PRODUCTION OF TELEVISION SIGNALS 



589 



serves a dual function, as the heavy mass, supported in this way, is 

 capable of little response to such mechanical vibrations as may be 

 transmitted through the cabinet and the walls of the amplifier shield. 

 With these precautions it has been found possible to make the effect 

 of all external disturbances of about the magnitude of the thermal 

 disturbances referred to in the first part of the paper. 



A schematic diagram of the amplifier tubes directly associated with 

 the photoelectric cell is given in Fig. 21. Attention has already been 



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Fig. 21 — Schematic of vacuum tube amplifier used with photoelectric cells 



called to the fact that the initial signal, that is, the time variation of 

 the light reflected from the scanned object, contains a direct current 

 component. The amplification of this direct current component is, 

 as has been stated, out of the question in any amplifier intended for 

 continued operation over long periods of time. The requirements as 

 to the range of frequencies to be transmitted, as discussed in the 

 preceding section, make it necessary to provide a circuit having 

 practically uniform efficiency from 10 cycles to above 20 kilocycles. 

 The relative phase shift of the several components must also be kept 

 very small. In view of the large amplification and consequent large 

 number of stages necessary, it has been thought impracticable to use 

 transformer coupling between all stages as the aggregate frequency 

 and phase distortion might well be greater than could be tolerated. 

 The so-called resistance capacitance coupling has therefore been used. 



