rate if you are a computer person — comes out twice as high. There are some historical reasons why this field rate had to be 50 or 60.

Since the mid-1930s the field frequency has been the same as the mains frequency, either 50 or 60Hz according to the frequency used in each country. This is for two very good reasons. 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, in days gone by, the smoothing of power supply circuits in TV receivers was not as good as it is today 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.

Now we have learned about the frame and field rates, the next task is to choose a horizontal scanning rate which will produce the degree of picture detail required and at the time 405-line television was developed, the maximum bandwidth that vision amplifiers could handle was around 2.3 to 3MHz. This in turn determined the number of lines in the picture, somewhere in the region of 400 to 450.

Quality control of cathode ray tubes (1960)

The time base circuits in television receivers had to be locked to synchronising pulses sent with the picture signal and generated at the studio; these are a kind of master "clock" that sets the timing of the whole system. For sound technical reasons, there had 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 time meant that this division process could only be made using odd numbers — they had to be integers and ideally 3, 5 or 7 to achieve maximum stability.

Let's take an example. The world's first regular all-electronic television service was British and used a vertical frequency of 50Hz (naturally) and a horizontal one of 10,125Hz (actually cycles per second in those days). How does 10,125 relate to 50? Easy! 50 * 3 * 3 * 3 * 3 * 5 = 20,250 which divided by 2 gives 10,125. To keep things simple you'll have to take my word that the integers need to be odd ones. The important thing is, that in this case we can get 25 frames per second, each existing of (3 * 3 * 3 * 3 * 5 =) 405 scanning lines.

What if we want higher definition? Then we increase one of the multipliers: we know that 3 * 3 * 3 * 3 * 5 gives us 405 and by changing the multipliers we find 3 * 3 * 7 * 7 gives 441 (a line standard used in several countries), 3 * 3 * 3 * 3 * 7 produces 567 (used for a while after the war in the Netherlands) and 5 * 5 * 5 * 5 gives us 625. You can ring the changes yourself and see what you get! If this all sounds completely over the top of your head, it doesn't really matter — rest assured your television will still work!

Line-store standards converter (625 to 405 lines) installed at BBC Television Centre (1964)

The original British 405-line standard was capable of producing extremely good pictures: in terms of black and white definition it produced more detailed pictures than are seen on most 625 line sets today, simply because the latter are manufactured with such coarse picture element grids that they throw away much of the detail in the PAL picture and disguise this loss of detail with colour. The 405-line standard was used in the UK, the republic of Ireland, Hong Kong and experimentally in several other countries. Most other countries before the second world war used 441 lines (Germany, the USSR and the USA) although France was out on a limb with 455 lines (most of the time). In 1940 the USA established its 525-line standard (NTSC). [1]

When were higher numbers of lines chosen? During the second world war both the French and the Germans made successful experiments with systems using more than 1,000 lines and there was a general feeling after the war in most European countries that it was time for a fresh start with higher definition — Great Britain and the USA decided to stick with what they had, however.

France opted for an 819-line system which was a bold move to capitalise on war-time research, to re-establish French pride and to protect French manufacturers from foreign competition — the pre-war Paris transmitter, now adjusted to 441 lines, struggled on until 1956 for those with older sets. The rest of Europe opted for 625 lines, a system devised in 1946 by two German engineers, Möller and Urtel (it appears that the Russians came up independently with a very similar system and if you had set anyone else the problem — to Europeanise the American 525-line standard — they would have come up with something pretty similar). In Geneva a Mr W. Gerber proposed this as an European system and it has remained in use until the present day.

Only the American 525-line system has had a longer continuous "innings", beating the original British 405-line system. The French 819-line system, also used in Belgium, Luxembourg, Monte Carlo and Morocco, finally died out during the 1980s, and the only other notable major change to note was the coming of NTSC colour (or is it color?) which necessitated that system to change the vertical frequency in America from 60Hz to 59.94Hz and the horizontal frequency from 15,750Hz to 15,734Hz.

A sixties advert for a Decca television set

Why did some European countries opt for 441 or 455 lines? Freedom of thought probably. Although the 405-line system was well established in Britain, there was no serious thought of international programme exchange before the war, so the need for total conformity was not a major factor. We have not yet achieved standardisation of computer operating systems or of driving on one side of the road, and there are always arguments in favour of whichever standard you favour. In theory 441 and 455 lines would give marginally better picture definition than 405 lines — marginally. [2]

Why did France choose 819 lines after the war? International standardisation of television systems was a low priority after the war. Britain was determined to stick to 405 lines, so as not to betray viewers who had already invested in a receiver, and France — as already mentioned — was determined to show the world that the French system was superior to everything else. Perhaps it was, but that decision — taken by M. Mitterand, incidentally — was to cost France dear in the end — as did the British decision to stick with 405 lines.

In August 1962 the BBC expressed hopefully that by April 1964 a start could be made with 625 lines on its new second channel in the London area, later to be extended to other regions. Indeed, after two years of testing and a campaign circling around two cartoon kangaroos named Hullaboo and Custard, on 21 April 1964 the first program was aired, with a delay of twenty-four hours due to a massive power failure in West London. With the advent of regular colour television transmissions — starting on 1 July 1967 on BBC2 and extended from 15 November 1969 to BBC1 and ITV — the 625-lines system gradually won over. The 405-lines system, however, endured for some years, finally fading from the British screens in 1985 — except with the hardy enthusiasts who still keep 405 alive!

Notes

1. NTSC was developed at the brink of the 1940s for monochrome television. The introduction of colour television brought some changes. In 1954 the USA adopted the NTSC (National Television System Committee) colour system. In Europe the French developed SECAM (Sequential Couleur à Memoire / Sequential Colour and Memory) in 1961. Germany followed in 1963 with PAL (Phase Alternation by Line). All these systems use interlaced scanning, NTSC has 525 lines with 60 fields per second, PAL and SECAM use 625 lines with 50 fields per second. Moreover all three systems are based on different colour encoding systems.

2. With high-definition television (HDTV) the world of television encountered some of the same problems. Japan already started experiments with this form of television in 1968. Their NHK-system (Nippon, Hoso, Kyotai) has an aspect ratio of 16:9, 1,125 lines per frame with a field rate of 60 fields per second. In 1986 the USA accepted this standard, but some TV companies protested because of conversion problems. They said to prefer a system of 1,050 lines, double the amount of the existing NTSC. Europe followed track within a few months with its own standard, Eureka EU95, also opting for a system with backward compatibility. In this case 1,250 lines (2 * 625) and a field rate of 50. By the way, nowadays digital television standards are based on 720 or 1,080 (visible) lines for an aspect ratio of 16:9 and 480 lines for an aspect ratio of 4:3.