PRODUCTION OF TELEVISION SIGNALS 



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envelope reaches 160 volts the tube fails to light. This corresponds to 

 a black area in the picture. When no picture signal is being received, 

 the amplitude of the unmodulated carrier wave causes the tube to 

 light at average brightness, corresponding to the locally introduced 

 d-c. component of the signal. It follows, then, that the amplitude of 



Fig. 28 — Diagrammatic representation of relation between modulated high 

 frequency wave impressed on grid type neon lamp and lamp characteristics. In- 

 tensity of glow is proportional to shaded area. 



the unmodulated carrier is fixed, as in a previous example, by the joint 

 requirements of two biases, that of the lamp and that of the signal 

 bias. 



There is a slight distortion inherent in this method due to the fact 

 that the light, which is proportional to the shaded area of the curve 

 of Fig. 28, is not strictly proportional to the amplitude of the envelope 

 with respect to the 160- volt limit. This is, of course, because these 

 peaks are portions of a sine wave and hence the time variation of the 

 glow resulting from any given carrier cycle is a function of its ampli- 

 tude. The effect is small, however, being most noticeable at low 

 values of illumination. 



In the case of the grid-lamp receiver the signal amplitude is adjusted, 

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