GB783022A - Colour television receiver - Google Patents

Abstract

783,022. Colour television. RADIO CORPORATION OF AMERICA. Aug. 17, 1955 [Sept. 14, 1954; Sept. 14, 1954], No. 23740/55. Addition to 780,389. Class 40 (3). A synchronous detector arrangement as described in the parent Specification, wherein the three colour-difference signals R-Y, B-Y and G-Y are derived by means of two synchronous detectors arranged to produce respectively both positive and negative versions of R-Y and B-Y and including a common coupling resistor across which is developed G-Y, is characterized in that the detectors each comprise an electric discharge tube including a control electrode, the chrominance sub-carrier being applied across the discharge tube and the locally-generated demodulating wave being applied to the control electrode. Fig. 4 shows such an arrangement, the synchronous detectors comprising triodes 53 and 73 with the chrominance sub-carrier applied individually to the anodes from secondary windings 43 and 45 of transformer 41. Locally-generated demodulating waves 47 and 67 of appropriate phases # 1 and # 2 (derived in practice from a tapped delay line, Fig. 5, not shown) are applied to the control grids via CR biasing networks 49, 54 and 69, 71. The phases are selected so that the signal R-Y is obtained at terminal 85 from across anode resistor 63, the signal B-Y is obtained at terminal 87 from across anode resistor 81, and the signal G-Y at terminal 86 as the combination -0.51 (R-Y) - 0.19(B-Y) appearing across the common cathode resistor 89. The triodes may be arranged to operate in class C, although reference is made to operation in a linear mode. Networks 65, 83 and 91 are provided to eliminate components at the subcarrier frequency. Due to interaction between the two detectors by reason of the common cathode resistor 89 it is necessary that the phases # 1 and # 2 of the demodulating waves should differ from the phase angles of the chrominance sub-carrier corresponding to R-Y and B-Y. In a typical design # 1 lags R-Y by 12.95 degrees and # 2 leads B-Y by 13.47 degrees. The amplitudes e 1 and e 2 of the sub-carrier waves applied to the anodes are in the ratio 1.41. According to a modification, Fig. 9 (not shown), interaction between the anode and grid signals in the two detectors due to interelectrode capacitances is neutralized by an additional capacitive coupling to the grids from the primary of the anode transformer supplying the chrominance sub-carrier. The Specification discusses the theory and design considerations of the invention at some length and also includes two complete colour receivers, Figs. 5 and 8 (not shown), incorporating the detector circuits. The detectors are arranged to operate at a high level whereby the output signals may be applied to modulate the display cathode-ray tube directly. Specification 774,441 is referred to.