. The Bell System technical journal . llion cycles. The circuit problem, therefore, involves the production of a highfrequency wave which varies in amplitude in accordance with theamplitude of the received picture signal. The solution has been con-veniently obtained by using a radio broadcast transmitter the voicefrequency circuits of which have been so modified that the extendedrange of frequencies required might be handled with minimum dis-tortion. The envelope of the 500-kilocycle wave modulated by the picturesignal, as shown in Fig. 28, is proportional to the signal amplitude plusa direct
. The Bell System technical journal . llion cycles. The circuit problem, therefore, involves the production of a highfrequency wave which varies in amplitude in accordance with theamplitude of the received picture signal. The solution has been con-veniently obtained by using a radio broadcast transmitter the voicefrequency circuits of which have been so modified that the extendedrange of frequencies required might be handled with minimum dis-tortion. The envelope of the 500-kilocycle wave modulated by the picturesignal, as shown in Fig. 28, is proportional to the signal amplitude plusa direct current biasing component of such magnitude that when the PRODUCTION OF TELEVISION SIGNALS 599 envelope reaches 160 volts the tube fails to light. This corresponds toa black area in the picture. When no picture signal is being received,the amplitude of the unmodulated carrier wave causes the tube tolight at average brightness, corresponding to the locally introducedd-c. component of the signal. It follows, then, that the amplitude of. Fig. 28—Diagrammatic representation of relation between modulated highfrequency 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 jointrequirements of two biases, that of the lamp and that of the signalbias. There is a slight distortion inherent in this method due to the factthat the light, which is proportional to the shaded area of the curveof Fig. 28, is not strictly proportional to the amplitude of the envelopewith respect to the 160-volt limit. This is, of course, because thesepeaks are portions of a sine wave and hence the time variation of theglow resulting from any given carrier cycle is a function of its ampli-tude. The effect is small, however, being most noticeable at lowvalues of illumination. In the case of the grid-lamp receiver the signal amplitude is adjusted,39 600 BELL SYSTEM TECHNICAL JOURNAL as f
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