PRODUCTION OF TELEVISION SIGNALS 573 



grid appears as a uniformly illuminated screen; but, when the high 

 frequency voltage is modulated by the received picture current, an 

 image of the distant subject is produced on the screen and his motions 

 can be followed just as in the smaller images formed on a disk. 



This method of presenting television images to a large audience 

 permits a very efficient use of the available energy to reproduce a 

 picture. The modulated current produces a glow discharge that 

 exactly covers an elemental area of the picture on the screen and is 

 viewed directly by the audience; consequently, there is absolutely no 

 loss of energy after the picture current has been converted into light. 

 In addition, each illuminated area of the screen responds to the 

 picture current in the same manner; the exterior electrodes are 

 exactly alike, and the use of a single tube assures the same pressure 

 and purity of neon throughout the grid. 



Fig. 9 shows such a screen set up for demonstration in an auditorium. 

 A loud speaker is mounted just below the screen and it is thus possible 

 for a large audience to both see and listen to a distant person at the 

 same time. 



Section II. The Television Signal Wave 



So far it has been assumed that the electrical signal wave is perfectly 

 transmitted between the conversion devices which transform the light 

 variations into electrical variations and back again. Perfect trans- 

 mission is, however, impossible with practical apparatus. There are 

 certain requirements placed upon the generated signal wave by the 

 characteristics of practical communication channels, and reciprocally 

 certain demands are made upon a transmission system by the inherent 

 nature of an adequate television signal. In addition to exploring 

 these mutual requirements experimentally it is desirable to analyze 

 them in such a way that, as far as possible, quantitative expression 

 may be given to them. This expression in the case of the signal 

 wave is best made by the methods of the Fourier analysis ; considering 

 the signal as made up of many sine wave components of various 

 frequencies. The requirement on the signal may then be described in 

 terms of these components and the requirements on the connecting 

 transmission system in terms of attenuation and phase characteristics 

 over a band of frequencies. These requirements will now be discussed 

 as a basis for the subject matter of the succeeding section of this paper 

 and of the following companion jjapers of this group on "Wire Trans- 

 mission Systems for Television" and "Radio Transmission Systems 

 for Television." 



The problems to be discussed may be conveniently considered 

 under three headings: 



