GB565710A - Automatic gain control for television receivers - Google Patents

Abstract

565,710. Automatic volume control. PHILCO RADIO & TELEVISION CORPORATION. May 18, 1943, No. 7891. Convention date, June 13, 1942. [Classes 40 (iii) and 40 (v)] In a television receiver adapted for the reception of video and synchronizing signals transmitted at different carrier frequencies, there is derived from the video carrier an automatic gain control voltage whose magnitude is determined substantially by the blanking level of the video carrier. Fig. 2 shows the last tuned I.F. transformer 15 of a television superheterodyne receiver feeding the second detector 17 and later stages (not shown). The I.F. output is also fed to circuits 19, 20 which together comprise a constant resistance circuit. The reactive elements of circuit 19 are seriestuned to the video I.F., while those of circuit 20 are parallel-tuned to the same frequency. Circuit 20 has a high Q-value and thus a voltage will build up across it corresponding to the video carrier strength and independent of the synchronizing pulses. This voltage is rectified at 21 and smoothed at 22, 23 to give an A.G.C. voltage for application to the R.F. and I.F. stages. In a modification, Fig. 3 (not shown), the I.F. is fed to a pentode in the output circuit of which is a critically coupled transformer of high Q value tuned to the video I.F. frequency and which feeds a diode to produce the A.G.C. voltage. U.S.A. Specification 2,347,248 is referred to.