Radio Broadcast (May 1928-Apr 1929)

JULY, 1928 WHAT HOPE FOR REAL TELEVISION? 127 and would obviously never register on the eye unless it happened to be repeated in successive pictures, and possibly not then. It would be so repeated, of course, unless motion had ensued in the intervening tenth of a second If there had been motion that part of the picture would be blank. This is the first difference we find between motion pictures and television. In the movies we see a blow start and see the arrival of the fist. We imagine the rest of it just as we see the successive positions of a speeding automobile and imagine the continuity. Only by speeding up the camera to get intervening snaps and presenting them in slow motion do we get detail. Each complete picture flashes at once. In television, because of subdivision into dots, only one dot is shown at a time and the mind will not retain this short flash during the remainder of the picture. We must make the screen retain the dot for us. A second distinction comes from the fact that we cannot enlarge the picture received in television with any increased detail in the result. We have chosen the minimum number of dots per square inch to give a passable image. If this is enlarged by a projecting lens, we simply sepa 48 times each second, arm energizes 50 segments across one row,2500 segments excited 18 times each second gives 45,000 light images per second Anode' Cathode '-Wire to Grid of Transmitting Amplifier CAMERA CATHODE RAY OSCILLOGRAPH THE SWINTON-CLARKSON TELEVISION CAMERA In "this device the person or object to be televised is located in front of the lens, the lens functioning to focus the reflected light from the object onto the plate. A stream of electrons from the cathode is attracted to the positively charged anode and a great many of the electrons pass through the hole in the anode plate and reach a group of photoelectric cells. In passing through the space between the hole in the anode and the plate the electrons come under the influence of the two coils A and B; coil A causing the stream to be deflected up and down the plate and coil B causing the stream to move back and forth across the cells. The image on the plate is scanned in this manner rate the dots. If we magnify the size ten times we'll have only 250 dots per square inch and only at a distance will this give the effect of a photograph. We haven't even the value of a printed half-tone where the dots are of varying size and shape as well as shade. Our dots are uniform except in shade. This may be overcome by the screen in the device described later. Now, getting back to the 3,600,000 impulses per second. This is equivalent to the modulation frequency in aural radio. The minimum frequency of the carrier would be about ten times that, or a frequency of 36,000 kilocycles, approximately eight meters. A larger picture or a better picture would drive us down to still shorter waves. A FOUR-INCH SQUARE IMAGE CUPPOSE, instead of a foot square, we make ^ the image four inches square. Even this would mean 40,000 dots for each picture or 400,000 impulses per second. Our carrier maximum would be 75 meters. Even suppressing one side band, the 400,000 modulation frequency Incoming " Image" signals from Photo-Electric cells at sender calls for a receiver to amplify evenly over a band of 400 kilocycles or as much as 40 of our present broadcast channels. This is for a tiny picture of poor quality and minimum speed. For any fast event, for a larger picture, or for even newspaper quality, what a complex receiver must be devised! One such station would blanket the entire broadcast spectrum. Go down in size and quality, if you will. A three-inch square picture with25 modulations to the lineal inch or 625 dots to the square inch, means only 56,250 impulses per second. The carrier could be as high as 535 meters but our tuning and amplification would be over five channels 10,000 cycles wide. To get within the legal separation of stations we can use a modulation of only 5000 impulses per second which, for a barely recognizable image, would give us only one square inch, remembering that we must send 10 pictures a second. What, then, is Baird in England doing, for example? Obviously the only thing he can do and basically the only thing that has ever been done in television. That is, to select as the object to be televised, something that has few gradations of light and shade and extremely slow movement. This is the human face. It is a familiar object, almost entirely white space with the shadows around eyes and nose very ill defined and their outline of no particular importance in recognition. The cartoonists have taught us that we need nodetail of afaceto recognizethe person. There is always some outstanding characteristic that suffices. Slight blurring would rather soften the result instead of spoiling it. Television is not achieved merely because seeing faces at a distance has been and will be accomplished. It was in recognition of this fact that the English publication Popular Wireless unsuccessfully sought to induce Baird to televise a simple cube in slow motion to win the sum of $5000 that magazine offered. That Baird ignored the challenge must merely mean that he, too, recognizes the limitations of his apparatus. There are other problems besides that of time in its relation to the defects of vision. There is the question of synchronizing the mechanically moving parts of transmitter and receiver. When things happen in the hundred-thousandth part of a second, there is need for absolute accord on both ends of the line. If the same power line is available at both ends, synchronous motors may be kept in step, but this exists only in few localities and over short distances. Synchronizing by this means it is not a real solution of the problem. THE REAL DIFFICULTY A S THIS brief review indicates, the real drawback is the fact that the picture must be subdivided and sent as a sequence of impulses. We would face similar difficulties with sound broadcasting if, for example, we had to send the whole of an opera selection as one blare of noise in a tenth of a second. It could be done, of course, by securing a sufficient number of musicians so that each need sound but one note. Then, at a given signal, every musician in this enormous 50 convolutions of Neon Gas filled Tube Potential sufficient to cause Neon Gas to glow THE BELL TELEPHONE LABORATORIES TELEVISION RECEIVER A large neon tube forms the basis of this television receiver. On the back side of the tube 2500 segments of tin foil are cemented, connecting by means of individual wires to 2500 segments on the commutator, which revolves in synchrony with the apparatus at the transmitter. The incoming signals, modulated in accordance with the shading of the subject being transmitted, are amplified and cause segments of the neon tube to glow with a brilliancy dependent upon the shading of the subject being transmitted orchestra would sound his note and go home. We would receive the opera selection but it would hardly be worth while. Yet this is exactly what the eye demands in television. One thought has been to divide the object into units. That is, in terms of pictures, not to send the whole picture over one carrier but, in effect, send, say, 144 pictures, each an inch square, and at the receiver these would make up into a single image one foot square. This was one of the first ideas suggested as long ago as 1880. Carried to its extreme, perfect results would be achieved but, within the limits of costs and apparatus, we merely multiply our troubles. Sixteen pictures 3 inches square might be managed if we could send anything but a crude 3-inch-square picture. It would be at 16 times the cost and 16 transmitters as well as 16 receivers would be needed and all would have to be synchronized. Along similar lines was Doctor Alexanderson's bundle Coil A f = 10 Perforated Stop Plate :with fixed opening Florescent Screen Deflecting Plate Wiretrom . Receiver Amplifier CAMPBELL SWINTON TELEVISION PROJECTOR This television projector, sometimes called a television receiver, uses an arrangement similar to that incorporated in the Campbell Swinton camera suggested in 1908. The electron stream from the cathode is caused to scan the fluorescent screen due to the action of the coils A and B, their intensity being varied by means of the two deflecting plates. Potential from the receiver amplifier is impressed on the deflecting plates and causes the number of electrons passing through the opening in the stop plate to vary in accordance with the image signals from the transmitter. The screen at the left of the camera becomes fluorescent under the action of the electron stream and the image then becomes visible to any one standing in front of the screen