Television is a science which offers unlimited opportunities to the engineer and the inventor. The television station at Alexandra Palace, London, was the first in the world to provide a regular daily service to the public
A STUDIO SCENE at Alexandra Palace. The performers work in the glare of powerful floodlights, and microphones are suspended above them. On the left is the booth in which the scanning apparatus is operated.
The television aerial tower at Alexandra Palace, in North London, is sufficiently futuristic in appearance to justify its connexion with one of the most recently developed branches of science. London’s television station was the first in the world to broadcast regular television programmes for entertainment purposes, and it has attracted the attention of scientists and engineers from all over the world.
Radio broadcasting is now so firmly established and so devoid of novelty that the radio engineer works quietly in the background, untroubled by the searchlight of publicity. The television engineer, however, is still experimenting, and everything concerned with the newer science is of tremendous interest to the public, which feels that the work being done to-day will mean much in the years to come. Television is still in the pioneering stage - a stage in which the research engineer finds unlimited scope for his skill.
Alexandra Palace stands on a hill 306 feet above sea level and is a notable landmark in London, visible from a considerable area of the Home Counties. The south-east tower has been entirely rebuilt and on the “stump” of the old tower there now stands a tapering lattice steel mast 220 feet in height. As the brick tower is 80 feet high, the summit of the mast is more than 600 feet above sea level. Height is essential for the successful transmission of television over long distances.
The British Broadcasting Corporation has leased more than 30,000 square feet of floor space from the Alexandra Palace Trustees. This space comprises three large halls in the south-east wing, the rooms above them on the first floor and the south-east tower. On the ground floor the halls have been converted for use as transmitter rooms. The first-floor rooms are studios, control rooms and apparatus rooms, and five floors of the tower house the offices.
This surprisingly comprehensive transmitting station includes also dressing rooms and make-up rooms for band and artists, a film-viewing room, a restaurant and kitchen, floors for the preparation and storage of scenery, and a boiler house for the heating system, which has been specially installed. The technique of television is such that a transmitting station is of a far more complex nature than an ordinary broadcasting station. The general atmosphere suggests a strange combination of a broadcasting station and a film studio. At the time when the Alexandra Palace station was built, it was necessary to make provision for two separate systems of transmission - the Baird and the E.M.I. systems, developed respectively by Baird Television, Ltd., and by Electric and Musical Industries, Ltd. The sound transmitter could be common to the two, as also could the aerials. Two separate transmitters and two separate studios, however, were necessary, and the layout of the Alexandra Palace lent itself admirably to these conditions.
The two main studios, one of which was used for the Baird system and the other for the E.M.I. system, were 70 feet long, 30 feet wide and 25 feet high. Their walls were covered with asbestos sheeting, which has a high degree of sound absorption. This was necessary, in view of the large area of the studios, to prevent the acoustic troubles which have long since been surmounted in ordinary sound broadcasting. The ceilings were treated with building board, again as used in broadcasting studios, and the floors were covered with black linoleum over which any other type of flooring might be laid.
Transmissions of different types require the use of varying numbers of microphones, and microphone points were installed in several positions in the studios. Portable stands of an expanding type were also installed.
THE CONTROL DESK AND AMPLIFIERS of the Baird transmitter, which was in use until February 1937. On the right is the power output stage. Immediately in front of the engineer, under a small semicircular hood, is a cathode-ray tube used for monitoring purposes. On its screen the control engineer may see a facsimile of the picture being transmitted.
Each studio was fitted with two stages. Because of the different requirements of the two transmitting systems for which they were designed, the detail arrangement of these stages varied. In one studio they were separated by a great steel lighting bridge, which allowed powerful batteries of lamps to be trained in either direction.
A large lighting switchboard for either studio permitted the engineers to maintain separate control over every lighting circuit. The whole arrangement was designed to give the absolute maximum of flexibility and to make possible the dimming or selective switching of any desired number of circuits. In February 1937 it was decided that the Marconi-E.M.I. system should be adopted, and the need for two separate transmitters and two studios was eliminated. The internal arrangement of the studios was modified and they may now be used simultaneously for transmissions which call for this arrangement.
All the lighting is of the ordinary incandescent lamp type, similar to that used in film studios. Spot and flood lighting are used, and many modifications are necessary to accommodate the lighting to the requirements of television.
Heard in South Africa
High up in the wall of the studios is a large plate glass window, behind which the producer and the control engineers operate. They have a comprehensive view of all that is going on within the studios, and can control the entire transmission from their point of vantage.
The studios are ventilated by extractor fans which suck the air through gratings fixed in the ceilings. The intakes for fresh air are openings in the upper part of the windows, and are fitted with filters to clean the air and to deaden outside noises. During performances the lower parts of the windows are covered with sound-proof shutters. Efficient ventilation is a necessity in television studios, as the powerful lighting used during performances radiates a considerable amount of heat. The maximum energy used for lighting may approach 50 kilowatts.
Near the centre of the ground floor section of the station is the room housing the sound transmitter. This operates on a wavelength of about 7·2 metres with an output of 3 kilowatts. It was this transmitter which astonished the whole radio world by making itself heard in Johannesburg and in Capetown, South Africa, although theorists had stated that the range of such a transmitter would probably not exceed twenty-five miles. The transmitter is built in four separate units, each housed in a metal cubicle. “Battleship grey and chromium plating” is the first impression that the visitor receives, and certainly all the Alexandra Palace equipment has an extremely well-finished appearance.
On ordinary broadcasting wavelengths it is necessary to restrict the width of the band of frequencies radiated by a transmitter, to avoid interference between one station and its neighbour in the ether. On the ultra-short waves used for television the necessity does not arise, and this sound transmitter has been specially designed to give an exceptionally high quality of reproduction, which is possible only when there are no restrictions at all on the band of wavelengths that may be radiated. The response of the transmitter is even, from 30 to 10,000 cycles a second, a band which contains all the frequencies necessary for the high-quality transmission of speech and music.
The use of ultra-short waves brings special problems in its train. One unit of a transmitter is far more prone to interact with another on these wave-lengths than on ordinary broadcasting wavelengths. Thus particularly efficient screening between the various units is necessary.
The outgoing energy is fed to the aerial, high up on the tower, by a specially designed system of feeders in concentric copper tubes. It is essential that the feeders should not radiate energy, but should merely convey it to the aerial from which it is to be radiated. This concentric system is the most efficient that has yet been evolved.
The main high-tension supply, at 6,000 volts, is supplied by mercury-vapour rectifiers, fed from a step-up transformer connected to the outside supply. The filaments of the valves are heated by direct current from a set of motor generators which have an output of 300 amperes at 20 volts.
In the centre of the sound transmitter hall is the control table, from which one operator is able to manipulate all the various power supplies to the transmitter. Switching operations are all carried out by remote control, and the inevitable precautions are taken to safeguard the staff and the apparatus. Should the power supplies be switched on in the wrong order, nothing will be damaged, because interlocking switch gear protects the apparatus.
“Persistence of Vision”
An exceedingly interesting device in use is known as a sequence-starting switch, which ensures that sufficient time elapses between the application of each successive power supply, and allows each bank of valves and other apparatus to become properly warmed before the main high-tension voltage is applied.
The large output valves are of the water-cooled type, and should the water supply fail the whole apparatus is automatically switched off by a special water-flow checking device. The transmitter cannot be started up again until the defect has been found and remedied. The staff cannot enter any of the transmitter units without switching off all high-tension supplies and earthing the apparatus, and the supply cannot be switched on again until all gates on the transmitters have been closed and locked. If these precautions and complications are necessary for the sound transmitter, which does not emerge from the realms of ordinary broadcasting practice, it may be imagined that the vision transmitters are even more complex in their nature. In practice, however, the radio portion of the vision transmitters is relatively straightforward. Its function is merely to transmit a steady “carrier-wave”, on which is superimposed the series of impulses which represent not sound, but pictures.
The complicated part of a television transmitter is the apparatus which translates a picture or a performance into electrical impulses which can be sent through the ether and reassembled at the receiver to form a true picture. This apparatus is in the studios, not in the transmitter halls.
AMPLIFYING STAGES of the Marconi-E.M.I. vision transmitter. The pictures have already been superimposed on the carrier-wave, and the function of the high-frequency units shown is to amplify the entire complex output from the earlier stages before it is passed on, by way of the feeder system, to the aerial.
In sound broadcasting the first link in the chain of transmission is the microphone. This is a surprisingly compact and simple piece of apparatus which converts sound waves into electrical vibrations. In principle it does not differ from the mouthpiece of the ordinary telephone. The electrical vibrations which proceed from the microphone are amplified and are finally fed to the transmitter, which superimposes them on the carrier-wave - the steady wave which is always radiating from the aerial of a broadcasting transmitter, even during the short intervals of silence between programmes. The receiver picks up the carrier-wave, modulated by the electrical vibrations corresponding to the sounds in the studio, and it converts these impulses back to intelligible sound.
The process of television, in a way, may be regarded as a parallel, but it is far more complex. There is no more difficulty in transmitting a chord played by a full orchestra than there is in reproducing a single note on a tuning fork. The chord played by the orchestra is transmitted as a composite whole. It is impossible, however, to transmit a picture as a whole - even if the picture is merely of such a simple object as a chessboard. Each light and dark square must be converted into the corresponding electrical impulse, and each impulse must be transmitted separately through the ether; and yet, at the receiving end, the impulses must be reassembled to give the impression of a chessboard and not a series of light and dark squares following one another.
The phenomenon known as “persistence of vision” makes television possible, just as it has made possible the cinematograph. For the television transmitter, radiating its picture of a chessboard, does transmit the light and dark squares in a sequence, and the receiver does pick them up in a sequence; but so quickly is this sequence reassembled that the eye sees, on the viewing screen, a complete chessboard.
Just as photographic reproductions in newspapers, viewed through a magnifying glass, resolve themselves into unintelligible groups of dots of varying size and intensity, so does a television picture consist of a tremendous number of light spots. Each spot is transmitted separately through the ether. The process involved, which may be carried out in several different ways, is known as “scanning” the picture.
The earlier systems of scanning used a rapidly-moving light-spot at the transmitting end. Were the chessboard to be transmitted, as an experiment, a light-spot could be arranged to travel from left to right along the top row of squares; then to begin again at the left-hand end of the second row, and each succeeding row in turn, until the sixty-four squares (thirty-two black and thirty-two white) had been transmitted as a sequence. Possibly the entire scanning operation would be repeated twenty-five times in one second.
At the receiver there would be a piece of apparatus which would reassemble these rapid alternations of “light” and “dark”, represented by electrical impulses for the light squares and periods of “silence” (to introduce the analogy of sound broadcasting) for the dark squares, into the facsimile of a chessboard.
The sound broadcasting analogy is useful for an understanding of the basic principles of television, and it is easy to imagine a brilliantly-lighted portion of the picture as representing an extremely loud sound, and a completely dark portion as dead silence. The various intervening grades of light and shade would be represented by all the sound intensities between pianissimo and fortissimo. By receiving the vision signal on an ordinary sound receiver it is possible to reproduce the transmitted picture in terms of sound; but at such a speed is the scanning operation carried out that it would be quite impossible to note the succeeding variations of light and shade by aural means.
FILM TRANSMITTER, incorporating a Marconi-E.M.I. Emitron camera (in the right foreground). Films are projected directly on to the screen of the camera, which is so arranged that negatives or positives may be run through the projector. When negatives are used the camera may be “inverted” to give a positive output.
The number of lateral sections into which the picture is divided by the scanning operation governs the amount of detail that can be reproduced at the receiving end. It also has a considerable influence on the complexity of the transmitting apparatus. The system at present in operation at the Alexandra Palace uses 405 scanning “lines”. No longer can the studio be equipped with a powerful spotlight which scans the picture. That simple method was satisfactory in the days of 30-line television, but is far too primitive for the modern system.
Thus it is that a piece of apparatus has been devised which is the exact counterpart of the microphone. It converts light-values into the corresponding electrical impulses (as the microphone converts sound-values), but it also scans the scene to be televised at the same time.
This apparatus when it was first developed, was popularly known as “The Electric Eye”, but it is generally referred to now as an Emitron camera. The scanning operation is now carried out not by a spotlight, but by a beam of electrons in a cathode-ray tube passing over a mosaic plate on which is reproduced, by an optical arrangement, the scene taking place on the stage. The mosaic plate is covered with minute light-sensitive particles, each of which is, in effect, a photo-electric cell. Thus the beam of electrons may be described as producing a separate reaction on each of these minute particles, in accordance with the degree of illumination caused by the particular part of the televised scene for which that individual particle is responsible.
The Emitron camera is sufficiently sensitive to be used either in normal daylight conditions or with illumination comparable with that generally used in film studios. The output “signal” from a television camera is so minute that it has to be amplified more than two million times before being passed on to the transmitter.
The initial amplifier is housed in the camera, which, in appearance, bears a certain resemblance to an ordinary film camera. This amplifier magnifies the minute signals from the light-sensitive plate of the camera, and they may then be passed along considerable lengths of wire before being amplified further and passed to the transmitter. Several such cameras are in use in the television studios. Two or more may be used for a single transmission, and the control-room engineer is able to “fade-in” from one camera to another for dissolving views and transition scenes.
THE TRANSMITTING AERIAL TOWER, standing on the rebuilt south-east tower of Alexandra Palace. The vision aerial system is suspended between the upper two sets of arms, with the sound aerial system below. Eight aerials and eight reflectors are used in either system to make the transmissions omni-directional. The lattice steel mast is 220 feet high, and its top is more than 600 feet above sea level.
The invention of the Emitron camera has been one of the most important steps in the progress of television, for all the complicated scanning apparatus is now contained in the one small unit. Further, it allows the use of cinema technique in the studios.
Films may also be televised, and the present transmissions make considerable use of them. It is an easier matter to “scan” a film than a complex stage scene, and so satisfactory are the present methods that films may be reproduced by television over a distance of several miles without a serious loss of detail. The Emitron camera is used for the direct transmission of films, which are projected straight on to the screen or “plate” of the camera.
A system known as the “intermediate-film” method was in use at Alexandra Palace until the end of 1936 for the transmission of scenes from outside or from the studio stage. The scene to be televised was first filmed by an ordinary cinema camera. The film was developed and printed within a few seconds and the film - not the scene - was scanned and televised. This system was one of the best until the Emitron camera reached its present high stage of perfection, and may conceivably be of some importance in the future for the televising of scenes which are unsuitable for the Emitron camera.
The vision transmitters at Alexandra Palace may be regarded as ordinary radio transmitters, which radiate, into the ether, the highly complicated signal which represents a scene cut into a large number of lateral lines and repeated twenty-five or fifty times in a second. They use a higher power than the 3 kilowatts of the sound transmitter, and, once more, a system of concentric feeders couples them to the aerial system.
The tower carries two separate aerial systems, that for the vision signal being at the top, with the sound aerial just beneath it. Either aerial consists of eight radiating elements and eight reflectors, arranged symmetrically round the tower. Aerial design has a considerable bearing upon the working range of a transmission on these ultra-short wavelengths, and much research work has been carried out by the B.B.C. Engineers.
The building of the aerial tower presented some interesting problems. The height of the steelwork above the brick tower is some 220 feet, and of this a height of 105 feet is tapered. The section is square, the sides of the square being 30 feet at the bottom and 7 feet at a height of 105 feet from the bottom At this point the section changes from a square to an octagon with 7-feet sides, suit the special arrangement of the eight pairs of aerials and reflectors.
During a gale the force of the wind on the upper sections of the mast is tremendous, and special means were adopted to transmit this load to the brick tower underneath. Four steel girders, each 30 feet long and 7 ft 6-in high, were placed on the top of the brick tower. The four legs of the mast were bolted to the corners of this square, and each comer was embedded in some 17 tons of concrete to act as a counterbalancing weight, and to absorb some of the vertical lift of each leg in gale conditions. At each comer a heavy bar, 50 feet long, was carried down inside the brick tower and, after having been subjected to a tension of some 30 tons, was firmly connected with the brickwork of the tower.
An extremely violent gale was experienced soon after the station had been put into regular operation, and it had the effect of temporarily putting one of the aerials out of action, but no damage was suffered by the mast or by the tower.
Two 5-in concentric feeders connect the aerials with the sound and vision transmitters. Since the unbelievably complex signal representing a stage scene or a film requires a much wider “slice” of the ether than does a transmission of a sound broadcasting station, it has been necessary to use the ultra-short waves for television. Only in this part of the radio spectrum is there sufficient space for television to be transmitted.
Problems of Short-Wave Transmission
This necessity has brought its own problems with it, chief of which is the limited distance over which wavelengths of 6 or 7 metres can normally be transmitted. As the wavelength becomes shorter, so does the radiated wave show a greater tendency to be absorbed by the ground over which it travels. These wavelengths, some years ago, were known as “quasi-optical”, because they appeared to travel little farther than an ordinary light beam transmitted from the same source. If the receiver could not “see” the transmitter there was a distinct possibility that it would not receive signals from it.
This theory has been exploded, and it has been found that signals transmitted from a favourable position, sufficiently high above surrounding country, cover distances considerably in excess of the optical range. The reception of the Alexandra Palace signals in South Africa has no bearing on this matter, since that could only be possible through the existence of a reflected wave which had left the earth’s surface, encountered one of the ionized layers in the upper atmosphere and been reflected or refracted back to a point many thousands of miles away on the earth’s surface.
The local range of the Alexandra Palace station, however, has proved to be greater than was anticipated. Twenty-five miles was the figure freely stated when the service was inaugurated, but sound and vision have been received at distances in excess of sixty miles.
Television and ultra-short-wave radio, now inseparably linked together, are two of the newest branches of the science of radio communication, and much research and development has yet to be done. They offer an unparalleled field for the ingenuity of the inventor and the engineer, and any kind of prophecy about the future is largely guesswork.
The London Television Station was one of the first practical results of the television engineer’s extraordinary skill, and is a monument to the amazingly rapid development of one of the most modern branches of engineering. In spite of this rapid development, it cannot be denied that television is still in its infancy.
CONTROLS FOR VISION AND SOUND at the desk in a control room. Continuous checking of the outgoing programme is carried out, and the engineers in charge see and hear this programme as it would be received by radio at a short distance. From control desks such as that shown, changes of scene, dissolving views and fadeouts may be arranged.