PRODUCTION OF TELEVISION SIGNALS 575 



pictures or scenes to be transmitted by the system. Let the first 

 be quite dark in general effect and require fluctuations in the signal 

 current of a certain average amount for its transmission. Such a 

 picture would have a low direct current component. Let the second 

 picture consist largely of medium grays and require about the same 

 fluctuations in signal intensity for its delineation. Such a picture 

 will have a medium direct current component. Let the third picture 

 be very light in general effect with such difference in light and shadow 

 as would require the same fluctuations in signal intensity as the 

 other two pictures. Such a picture would have a relatively high 

 direct current component. In passing through a resistance-condenser 

 coupled amplifier, the signals for all three types of pictures would 

 be changed from fluctuations superimposed upon direct current to 

 alternating currents, all of about the same average value. 



At the receiving end of the circuit the direct current component 

 may be reinserted by superimposing the alternating current fluctuations 

 upon a fixed value of direct current such as the steady state current 

 in the last amplifier tube. This direct current component would give 

 the best average results if it corresponded to that suitable for the 

 gray picture, which would, of course, then be most nearly correctly 

 reproduced. However, most of the detail of the dark and light 

 scenes would also be reproduced though the tone values would be 

 distributed about a medium gray. Fortunately a change in character 

 of this kind has proven for the most part unimportant. Where it is 

 important it can be taken care of very simply by providing, at the 

 receiving end, means, either manual or automatic, for changing, in 

 accordance with the type of scene being transmitted, the magnitude of 

 the unidirectional current upon which the received alternating current 

 is superposed, which amounts simply to the restoration of the direct 

 current. 



In the case of scenes which are changing, however, frequencies 

 lower than picture frequency will in general be generated and their 

 suppression may be expected to affect to some degree the perfection 

 of the picture. In effect, these frequencies are analogous to changes 

 in tone values in the case of still pictures and their elimination results 

 in fluctuations in the apparent brightness of the image. This effect 

 is not disturbing with many types of subjects, as for example in the 

 reproduction of the face. 



One remarkable result of not transmitting the direct current 

 component of the signal in the case of the reflected beam method of 

 scanning is that the television transmitting apparatus can be located 

 and operated in a well-lighted room, for if this general illumination is 



