552 BELL SYSTEM TECHNICAL JOURNAL 



Characteristic Problems of Television 



The problem of television in its broad outlines is that of converting 

 light signals into electrical signals, transmitting these signals to a 

 distance, and then converting the electrical signals back into light 

 signals. Given means for accomplishing these three essential tasks, 

 the problem becomes that of developing these means to the requisite 

 degree of sensitiveness, speed, efficiency, and accuracy, in order to re- 

 create a changing scene at a distant point, without appreciable lapse 

 of time, in a form satisfactory to the eye. 



A convenient starting point for the discussion of television is the 

 human eye itself. In this an image is formed upon the retina, a 

 sensitive screen, consisting of a multitude of individual light-sensitive 

 elements. Each of these elements is the termination of a nerve 

 fibre which goes directly to the brain, the entire group of many million 

 fibres constituting the optic nerve. A theoretically possible television 

 system could be made by copying the eye. Thus a large number of 

 photosensitive elements could be connected each with an individual 

 transmission channel leading to a distant point, and signals could be 

 sent simultaneously from each of the sensitive elements to be simul- 

 taneously used for the re-creation of the image at the distant point. 

 The number of wires or other communication channels demanded in 

 a television system of this sort would be impractically large. For 

 practical purposes, reduction of the number of transmission channels is 

 made possible by the fact that, while in vision all parts of the image 

 on the retina are simultaneously and continuously acting to send 

 nerve impulses, the inertia of the visual system is such that a sensation 

 of continuity is obtained from discontinuous signals, provided these 

 succeed each other rapidly enough. Due to the phenomenon of 

 persistence of vision, it is immaterial to the eye whether the whole 

 view be presented simultaneously or whether its various elements be 

 viewed in succession, provided the entire image be traversed in a 

 sufficiently brief interval, which for purposes of discussion may be 

 taken as 1/1 6th of a second or less.^ We thus have available in 

 television the same artifice which is used in the much less exacting 

 problem of transmission of pictures over a telephone line, that is, 

 of scanning, or running over the elements of the image in sequence, 

 instead of endeavoring to transmit all of the elementary signals 

 simultaneously. The development of a television system therefore 



^ This figure of l/16th of a second, commonly quoted in discussions of this sort, 

 is a convenient one, although the frequency of image repetition necessary to extinguish 

 "flicker" is actually proportional to the logarithm of the field brightness. A some- 

 what higher rate of image repetition was used in the final television apparatus. 



