A AIULTI-CIIANNEL TELEVISION APPARATUS 35 



discs. Perhaps the most pertinent conckision from this survey is that 

 the disc, while ciuite the simplest means for scanning images of few 

 elements, is entirely impractical when really large numbers of image 

 elements are in question. As yet, however, no practical substitute 

 for the disc of essentially different character has appeared. 



Turning now to the photoelectric cell. The question of adequate 

 sensitiveness to handle a large number of image elements is intimately 

 connected with the method of scanning, as has just been brought out, 

 so that no simple answer is possible. It is, however, probable that a 

 very considerable increase in sensitiveness over anything now available 

 must be anticipated, whatever scanning device is adopted. In the 

 matter of frequency range there is definite information.- In cells 

 depending on gas amplification (such as argon or neon) a characteristic 

 behavior is a falling off of output with frequency, greater the higher 

 the voltage used, which, becoming noticeable at about 20,000 cycles, 

 may at 100,000 cycles be so considerable as to constitute a practical 

 block to transmission. Vacuum cells are free from this failing, but 

 are much less sensitive. Systematic experiment and development of 

 photoelectric cells with particular reference to extending their range of 

 frequency response is indicated as a necessary step in the attainment 

 of a many-element image. 



Taking up next the circuits associated with the photoelectric cell, we 

 find, in general, that the higher frequencies progressively suffer from 

 the electrical capacity of cells and associated wiring and amplifier 

 tubes. This in turn calls for auxiliary equalizing circuits, with 

 attendant problems of phase adjustment, and for increased amplifica- 

 tion. Amplifiers capable of handling frequency bands extending from 

 low frequencies up to 100,000 cycles or over offer serious problems. 



Communication channels, either wire or radio, are characterized by 

 increasing difficulty of transmission as the frequency band is widened. 

 In radio, fading, different at different frequencies, and various forms of 

 interference stand in the way of securing a wide frequency channel of 

 uniform efficiency. In wire, progressive attenuation at higher fre- 

 quencies, shift of phase, and cross-induction between circuits offer 

 serious obstacles. Transformers and intermediate amplifiers or re- 

 peaters capable of handling the wide frequency bands here in question 

 also present serious problems. 



At the receiving end of the television system, conditions are similar 

 to the sending end. The neon glow lamp, commonly used for re- 

 ception, is already failing to follow the television signals well below 

 40,000 cycles, and, in the case of the 4,500-element image above 



2 Loc. cit., p. 456. 



