Patent GB-780389-A (GB)
Title: Colour television receiver
IPC: H04N9/66
Filed: 1955-06-15
Abstract: 780,389. Colour television. RADIO CORPORATION OF AMERICA. June 15, 1955 [July 9, 1954; July 28, 1954; July 28, 1954], No. 17297/55. Class 40 (3). [Also in Group XL (c)] The colour-difference signals R-Y, B-Y and G-Y, Fig. 2, corresponding to different phases of a chrominance sub-carrier wave are derived by means of a circuit arrangement utilizing only two synchronous detectors which produce directly the R-Y and B-Y signals, each of the detectors being arranged however to produce also an output of opposite polarity and the G-Y signal being obtained by coupling means which combines these two outputs. The invention is described in relation to a transmission in which the luminance signal Y is of the form 0.59G + 0.30R + 0.11B and the R-Y and B-Y signals are conveyed as quadrature modulations of the sub-carrier. The G-Y signal is obtainable from -0.51 (R-Y) - 0.19 (B-Y). In a first arrangement, Fig. 4, the synchronous detectors 87, 89 each comprise a pair of oppositely-connected diodes with the sub-carrier wave from terminal 60 applied in the same phase to both diodes and the local sub-carrier demodulating wave of same frequency and appropriatley different phase for each detector applied in phase opposition to the two diodes from centre-tapped transformers 101, 119. Directly in series with each diode is a CR bias network which allows it to conduct substantially only at the peak of the demodulating wave. The outputs from detector 87 are developed across CR networks 93 and 83, 85, that across 93 being of positive polarity and that across 83, 85 negative, whilst the corresponding outputs from detector 89 are developed across CR networks 97 and 83, 85, network 83, 85 being therefore common to both detectors. The demodulating waves to 87 and 89 are phased so that the detectors produce respectively the B-Y and R-Y signals. + (B-Y) therefore appears at terminal 68, + (R-Y) at terminal 70 and a combination of - (B-Y) and - (R-Y) at terminal 66 from across the common CR network 83, 85. By appropriate selection of the impedances of the CR networks 93, 97 and 83, 85 the output at 66 may comprise- 0.51 (R-Y)- 0.19 (B-Y = G-Y). Due to interaction between the two detectors resulting from the common CR network 83, 85, the phases of local demodulating waves need to differ from a strict quadrature relationship and in one particular construction may correspond to phases A and C as shown in Fig. 2, where A lags the R-Y phase by 13 degrees and C lags the A phase by 50.6 degrees. The local demodulating waves are derived from stage 131. The parallel combination of parallel circuit 120 and series circuit 110, 121 is brought to resonance at the subcarrier frequency and resistor 122 is adjusted to control the relative phase in the two parallel arms. In a second arrangement, Fig. 5, the two detectors comprise triodes 151, 153 arranged for class C operation. The sub-carrier is applied in common to the grids from terminal 60 and the local demodulating waves are applied from terminals 138, 142 to tuned circuits 163, 165 in the cathode loads. Detector 151 is arranged to produce + (B-Y) at terminal 144 from across anode load 157 and an output - (B-Y) of opposite polarity across cathode load 167, 169. Similarly detector 153 is arranged to produce + (R-Y) at terminal 146 from anode load 159 and -(R-Y) across cathode load 173, 175. Resistor 171 couples the two cathode circuits and by appropriate choice of resistors the output taken from the tapping on 175 may comprise - 0.51 (RÀY) - 0.19 (B-Y) = + (G-Y). Stage 155 is an in-phase amplifier whereby + (G-Y) is obtained at terminal 148. Dependent upon the value of resistor 171 a degree of interaction occurs between the two detectors and it is necessary for the demodulating waves applied at 138 and 142 to depart from their normal quadrature relationship, for example the B-Y wave may lead its normal phase by 30 degrees and the R-Y wave lag by 10 degrees. A third arrangement, Fig. 6 (not shown), generally similar to Fig. 5, employs a common resistor in the cathode of circuits of the two detectors to combine their outputs and from across which the signal + (G-Y) is obtained. The in-phase amplifier is dispersed and the circuit utilizes only a double-triode for the two detectors.