GB697897A - Improvements in circuit-arrangements for automatic gain control - Google Patents

Abstract

697,897. Television. PHILIPS ELECTRICAL INDUSTRIES, Ltd. Feb. 1, 1952. [Feb. 5, 1951], No. 2750/52. Class 40 (iii). [Also in Group XL (c)] An automatic gain control (A.G.C.) circuit for a television receiver for negatively-modulated signals, in which the D.C. component is lost after the detector stage and subsequently restored, is characterized in that a stage of the receiver prior to the detector is periodically cut off, the A.G.C. potential being derived by peak detection of the modified video signal so obtained and subsequent to restoration of the D.C. The signal detector 7 is direct-coupled to a video amplifier 10, which is R.C. coupled to a further amplifier 13 direct coupled to the picture tube 17. Pulses derived from the line scanning generator 23 are applied to an I.F. stage 5 so as to cut it off for a short period (32, Fig. 3) subsequent to each line synchronizing pulse 28. This provides a reference level (i.e. zero carrier level) upon which D.C. restoration can be effected in the valve 13, which is provided with grid-leak and condenser bias for this purpose. A peak rectifying diode 19 is coupled to the anode circuit of the valve 13 and produces across load resistance 20 a D.C. output proportional to the height of the synchronizing pulses with respect to zero carrier level. The potential across resistance 20 is applied as A.G.C. to stages 2, 6 of the receiver. To prevent the impulses 32, which comprise " peak white " picture modulation, from appearing on the tube 17 corresponding blocking pulses are applied to the grid 27 from the generator 23.