A Multi-Channel Television Apparatus * 



By HERBERT E. IVES 



A bar to the attainment of television images having a large amount of 

 detail is set by the practical difficulty of generating and transmitting wide 

 frequency bands. An alternative to a single wide frequency band is to 

 divide it among several narrow bands, separately transmitted. A three- 

 channel apparatus has been constructed in which prisms placed oyer the 

 holes in a scanning disc direct the incident light into three photoelectric cells. 

 The three sets of signals are transmitted over three channels to a triple elec- 

 trode neon lamp placed behind a viewing disc also provided with prisnis 

 over its apertures so that each electrode is visible only through every third 

 aperture. An image of 13,000 elements is thus produced. For the suc- 

 cessful operation of the multi-channel system, it is imperative to have very 

 accurate matching of the characteristics in the several channels. 



IF, 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 ele- 

 ments increases directly with the area of the image. The number of 

 such indistinguishable elements in everyday scenes, in the news 

 photograph, or in the frame of an ordinary motion picture is aston- 

 ishingly large. An electrically transmitted photograph 5 inches by 7 

 inches in size, having 100 scanning strips per inch, has a held of view 

 and a degree of detinition of detail, which, experience shows, are 

 adequate (although with little margin) for the majority of news event 

 pictures. It is undoubtedly a picture of this sort that 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 Syi 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 that the type of image successfully transmitted falls very far 

 short of the finely detailed picture just considered. Probably the 

 most satisfactory example of television thus far demonstrated is the 



* Jour. Optical Soc, Jan., 1931. 



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