GB525284A - Automatic control of television receiving systems - Google Patents

Abstract

525,284. Automatic control of television receiving apparatus. HAZELTINE CORPORATION. Feb. 16, 1939, No. 5214. Convention date, March 15, 1938. [Class 40 (v)] In a television system in which the intensity of general illumination is transmitted by the average intensity of the carrier, the signal during the frame retrace period, which is modulated by synchronizing pulses only, is used to provide a controlling potential which is responsive to the intermediate amplitude-level of the modulation envelope of the signals, but independent of its " timing " characteristics. The derived potential may be used for automatic volume selectivity, or tuning control, but as described is applied to volume control only. The signal output from detector 15, Fig. 1, is applied to valve 20, and if the input signal is as shown in Fig. 2 the anode current of valve 20 is as indicated in Fig. 4. The time period of condenser 26 and resistance 25 is long compared with the line blanking periods, but short compared with the frame blanking periods, so that the voltage developed across condenser 26 is of the form shown in Fig. 5. This voltage is subtracted from the anode voltage supplied at B so that a voltage as indicated in Fig. 6 is obtained by peak rectifying anode 21 reversed by valve 22 and applied through smoothing filters 30, 31 as a control voltage to the radio frequency amplifier 12, the oscillation modulator 13, and the intermediate frequency amplifier 14. Battery 33 is provided to compensate for the +B voltage. The control voltage is fairly steady and as it is derived from the unmodulated parts of the carrier, it is independent of light modulation components. The diagrams A in Figs 2, 5 and 6 indicate a signal of normal strength and those at A<SP>1</SP> a faded signal and it will be seen that the control voltage P, P<SP>1</SP> varies with fading.