562 BELL SYSTEM TECHNICAL JOURNAL 



Application to Television 



As our first illustration was drawn from one of the earliest forms 

 of electrical communication, the submarine cable, it may be fitting 

 to use as the last what is probably the newest form, namely, television. 

 Here the information to be transmitted exists originally in the form 

 of a magnitude which is a continuous function of both space and time. 

 In order to determine what line facilities are needed to maintain a 

 constant view of the distant scene we wish to determine the line- 

 frequency-range required. This we know to be measured by the 

 total information to be transmitted per unit time. 



In the systems of television which have been most successful the 

 method has been similar to that of the motion picture in that a suc- 

 cession of separate representations of the scene is placed before the 

 observer and the persistence of vision is relied upon to convert the 

 intermittent illumination into an apparently continuous variation 

 with time. The first step in determining the required frequency- 

 range is to determine the information content of a single one of the 

 successive views of the scene. This may be determined exactly as 

 for a still picture. The required degree of resolution into elementary 

 areas and the required accuracy of reproduction of the intensity within 

 each aiea determine an effective number of selections and a number 

 of primary symbols available at each selection. These determine 

 a minimum product of wave-number-range by distance. This in 

 turn is equal to the product of line-frequency-range by time which 

 must be available for the transmission of a single view of the scene. 

 The time available is set by the fact that flicker becomes objectionable 

 if the interval between successive pictures exceeds about one sixteenth 

 of a second. Thus we have only to divide the product of wave- 

 number-range by distance for a single picture by one sixteenth to 

 obtain the line-frequency-range necessary to maintain a continuous 

 view. 



In the result just obtained an important factor is the interval 

 necessary to prevent flicker. The tendency to flicker is, however, 

 the result of the particular method of transmission. If it were practical 

 to eliminate this factor, the required frequency-range might be some- 

 what different. We might, for example, imagine a system more like 

 that of direct vision in which the magnitude-time function representing 

 the intensity variation of each individual elementary area is trans- 

 mitted over an independent line and used to produce a continuously 

 varying illumination of the corresponding area of the reproduced 

 scene. The frequency-range required on any one of these individual 



