TELEVISION

Evening Post, Volume CXVI, Issue 77, 28 September 1933, Page 21

TELEVISION

A NEW IDEA AT LAST

At last it looks as though .a .new and promising line of attack on the television problem has been discovered. A vast amount of experimenting has been done, over many years now, without making muck progress towards really satisfactory, results. This work must have cost, an enormous amount of money, but so important is the quest, and so fascinating the inquiry, that there has been plenty of money and energy to bo applied to it. Progress has been hampered by two fundamental difficulties, both related to the fact that for the production of a detailed image an enormous number of "elements" of the- picture have to ba transmitted. On the one hand , this necessitates very high frequency modulation of tho carrier wave by which the imago is broadcast, and this results in each transmission occupying a wide band of the radio frequency spectrum. It has long been recognised that this difficulty has removed the ■possibility of television on tho wavelengths used for regular broadcasting, but the short and "ultra-short" wavelengths afford promise of practicability. On the other hand tho problem of subdividing the image into elements has been attacked chiefly by mechanical means, and no mechanism has been devised that conies anywhere near doing the job. The well-known "scanning disc," an invention that long antedates radio, has been used in one form or another. Exploiters of this system' have made enthusiastic claims for success, but the wary have always sprinkled plenty of salt on them before taking, an.d the scanning "disc can be written down as falling far short of what is wanted. It has been able to deal only with small pictures coarse in detail, and magnification of the result only emphasises the coarseness.

The limit, of a purely mechanical scanner being evident, attempts hayc been made to utilise electronic discharges, ia what is called tho cathode ray tube. This device, however, has i been u^ed more as a receiver than as a ; transmitter. In tho cathode ray tubo of tho usual form, electrical elements are so arranged that v a stream of electrons •• attracted from a-hot cathode are impelled by a highly positive anode at such a velocity that they shoot past the anode to the end of the tube and there strike a treated screen surface which, glows under the impact. As they first pass through a tube, the electrons, form a narrow beam, so that-only a spot on the screen glows. Additional electrodes inside the tube, or coils'outside it, are used to deflect .the beam, and as it is' weightless and its velocity is enormous,-these deflections can be made to succeed one another at extraordinarily high speed.

The cathode ray tube has now been combined with photo-electric material into an entirely i.ne.w 'television- transmitter of which the possibilities seem most promising.'

It is characteristic of somo photoelectric substances that they acquire a electric charge when illuminated; and an electron beam, being composed of negative electrons) will immediately neutralise such a charge. These two facts are the basis of tho invention. A screen consisting of a sheet of insulating material, coated with a conducting layer, on the back, has its front covered with a mosaic of minute plates, of photo-electric material. The electron beam can, by external controls, be made to play over the whole mosaic surface. Outside the tube is a photographic lens, which focuses a. scene upon the mosaic screen. The plate at the back of the screen is connected to an amplifying system.

Now if a picture is formed on the screen by the lens, every element in the mosaic will acquire a charge, proportionate in quantity to the light falling upon it, and the aggregate charges will induce an equal charge upon the back plate. The discharge of any one of the elements will alter the charge on the back plate, and will thus operate the amplifier. If they are discharged in sequence the amplifier will receive impulses corresponding to the illumination of the discharged elements in the same order.

The electron, beam is, by means of suitable electric oscillatorSj made to sweep, or scan, the illuminated surface, and as this can be done at a rate enormously higher than is possible with any mechanical scanner, the picture can be analysed with a fineness which is limited only by tho sizo of tho elements of the mosaic and the fineness of, the electron beam. The resulting output can, of course, be used in established, ways to modulate a carrier •wavc-j and on reception the modulation can operate a reproducer, which may also be a cathode ray device.