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.

The post How it works… A question of lines appeared first on THIS IS REDIFFUSION from Transdiffusion.

This is the first of several articles to be printed in Fusion” about the engineering side of television broadcasting. Suggestions for future subjects will be welcomed. The word “genlock’ has been frequently used, perhaps far too often by people who do not really know what it means. Here the assistant head of engineering, basil bultitude, explains what it is, why it is used, and the advantages of other forms of generator locking.

To start with, the basic system of television transmission must be understood. A television picture is made up by one small spot of light travelling over the picture screen. It first starts at the top left-hand corner and rapidly travels across the screen in a series of straight lines. With each successive line the spot moves slightly down the screen, and eventually, when 405 lines have been completed, the whole screen has been covered and one picture is said to have been transmitted.

The time taken to transmit one picture is a twenty-fifth of a second. Because of flicker, inherent in this system, two half pictures each of 202½ lines, double spaced, are actually transmitted. Each half picture takes a fiftieth of a second to transmit and is termed ‘one frame’. The light spot varies in intensity related to the brightness of the original scene; although there is only one spot of light on the screen, it is moving so fast that persistence of vision of the eye enables us to ‘see’ a whole picture.

It is obviously necessary for the spots on all television sets to be in exactly the same area of picture at any instant. For this reason, at the end of each line and frame, synchronising pulses are transmitted. These synchronising pulses ensure that the spots in the receivers and in the cameras start a new line or frame at precisely the same time.

Because of interference from the electrical power mains which makes itself felt on the received picture, it is desirable that the frame speed be related to the mains speed (frequency), i.e. fifty per second.

To achieve this, the synchronising pulse generators (SPG for short) at the studio end are locked to the mains. One might think that if several SPGs were locked to the same mains supply, their outputs would be identical. Unfortunately this is not so.

The SPG mains locking system is not rigid but is in fact slightly springy. This means that two such generators locked to the same mains supply will be continuously varying slightly in speed, one with the other, depending upon the fluctuations of mains, and upon the ‘springiness’ of their lock.

Consequently, a picture generated by a camera fed from one SPG may be running at a slightly different speed from the picture of another camera fed from another SPG. This is insufficient to cause any noticeable mains interference, but it is sufficient to prevent superimposition or mixing of these two pictures.

This is, of course, a definite requirement for a television studio, and is achieved by driving all the cameras, etc, within one building from one SPG. At Wembley the SPG is situated in the central control room and at Television House in the master control room.

Sooner or later a situation must occur, when it would be desirable for a picture signal from Wembley to be superimposed upon a picture signal from Television House. The method of achieving this is ‘genlock’. In this system, the SPG at Television House is not locked to the mains but is in fact rigidly locked to the synchronising pulses from the Wembley generator.

Thus the degree of mains lock affecting the Wembley generator will also affect the Television House generator, and in consequence their synchronising pulses should be identical. The pictures from Wembley may then be superimposed on those at Television House. It is obvious that the Television House SPG may be locked to any incoming signal operating on British standards, but only to one at a time.

Thus, it would not be possible to mix pictures from Wembley, Television House and ITN together. If, when a Television House studio or telecine was on the air the SPG was then genlocked, a considerable disturbance would be seen on the pictures. In order to superimpose, one must genlock to a source before ‘taking it’, and because this cannot be done while Television House is on the air, a serious limitation is placed on its use. Attempts have been made to genlock more quickly by a system of automatic genlocking to reduce the picture disturbance. Nevertheless, a picture disturbance of up to four seconds may still take place. Automatic genlocking facilities are now available at Television House. The actual mixing or superimposition may only take place on the station whose SPG is genlocked.

It would be possible for Wembley to genlock its generator to ITN and Television House to genlock to Wembley, thus making a sort of ‘chain genlock’, then all three SPGS. would be running at exactly the same speed. This would mean extending lines from ITN to Wembley in order to carry the locking signal and would mean that ITN could only be mixed at Wembley.

This limitation occurs because of the different path lengths of the signals. The signals travel at roughly the same speed as light, i.e. 300,000,000 meters per second, and quite obviously a synchronising pulse that went from ITN to Wembley and then to Television House would arrive later than the picture signal that went from ITN direct to Television House. Within any television studio centre the path lengths of the various signals are carefully arranged to be the same, and this process is called ‘timing the station’.

From the above it will be seen that the genlocking system, automatic or otherwise, falls far short of the ideal of being able to mix anything anywhere. From our own company’s point of view, this imposes a very serious handicap. Our programmes may come from Wembley, Television House, outside broadcast vans and other programme contractors, all working, of course, on separate synchronising generators. The commercials always come from Television House, thus twice every fifteen minutes a different set of synchronising pulses are fed to the transmitter and of course to the home receivers. The receivers thus have to take up a new sweep speed suddenly, on the cut to and from the commercials and this nearly always results in a ‘frame roll’.

Towards the end of 1958, Associated-Rediffusion engineers attempted to rectify this situation by designing at new form of genlock called Slavelock. In this system the Wembley SPG was locked by a new method to the Television House SPG so that the two stations behaved, electrically, as one. This meant that the Television House SPG could then genlock to other sources, one at a time and the Wembley generator would automatically follow.

The necessary apparatus was built and in June of this year the two stations were locked for the first time by this method. Unfortunately the results were not perfect, the picture verticals from Wembley being slightly ragged. This was due to inaccurate timing. Work had to be stopped on this project because of other commitments, which is sad when we were so near our goal. However, perhaps in the future we may again start melting the solder on this apparatus.

The above is by no means the final answer, it only permitted sources to be mixed or superimposed at Television House and not, say, at Birmingham. Other broadcasting organisations throughout the world are working on the genlock problem. The BBC some months ago suggested that, if instead of locking to the mains, they locked their individual generators to a quartz crystal, they could simplify the problem.

Another idea of achieving a ‘synchronous network’ is that each incoming signal should go through a Standards Converter. A Standards Converter is basically a television camera looking at a television picture monitor. In this system the incoming picture is displayed on the monitor and the camera operated from the local SPG, thus the final picture from the camera is in lock with the rest of the station. This system has undoubtedly much to recommend it. The loss of picture quality in modern Standards Converters of good design is much less than is commonly supposed. However, it is certainly expensive; a Standards Converter costs approximately £20,000.

This is a complex story, and in order to try to explain it I have had to take some technical licence with the explanation, and for this I hope I may be forgiven by my engineering colleagues.

The post How it works… The Problem of Genlock appeared first on THIS IS REDIFFUSION from Transdiffusion.

It would be fair to say that in recent years television broadcasts have become more complex, more polished and operate at a faster pace; this process must continue and, so far as television studios are concerned, will probably culminate in the ‘Spectacular’. Spectacular drama and light entertainment broadcasts have been shown in the past only via the film medium in this country. They have never been televised ‘live’ due to the lack of studio space and facilities. It is to remedy this omission and to present Spectacular British Broadcasts that Associated-Rediffusion has decided to lead the way by building Studio 5 at Wembley.

The Studio and its associated service areas are to be built on a site at present used for car parks and film vaults. The building will contain a studio of approximately 14,500 square feet floor area, production, camera, sound and lighting control rooms, a new canteen, entrance hall, visitors’ lobbies, dressing rooms, make-up rooms, etc.

The main feature of the Studio is to be a partition which may be lowered or raised, thus enabling the Studio to be divided into two studios of approximately 6,700 square feet floor area each. These studios will be known as Studio 5A and 5B, and will have their own associated control rooms, make-up rooms, etc. If the partition scheme is successful it will represent a major engineering achievement.

One great difficulty in planning any enterprise of this magnitude for television is the fact that production techniques and demands will most certainly have changed by the time the installation has been completed. Many traditional broadcasting practices have thus been discarded and new ideas put in their place. One example of this is to be found in the production control rooms, where the control desk is to be curved following the principle of the round table. This idea has two particular advantages to recommend it; no matter where a person sits the rest of the company is always in front of him, and also his immediate neighbours do not obscure his view or that of any others at the desk. Seated on the outside of the curve the control room personnel look inwards towards the picture monitors. The picture monitors will be 21-inch screen diagonal, because in the 1960’s it is estimated that 21-inch receivers will be the most usual for British homes, and it will be essential for the director to get the same sensation of perspective and impact as his audience.

The control room monitors will all be below eye level, so minimizing the angle between script and monitor, and avoiding the usual neck stretching which is a feature of the traditional design.

The control of the production lighting will be very comprehensive. The lamps will be raised and lowered by remotely controlled motorized hoists and the intensity of the illumination controlled from lighting consoles. The installation is designed to cope with any production likely to be brought to the studio. When the studio is used as one, the control of all the production lighting will be from one console position. Eight modern television 4½-inch Image Orthicon cameras will be available for the studio as a whole. These cameras will be a little more than half the weight of the present cameras and will open a new avenue for our cameramen in handling techniques. Two studio zoom type lenses will be available for the whole studio. When the partition is lowered, four cameras will normally be associated with each studio but it would be possible to have, say, five in one and three in the other. The design is such that should a camera channel fail in Studio 5A it will be possible to have a replacement from Studio 5B and vice versa within a matter of a minute or so.

By the time Studio 5 is completed, tri-alkia Image Orthicon camera tubes will almost certainly be available, this in simple language means that the cameras will be much more sensitive than those of today and will enable directors and lighting supervisors to create new visual moods and effects. An Inlay desk is planned for each production control room. This will have the facilities for inlay, overlay, scanning of 2 × 2 transparencies and roller captions.

Two separate sound control rooms are planned, each with a newly designed sound desk, four multi-speed gram units and a tape recorder. The sound installation will give maximum flexibility, and with very little alteration will give facilities for stereophonic sound. It is hoped that a new type microphone zoom will be available giving a full 360° rotation, and a seat for the operator.

A fully equipped make-up room will be situated adjacent to each of the two studios. This follows modern practice by getting make-up as near to the studio floor as possible. Each is to have seven make-up positions, with all the necessary wash-basins, hair dryers, etc.

By 1960 it is envisaged that the scenery will be of relatively lightweight construction, and a great deal of this, on the faster type productions, will most probably be ‘flown’. For this purpose eighteen power-operated hoists will be available plus the usual skids, etc. The hoists will be operated from a control panel on the studio floor, or from a portable handheld control board.

It is planned to have a maximum audience of 500 in the studio on certain productions, and it is hoped that 27-inch monitors will be available enabling them to view the televised show. The strain of television inevitably tells on the studio operators after a period of time. In planning Studio 5 considerable thought, time, tears and sweat have been expended in trying to reduce the operational fatigue experienced by the studio personnel today. It is hoped that this will be repaid by better and more polished productions, for although Studio 5 will, without doubt, be the finest television studio in England, it will rest with the operators and artists as to whether this venture will be a success or not.

The post Associated-Rediffusion’s new studio appeared first on THIS IS REDIFFUSION from Transdiffusion.