Patent GB-1189193-A (GB)
Title: Colour Signal Processing Circuits
IPC: H04N9/67
Filed: 1967-05-12
Abstract: 1,189,193. Colour television. R.C.A. CORPORATION. 12 May, 1967 [23 May, 1966], No. 22246/67. Heading H4F. Relates to a colour television receiver in which the chrominance component of the received video signal is amplified by tube 30 and passed via a saturation control 49 to the input of two demodulator pentodes 60 and 70, which are fed with differently phased reference oscillations from source 19 synchronized by the burst component, the two colour difference signal outputs of pentodes 60 and 70 being matrixed by three amplifying triodes 80, 90 and 100 to derive the third colour difference signal. Positive going pulses which occur during line retrace intervals from cathode follower 50 which is triggered by positive horizontal blanking pulses from the deflection circuits, are passed via diode 57 to cut off tube 30 during the retrace intervals so that it does not pass the burst component (see later). The colour difference signal output of demodulator 70 is developed across a split anode load comprising resistor 74 shunted by the series combination of resistors 76 and 59. A corresponding split anode load for demodulator 60 comprises resistors 64, 66 and 59. The outputs of demodulators 70 and 60 are coupled to the grids of their associated colour difference amplifier tubes 90 and 80 via capacitors 92 and 82, and a negative feedback path is established between the anodes and grids of the tubes by means of resistors 96 and 86. The third colour difference amplifier tube 100 receives a combination of anode outputs of tubes 90 and 80 via matrixing resistors 110 and 112. A split " simulated demodulator load " is provided in association with the grid circuit of tube 100, to match the grid circuitry of tubes 90 and 80 comprising resistor 114 shunted by series resistors 116 and 59. The cathodes of tubes 80, 90 and 100 are all grounded. The split (real and " simulated ") demodulator loads are provided so that the positive pulses from amplifier tube 50, in addition to cutting off the chrominance amplifier 30, also appear across the shared resistor 59 and are fed via the respective resistors 76, 116 and 66 and associated capacitors 92, 102 and 82 to the control grids 93, 103 and 83 respectively to establish a stable line interval bias at these grids without introducing thereby any disturbing colour difference signal cross-coupling. The pulses from tube 50 thus establish the operating points of tubes 80, 90 and 100 which directly affect the colour image reproducer biases due to the D.C. coupling between matrix tubes 80, 90 and 100 and the reproducer. During the line intervals when blanker tube 50 is non-conducting, chrominance amplifier tube 30 returns to its conductive condition and diode 57 is rendered non-conducting, thus isolating the cathode follower 50 output from the tube 30 during line intervals. This isolating function is important since during the line interval tube 30 is subjected to wide impedance variations due to colour killer or automatic chrome control circuitry'and without the isolation provided by diode 57 such impedance variations could disturb the matrix tubes bias development. The negative pulses developed at the anodes of tubes 80, 90 and 100 may be used to effect reproducer blanking.