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.

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As things now stand at the time of going to press, Government policy is to broadcast colour transmissions using a 625-line standard, to be radiated by the BBC on an Ultra High Frequency in Bands IV and V.

This decision will effectively preclude the 47,000,000 viewers who normally watch the Independent Television channels from receiving colour transmissions on their normal channels and means that the programmes will reach the minimum audience at the maximum, indeed astronomical, cost.

These questions and answers by general manager, john mcmillan attempt to discuss a possible alternative in terms comprehensible to the non-technical reader.

The technology of the subject is such that any discussion of colour must additionally cover the frequency bands to be used for transmission and the line standard. These things are closely connected.

1 There is much talk regarding the relative quality of 625 and 405-line television pictures. What determines the quality of a picture?

The quality of a television picture is fundamentally determined by the amount of ‘information’ transmitted, and received. More ‘information’ – better pictures.

The ‘information’ in its turn is limited by the frequency bandwidth of the video signal. More bandwidth – better pictures.

The line standard adopted affects the bandwidth. More lines – more bandwidth.

The more bandwidth used, the fewer transmitters can be accommodated in a particular slice of the available frequency spectrum allocated to television broadcasting.

2 What line standards are in actual use today?

In order of theoretical ‘goodness’ and somewhat simplified, they are as follows:

3 We seem to be right at the bottom of the league table. Surely a change to a ‘better’ standard is most desirable?

Not so. It so happens that the British Standard as specified by Messrs. Schoenberg and Blumlein of EMI some 36 years ago was a singularly good choice in all respects. It is a fact that it is possible to transmit enough information to give a first class picture on a 3 Mc/s video bandwidth.

4 What about the actual line structure? Does not 405 lines give a coarse-grained picture?

Viewed at the proper viewing distance, the line structure of any standard is virtually invisible.

5 So you think 405 lines is an adequate standard?

Yes. If a 405-line receiver has a good aerial and is properly tuned it gives a very good picture. Above all things, a 405 lines system is already in being and gives excellent coverage with the minimum number of transmitters.

6 Why did the Government of the time elect to move to a 625-line standard?

Largely due to a desire to operate on a common European Standard and so facilitate interchange of programmes.

This was a nice idea rather than a useful practical facility. It is no longer a nice idea because Europe has now decided on two different methods for colour.

7 BBC 2 transmission is often heavily criticised. What are the facts?

Of the frequency bands available under international agreement for TV broadcasting two Bands. I and III are VHF and two Bands, IV and V are UHF.

The transmission and reception characteristics are quite different.

On VHF the BBC attains 99.5 per cent coverage of the country with 30 main stations and 62 major fill-in stations.

Similar figures for 97 per cent coverage by the ITA are 32 main stations and 30 fill-in stations. For UHF and an estimated coverage of 95 per cent some 64 main stations and 250 major fill-in stations are required.

To extend this to 97 per cent a further 1,000 minor fill-in stations are estimated to be necessary. Further extension to a higher coverage figure is considered to be economically impractical.

At 97 per cent coverage over 1,500,000 people will be without television on UHF due to local screening difficulties.

Present official policy is to abandon the present economic Bands I and III and move all television broadcasting to Bands IV and V. Surely a most unsound scheme requiring a huge increase in cost for an inferior result.

8 If the VHF bands are so effective and economical, why does the Government not convert them to 625 lines?

Because the additional Channel bandwidth required for 625 lines (8 Mc/s) restricts the number of transmitters which can be accommodated in the space available to a level which would not give the high percentage national coverage deemed to be necessary for mass viewing.

9 So if we are to adopt 625 lines for colour we will positively have to go to UHF and accept the costs and consequences?

Yes. However, it must be re-emphasised that on all counts reception in lay hands is much worse on these bands. They have never been a real success in any country (USA, UK. Germany).

10 Is there any way out of this?

There is indeed. We could transmit colour on 405 lines on the existing VHF channels and forget about Bands IV and V except for additional programme services if, indeed, this country feels it can afford them.

11 Are there any technical difficulties?

None at all. On the contrary, certain features of the 405 lines transmission characteristic (positive modulation, amplitude modulated sound) are peculiarly suited to the transmission of colour. This is by far the cheapest, quickest, and most efficient way of getting colour TV off to a flying start with an immense potential audience on both BBC and ITV channels.

12 It is being said that the compatible picture in monochrome is inferior on 405 lines. Is this true?

Practically speaking, no. All things are. however, relative. It is true that compared with 625 lines the black-and-white compatible picture is, technically, slightly inferior as indeed is the case in normal black-and-white transmissions.

This, however, is a third order effect and would quite certainly not be noticed by the normal viewer.

All compatible black-and-white pictures from a colour channel are slightly inferior to those from a monochrome channel.

This is the inevitable price of introducing colour. It is of no practical importance.

13 Is it true that this country will not be able to sell colour receivers overseas unless we adopt 625 lines UHF standards for Great Britain?

That allegation is false. A receiver designed and originally manufactured for VHF 405-line colour and UHF 625-line black-and-white can be produced at minor additional expense in the factory for any other system. Our export prices would still be competitive. Incidentally, one of the main arguments some years ago for the adoption of a UHF 625-line black-and-white standard in this country was the same export argument. It was adopted for BBC 2 but there is no evidence of large export results.

14 If the existing BBC 1 and ITV VHF networks were converted to colour as you suggest what would happen to BBC 2?

It would stay in black-and-white … at least for the time being. Thus viewers would have two reliable colour programmes and one black-and-white instead of the present plan which provides for one unreliable colour service and two black-and-white.

The post The case for 405 line VHF colour appeared first on THIS IS REDIFFUSION from Transdiffusion.

‘Maintaining all the electrical equipment and installations at Wembley and Television House could be compared with the old story of looking after the Forth Bridge: when you finish, you have to go back to the beginning and start all over again’, says Bernie Finch, electrical supervisor. There are 62 electricians, 16 covering maintenance at Wembley and Television House. The other 46 cover all studio production lighting.

To many people the world of electronics, or just plain electricity means no more than being satisfied when the light comes on at the flick of a switch, or the television warms up and presents its picture without problem. Bernie Finch’s world, however, covers far more than just looking after light switches. ‘In the section, we’re responsible for ventilation, air conditioning, supplying power to outside broadcast units, and for all lighting, both for offices and studios. Really, we’re supplying a service to everyone.’

The power which Bernie and his team watch over, totals 2,800 kilowatts – enough to meet the needs of a small town. The intake of studio 5 alone accounts for 1,500 kW, which would supply a medium-sized village with light and heat.

‘When a company such as Rediffusion depends so much on electrical power, you have to be prepared for any emergency. For instance, when a recent power cut blacked out Television House and a large slice of the city, it was only the fact that there is an emergency alternator installed in the basement of Television House that saved the day. Within 40 seconds of the power cut the alternator was switched on, programmes could continue and essential lighting was restored. The alternator itself runs on diesel fuel, and can supply power for two days. Fully loaded, it could run for weeks.’

A little known aspect of the electrical section’s work is the supplying of special effects – especially explosions. Bernie himself has been responsible for many of the explosions that have wrecked cars in some of Rediffusion’s series.

It could be said that where the work of Bernie Finch’s section finishes, Jim Crossley’s begins. Jim Crossley is supervisory engineer (maintenance), and his men service the company’s technical equipment.

Says Jim Crossley: We look after the internal workings of cameras, sound, telecine, and film department equipment, monitors and master controls. We also maintain the outside broadcast equipment.

‘Outside broadcasts are perhaps one of the most interesting aspects of electronic maintenance. During the last general election, we converted a railway carriage on the train bringing Mr. Wilson back from Liverpool into a control room and video taped an interview which was shown during the election programme before the train had arrived in London.

‘The outside broadcast coverage of the World Cup series brought special problems as for Eurovision, all programmes have to go out on the 625 line system. Our equipment is, of course, 405 line, and we had to do a conversion job. I think that it’s to the credit of our people that the coverage went so well.’

Many programme gimmicks are designed and made by electronic maintenance. Requests vary from a simple clapometer (applause level indicator) for ‘Double Your Money’ or cues and communications for creepie-peepie cameras to the very complex. These have included designing and making a whole range of special sound equipment in a month for ‘Ready, Steady, Go!’ and rigging Studio 9 for a general election programme with all the hundreds of extra facilities required.

The main job, however, is to maintain the technical equipment. The company has probably lost less air time and had fewer faults on transmission than anybody else and that is the highest tribute to the work of the maintenance sections.

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