How it works… A question of lines

Television companies operate on different television standards throughout the world. A fundamental difference in these standards is the number of lines used to constitute the picture. In the United Kingdom 405 lines are used, in the United States 525, in Europe generally 625 and in France and Belgium 819 lines.

At the time of writing a Television Advisory Committee is sitting to discuss the whole question of television standards in this country. What are the reasons for different television standards being adopted throughout the world? All television companies operate a system of interlaced scanning. This means (see How it Works, No. 1) that two separate frames are transmitted in order to compose one complete picture. The interlaced scanning system requires that the number of lines shall not be exactly divisible by two. Thus, in our own system, 405 lines divided by two becomes 202½, and in the 525 system it becomes 262½. This gives the first reason why these magic numbers have been chosen.

With the advent of the cathode ray tube and the start of the BBC television service in 1936, A. D. Blumlein and other early television engineers, decided to use 405 lines to make up a picture. This was a tremendous step forward from the low definition system of the previous days.

To discover why Blumlein chose 405 lines, we have to consider the circuits and equipment available to him in 1936. First, the early cathode ray tube’s spot size, which determines the width of a scanning line, was quite big. It would obviously be nonsense to increase the number of lines so much that one line of picture was falling on its neighbour because of its width. Next, the large cathode ray tubes were then no bigger than 12 inches and as the eye can only register detail which subtends to it an angle of greater than one minute, it was obviously unnecessary for him to place the lines of the picture too close together. It was estimated that if a person sat approximately six feet away from a 12-inch tube, it would need approximately 400 lines for the line structure on the picture to become unobtrusive.

Obviously the more lines used, the greater the vertical definition of the television picture would be – in the same way as a greater number of dots in a newspaper photograph increases the definition or sharpness of the picture. But this is not the only consideration.

Definition has also to be considered in the horizontal direction, i.e. along each scanning line. In fact a line of picture is similar to the line of dots which makes up a newspaper photograph, each dot, which we will call a picture element, being spaced from its neighbours by a distance equal to the space that you can see between two lines on a television picture. If it were a square picture and the vertical definition was equal to the horizontal definition, the total number of picture elements would be equal to the product of 405 vertical elements and 405 horizontal elements, i.e. a total of 164,025 elements.

As 25 pictures are transmitted a second, the total a second becomes an astounding 4,100,625 elements. One cycle of alternating electric current can accommodate two elements and so a frequency bandwidth of approximately 2,050,000 cycles must be transmitted for a square 405-line picture. A correction must be made to this calculation because the picture was not square but a rectangle of ratio 5:4 and in fact 2,500,000 cycles had to be transmitted.

This frequency of 2.5 megacycles was exceptionally high in those days and it was very difficult to design electronic circuits capable of passing that bandwidth of frequencies without appreciable distortion.

The camera tubes in use then were high velocity devices manufactured by E.M.I. and called Emitrons. They were, however, insensitive by modern standards and produced
various spurious effects on the pictures, such as severe tilt and fuzz.

It was unnecessary to raise the number of lines above 405, because it would not have resulted in any apparent increase in picture quality due to small receiving tubes and insensitive cameras, and the fact that the large bandwidth would have had to be increased still further. Why were 405 lines used not, say, 401? The line frequency must be related to the mains frequency to avoid a hum disturbance on the picture (see How It Works, No. 1). The nearest number of lines to the calculated 400 that would meet this requirement was 405.

When American television started the number of lines could not be exactly the same as ours because the mains frequency in the United States was 60 cycles per second compared with England’s so cycles per second. Our experience enabled the Americans to increase the vertical definition of their system slightly. So they chose a standard of 525 lines. However, the overall definition of their system is still no greater than ours.

At the end of the last war the BBC television service re-started, using the original broadcast equipment at Alexandra Palace. While circuit techniques had advanced greatly, big strides had still to be made in camera tubes and receivers. So it was decided that no great advantage would be gained by changing the original line standard drastically. At that time television receivers were very expensive – it was not until one firm marketed a receiver which sold for approximately £45 that television really began to spread rapidly. Even so, this receiver had only a nine-inch tube.

In the early 1950’s the aspect ratio (i.e. the ratio of the height to the width of the picture) was changed from the original 5:4 to 4:3. This change was made to accommodate film more easily on television. But it resulted in the necessary bandwidth being increased to 3.1 megacycles, in order to get the equivalent definition. Even today there are still receivers which cannot resolve three megacycles. One dare not think what they would be like if it were necessary to resolve a still higher frequency associated with an increase of lines.

Within the last decade new and more sensitive camera pickup tubes have become available. These tubes, together with modern circuit techniques and components, coupled with the fact that television receivers have now progressed beyond the stage of a 12-inch picture, make the limitations of the 405-line system apparent.

When television services started on the Continent, they gained tremendously from the lessons learnt in both Britain and America and a standard of 625 lines was selected. This appears to give a superior picture to 405 lines, provided that receivers are designed to accommodate the increased bandwidth.

I believe the Continental countries have been right in adopting a higher standard. In France and in Belgium they developed a super-definition system of 819 lines. This has much to recommend it – it provides an adequately fine line structure to accommodate the very large picture tubes of the future, but unfortunately there is the disadvantage of an enormous bandwidth.

The question of bandwidth is a problem in itself. There is only a certain amount of spectrum space on the air available for television. As more and more transmitters come on the air it is vital that the amount of spectrum space they use is kept to a minimum so that they will not interfere with each other. The higher the line definition and the greater the bandwidth required by the transmitters, the smaller the number of transmitters that can be accommodated in the available spectrum space. Should more transmitters come on the air in this country, or our line standard change, it will be necessary to utilize at least part of the two remaining television bands, i.e. Bands IV and V.

The situation now is reasonably clear. In this country we have developed a high definition system of television which can produce as good a picture as any other television service in the world. But this may not be so in the future. Within the next five years our pictures may be inferior to those obtainable in many other countries. A common standard for all European countries is obviously desirable.

Programmes could then be exchanged freely without the difficulties and degradation caused by standards converters. However, a change from 405 lines to 625 lines would necessitate the modification of most of our studio apparatus and every television receiver in the country, at some considerable cost. This would not be a very popular decision with someone who had just bought a new receiver.

The alternative is to build more transmitters and radiate our pictures on 405- and 625-line standards simultaneously. If that were done, we could, over the next five or seven years, gradually change television receivers from one line standard to another. However, it would mean a big capital cost to both the BBC and the ITA as each of their present transmitters would have to be duplicated.

Further, as we would have to double the number of television transmitters on the air, the shortage of spectrum space would mean that bands IV and V would have to be used. The extent of the propagation difficulties to be met in these bands is still being studied. The range of transmitters operating in these bands would probably be much more limited than those at the moment. So it is not a simple matter of purely doubling the number of existing transmitters. It may mean that the number would have to be trebled.

To add to the difficulties a third or fourth television programme network is possible, each requiring a completely new series of transmitters. Colour must also come. It depends on the system of colour transmission used whether colour signals can be sent over the present transmitters or not. If not it would mean a further complete chain or chains of television transmitters taking the air.