Patent GB-793691-A (GB)
Title: Chrominance-signal component-selection system
IPC: H04N11/14
Filed: 1955-11-16
Abstract: 793,691. Colour television. HAZELTINE CORPORATION. Nov. 16, 1955 [Dec. 8, 1954], No. 32867/55. Class 40 (3). A phase-selective circuit responsive to a phase and amplitude modulated colour sub-carrier develops in two output circuits a first pair of signals derived along a first pair of axes, the circuit including means for intercoupling the two outputs so as to develop a second pair of signals representative of components derived along a second pair of axes. The circuit as illustrated in Fig. 3 comprises a pair of pentodes 35, 39 with the sub-carrier at 3.6 Mc/s. applied in the same phase to the control grids of both pentodes from terminals 30 and a 7.2 Mc/s. reference signal applied in one phase to the control grid of pentode 35 from terminals 57 and in opposite phase to the control grid of pentode 39 from terminals 40. The anode output circuits 45, 46 are tuned to 3.6 Mc/s. and are intercoupled by a circuit 47, 48 series resonant at 7.2 Mc/s. so as to be capacitive at 3.6 Mc/s. The sub-carrier is considered to comprise two quadratum components 15, 16, Fig. 3a, along axes from which it is desired to derive the final pair of signals. The 7.2 Mc/s. signals are of sufficient amplitude to drive the pentodes between cut-off and conduction whereby they effectively sample the sub-carrier. The resulting anode currents (1-K)I3 and (1-K)I4, which represent the first pair of signals, have a quadrature relation at 3.6 Mc/s. and are oriented at 45 degrees with respect to 15 and 16 by appropriate phasing of the 7.2 Mc/s. signals. From Fig. 3a, (1-K)I3 = (1-K)(0.707 15-0.707 I6) and (1-K)I4= (1-K)(0.707 15+0.707 I6). By adding and subtracting these two signals by means of coupling circuits 47 and 48, outputs of (1-K) 1.414 I5 and (1-K)1.414 I6 corresponding to the desired second signals, are obtained. This is shown at N and M where the 90 degree phase advance introduced by the coupling circuit is illustrated. The circuit provides a gain of 1.414 over the corresponding signals (1-K)I5 and (1-K)I6 which could be derived directly by appropriate phasing of the 7.2 Mc/s. signals and omission of the coupling network. The operation as considered above ignores the quadrature components KI3 and KI4 which appear in the anode currents due to the sampling action occupying an appreciable interval of time. These components may, however, be effectively eliminated by tuning the anode circuits slightly below 3.6 Mc/s. The signal components coupled from one anode to the other are then shifted by more than 90 degrees as indicated in Figs. 3b and 3c whereby the respective I3 and I4 components combine and produce resultants along axes a and b which, when added, differ only slightly from the desired signals (1-K)I3 and (1-K)I4. The two outputs representing components along the desired second set of axes appear separately at terminals 50 and 51 so that the circuit functions as a so-called " axis selector." As such, the circuit is applied to a colour receiver, Fig. 1 (not shown), arranged generally as described in Proc. I.R.E. January, 1954, pages 299-308, so as to supply signals to a cathode-ray tube which utilizes colours in the sequence R,G,B, G, R, &c. One output of the circuit is applied directly to the cathode-ray tube (along with the luminance signal Y and a Y-M correction signal), whilst the other output is applied to a second harmonic sampler and thence to the tube. Specifications 772,792, 784,055 and 793,692 are referred to.