A Multi-Channel Television Apparatus

F, in a received television image, the individual image elements are, as they should be, of such a size as to be just indistinguishable, or unresolved, at a given observing distance, the number of image elements increases directly with the area of the image. T h e number of such indistinguishable elements in everyday scenes, in the news photograph, or in the frame of an ordinary motion picture is astonishingly large. An electrically transmitted photograph 5 inches by 7 inches in size, having 100 scanning strips per inch, has a tield of view and a degree of definition of detail, which, experience shows, are adequate (although with little margin) for the m a j o r i t y of news event pictures. It is undoubtedly a picture of this sort t h a t the television enthusiast has in the back of his mind when he predicts carrying the stage and the motion picture screen into the home over electrical communication channels. In this picture, the number of image elements is 350,000. At a repetition speed of 20 per second (24 per second has now become standard with sound films) this means the transmission of television signals at the rate of 7,000,000 per second,-- a frequency band of 3>i" million cycles on a single sideband basis. This may be compared to the 5,000 cycles in each sideband of the sound radio program, or it may be evaluated economically as the equivalent of a thousand telephone channels. When we examine what has been achieved thus far in television, we find t h a t the type of image successfully transmitted falls very far short of the finely detailed picture just considered.