“Television is the art of instantaneously producing at a distance a transient visible image of an actual or recorded scene by means of an electrical system of telecommunication.”

The wording of the British Standards Institution’s definition of our work, gives a somewhat unimaginative description of an art form that has, perhaps, a most complex method of construction to produce small black- and-white shadows, yet also has a high degree of human attraction.

The first process in any television system the creation of an optical image of the scene to be transmitted. This optical image is formed in the case of cameras by the camera lens, and is focused by it on to the face of the pick-up tube inside the camera. It is then the function of the pick-up tube to convert the light image into an electrical current which varies in magnitude, in proportion to the amount of light reaching the tube from each part of the scene at any given time.

The exchange of light for an electrical current is the principle upon which a photographic exposure meter works. The tiny light-sensitive cell in an exposure-meter gives a small electrical current in direct proportion to the average light value of the subject which it is seeing and registers this visually on a scale.

This device, invaluable for photographers, is useless for television because it cannot convey information about the detailed structure of the scene. In the past many methods were tried, using the light-sensitive or ‘photo-cell’ as it is called, to break up the picture into small parts and to allow the photo-cell to look at one part at a time in strict sequence and to register a separate value of current for each. This is similar to the way in which the eye ‘scans’ a printed page, gleaning different information word by word and line by line.

Baird scanned his scene by means of an opaque rotating disc containing a series of holes in the form of a spiral placed in front of his photo-cell. The spiral was arranged so that, as the disc revolved, each hole in turn swept across the picture, allowing the photo-cell to see a small, continuously varying part of the scene. When the last hole had completed its scan across the bottom of the picture, the first appeared and started the operation at the top again. The current from the photo-cell was proportional at any instant to the amount of light reflected through the hole from the scene.

Another method reflected light from the scene by means of a revolving drum containing mirrors set on the outside in a staggered formation. When the drum revolved at high speed, each mirror reflected into the photo-cell light from a slightly different part of the scene each time.

These mechanical methods, however, suffered from extreme clumsiness of operation and poor definition because of their inability to scan the scene in small enough elements. A ‘major break-through’, as the popular press would now call it, was the innovation of a vacuum tube with electrical scanning.

This tube, rather like a large valve, was developed in this country in the early 1930’s and brought the first high- definition television system in the world into operation in 1936. The tube was called an Emitron or Iconoscope and was cumbersome, insensitive and, by today’s standards, somewhat crude in operation, but was in fact the parent of most present-day camera tubes.

In appearance it consisted of a cylindrical glass tube about 14 inches in diameter, one end of which blossomed into a bulb about 7 inches in diameter. The bulb had a flat window on one side and inside, parallel to the window, was mounted a ‘target’ upon which the optical image of the scene formed by the lens was focused. The tube joined the bulb at a slight angle and contained a device like a gun, which in effect fired a tiny stream of electrically-charged particles at the target.

This fine beam of charged particles called ‘electrons’, was made to sweep over the target in straight lines one below. the other, starting from top left of the picture through to bottom right and back to top left again. This sweeping or scanning could be achieved electrically at enormous speed and in fine detail because electrons are so small that they are almost weightless. At first sight this may appear to be unconnected with our original problem, which was the inability of the photo-cell to distinguish between the brightness details of a picture. In order to see in what way this can be useful, we must consider more closely the function of the target.

The target (so called because the optical image lands there as well as the electron beam) is made up of a sheet of mica covered on the front with a mosaic of many thousands of tiny photo-cells, each one insulated from each other and able to work independently. The mica sheet is then backed by a metal plate.

The situation is now that the optical image projected on to the target by the lens is not falling on one photo-cell but on many thousands so that each one is receiving an amount of light in proportion to the brightness of that particular element of the original scene. Part of our problem has, therefore, been solved, as the scene can now be divided into many small elements and each photo-cell can give a separate value of current.

As always, of course, there is a snag. What was an advantage at first sight now becomes a problem, because as soon as light falls on to the mosaic all the photo-cells merrily start to produce a current which they store rather like a battery. This current is not all required at once, and this is where the beam of electrons comes into its own.

As it scans the mosaic, it acts like a switch and touches each photo-cell in turn, causing them to discharge their stored current to the metal plate at the back of the target.

This current can be collected and forms the signal output of the tube representing at any given instant the discharged current of one photo-cell and, therefore, the electrical equivalent of one picture element. As this is changing extremely quickly there is a continuous output from the tube which is called the ‘video signal’. This signal is amplified and processed in many ways to become the transmitted picture, which when re-created by the receiver builds ‘a visual image of an actual or recorded scene…’ as in the definition.

This then was the first attempt at high definition tele- vision and many improvements followed fairly rapidly. Perhaps the most significant was the separation, in the pick-up tube, of the two functions performed by the target of electrical image creation and scanning. This was achieved by placing in front of the target a semi-transparent sheet having photo-electric properties which gave an electrical magnification and greater sensitivity. This type of tube was called an image iconoscope. The construction of the target was also modified to have a greater electrical storage, i.e. a larger battery, which was then scanned in much the same way as before.

These tubes were in use for a number of years and were in fact still used after the war.

Still further increases in sensitivity have been made and more efficient and complex tubes are now used by most television broadcasting organisations. The ‘image orthicon’ which is used for television broadcasting cameras, is perhaps the most versatile and widely used tube today. It has such sensitivity that in certain cases it will give a reasonable picture by moonlight. Its method of signal production does, however, differ from the image iconoscope but the same basic principles apply to both.

Another new tube, the ‘Vidicon’, is used extensively in telecine machines and industrial cameras and indeed some of the most significant developments of recent years have been in the field of vidicon tubes which, because of their small physical size and relative cheapness (£25 as against £450 approximately for an image orthicon), have been much favoured and will doubtless become increasingly useful in a wider field when certain fundamental snags are overcome.

Television cameras have many uses in fields other than broadcasting and one of the most significant has been their introduction into the field of medicine. Here, their use is obvious as a means of allowing many hundreds of students and nurses to view on closed circuit a major surgical operation, often in colour, which only a few at a time could normally watch in an operating theatre.

Additionally, because of their sensitivity under certain modified conditions of working, television cameras can be used to intensify an X-ray image in order to keep the ‘dosage’ of X-rays to a minimum for the patient while still providing a satisfactory picture under deep penetration conditions.

The use of television cameras in space-satellites is only just beginning and recent developments have been rapid indeed, as has been similar progress for the armed forces. So, from the humble photo-electric cell and Baird’s scanning disc of the 1920’s, has grown a world-wide industry employing many hundreds of thousands of people of varying skills which, I think it is fair to say, can give pleasure, entertainment, instruction and a means of research on a scale quite undreamed of even 30 years ago.

Of the future we can only make an intelligent estimate based on the rate of growth to date and the present state of the art and science.
That international exchanges of vision and sound will take place on a day-to-day basis and that trans-Continental networks will be available for world events is obvious from present indications.

That many future transmissions will be in colour and possibly stereoscopic as well would be a natural extension to the reality of today’s programming.

That more and more use of closed circuit viewing will be likely perhaps for specific programmes on a wired ‘pay as you view’ service, and eventually as an added facility to the telephone service.

Whether television could be made cheap enough for domestic use ‘See if junior is asleep in the nursery’ type of thing is debatable at present. Certainly a flat picture-frame type of viewing tube is possible and indeed in development at present.

That we shall explore, remotely at first, the outer edges of space and the depths of the oceans is again highly likely. In all these projects, however, the television camera tube has played and will continue to play a vital part in what is, perhaps, the most exciting medium of communications of our age.

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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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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.

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The first use of the word telecine seems to have been lost in the mists of television antiquity. It indicates the conversion of any type of film picture to a television signal suitable for transmission.

It may surprise most people to learn that the one thing that television cannot do is to transmit a complete television picture.

This may sound utter nonsense until it is remembered that the television picture we see only really exists in the mind of the viewer because of nature’s foresight in providing the human eye with a degree of persistence of vision.

Once this faculty of the human eye was recognised it became possible to evolve a number of systems capable of producing animated pictures from a series or sequence of completely still pictures – each picture had a slight positional displacement of image corresponding to what would produce normal movement.

As practically everyone must know, the projection of a picture with movement, whether it is through a home movie or a cinema projector, is the result of a series of transparent still pictures being shown in sequence.

Each time a picture is flashed on to the screen it is a complete picture in every sense of the word. But with our present television system it is not possible to transmit at any one time a complete picture, so an alternative method had to be evolved. In this television system a picture is continuously being built up on the viewing tube, by a very fast-moving, single spot which sweeps from top to bottom in a series of horizontal lines. The brightness varies according to the picture information it carries.

The television system, therefore, examines the picture being televised in a systematic manner by scanning every point of it and at the same time sending out a signal corresponding to the tonal value of that part of the picture being examined at that particular instant.

So cinema film projectors and their television counterparts have, for the most part, little in common with each other.

The name telecine has now become synonymous with the transmission of all film material, and all film transmission equipment is now known generally as the telecine machine or film scanner.

At present there are two alternative methods of transmitting a film picture.

The first is known as a ‘Vidicon Storage System’ and the second as the ‘Flying Spot System’. Without going into too much technical description, the Vidicon Storage System basically consists of the normal type of intermittent motion film projector which throws its optical film image on to the face of a small, special television camera pick-up tube called a Vidicon.

The optical images formed on the Vidicon tube face instantly form an equivalent electrical image on a photo-conductive layer called the target, and from then on the electrical image is examined or scanned in practically the same way as an ordinary television studio camera tube transforms a live studio scene into a television picture.

The telecine projector shutter has to be modified from its usual method of operation so that the optical image of each film frame is flashed on to the vidicon tube face during a very short period – called the frame blanking period. This is the precise period of time allowed for the fly-back of the electronic scanning beam.

The vidicon target has the fortunate property of being able to retain its electronic image long enough (hence the term storage system) for one complete scanning sequence to take place and thereby produces all the necessary information to build up a television picture.

While the retained electronic image is being scanned within the vidicon tube, two other important operations are simultaneously taking place. They are: 1. The shutter is being made to cut off the light source completely, and 2. The projector mechanism is utilising this longish period of scanning time to move the film forward on to its next stationary frame position so that it can be subsequently flashed on to the vidicon tube face.

The second method utilises the flying spot principle and here we use a very much more straightforward scheme. Basically we illuminate only a very small part of the film picture at a time. This is done by passing through the film a small intense point of light produced by a special type of cathode ray tube called a flying spot scanning tube.

This intense spot of light scans the film picture area completely and emerges on the other side of the film with a variation in brightness corresponding to the density of the image it has had to pass through.

The modulated spot of light is then directed on to a device known as a photo-electric multiplier cell. This device reacts to light by generating electric currents directly proportional to the strength of light falling on it.

It is these changing amounts of electrical energy which, when transmitted in proper sequence, tell the home television receiver when and where to produce whites, greys or black tones.

The Associated-Rediffusion telecine section is very versatile in that it can handle practically any combination or type of film material available, be it either 35 mm or 16 mm. At present the telecine section is equipped at Wembley with one Cintel, two E.M.I. flying spot scanners and one R.C.A. vidicon film channel, known respectively as Cintel 6, E.M.I. 1, E.M.I. 2 and R.C.A. 4 machines. Television House is equipped with one E.M.I. and two R.I. flying spot scanners, besides one R.C.A. Vidicon film channel. These are known as E.M.I. 5, R.I. 7, R.I. 8 and R.C.A. 3 machines.

The standard or professional film is 35 mm, while 16 mm is generally known as substandard.

The 35 mm film can be either married, that is with picture and sound all on one film, or double-headed, sometimes called unmarried. This means that the picture and sound information is on two separate film reels but, of course, carefully synchronised. The separate sound track can be either magnetic or optical.

But now we are moving away from our subject. I hope the above has helped you to understand telecine a little better.

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