PRODUCTION OF TELEVISION SIGNALS 587 



electric signal wave corresponding to this initial luminous wave in its 

 relative instantaneous amplitudes. Certain important relations be- 

 tween the characteristics of the signal wave and the resulting image 

 have been pointed out. There remains the question of obtaining an 

 electric signal wave suitable for long distance transmission and of 

 providing for the control of the illumination at the receiving terminal 

 by the electric signal wave as delivered by the transmission medium. 



In the use of wire lines for television it is fortunately true that a 

 suitably prepared open-wire circuit possesses a frequency range 

 sufficient for the transmission of all the essential components of the 

 signal wave. Details regarding the characteristics of the wire circuits 

 are given in a companion paper by Messrs. Gannett and Green, from 

 whose work are obtained data essential to the design of the terminal 

 equipment. These data fix the power level at which the signal should 

 be delivered to the line and the power level which will be available at 

 the receiving end. When the transmission is by radio it is, of course, 

 necessary to effect a frequency translation in order to secure a wave 

 suitable for radiation and transmission through the ether. In this 

 case, however, the radio system, which is described in a paper by 

 Mr. E. L. Nelson, when considered as a whole may be conveniently 

 taken as a system capable of the transmission of a signal wave occu- 

 pying the same frequency range as that supplied to the wire circuits. 

 In fact the design of the radio system is such that it may be used inter- 

 changeably with the wire line in so far as the remaining electrical 

 terminal equipment is concerned. 



The tenninal circuits, then, fall into two groups: first, those used 

 at the transmitting terminal for building up the wave controlled by 

 the time variations in light to the power level required by the line; and 

 second, those used at the receiving terminal to bring the wave de- 

 livered by the line to the proper form for controlling the luminous 

 sources from which the received picture is built up. 



Transmitting Circuits 



Starting with the photoelectric cell in which the initial luminous 

 signal wave is converted to an electric signal wave, we are interested 

 in the magnitude of various pertinent constants. The cell may be 

 considered for our purposes as an impedance, the value of which is 

 determined by the quantity of light reaching it. With no illumination 

 at all this impedance is almost entirely a capacitance of the order of 

 10 m.m.f. When the cell is illuminated this capacitance becomes 

 effectively shunted by a very small conductance which is roughly 

 proportional to the square of the voltage between the electrodes. 



