Patent GB-809334-A (GB)
Title: Improvements in or relating to colour television apparatus
IPC: H04N9/24
Filed: 1955-12-19
Abstract: 809,334. Automatic control systems for cathode-ray tubes. MORLE, C. W. (Philco Corporation). Dec. 19, 1955, No. 36317/55. Class 38 (4). [Also in Groups XL (b) and XL (c)] Relates to a colour television receiver comprising a single gun C.R.T. 10 with a screen 9 of repeated red, blue and green vertical phosphor stripes scanned transversely thereto in which (1) red, blue and green colour signals are applied sequentially to intensity modulate the scanning beam by means of a colour sampling circuit, the sampling circuit, Figs. 1 and 5, or the beam scanning circuit, Fig. 6, being controlled by a local reference oscillator or (2) a luminance signal is applied to the cathode of the C.R.T. and a chrominance signal comprising a phase-modulated sub-carrier of frequency equal to that of a local reference oscillator is applied to the grid of the C.R.T.; in either case the frequency of the reference oscillator is equal to the stripe recurrence frequency. According to the present invention the frequency or phase of the reference oscillator is controlled by the differential output from a pair of photo-cells responsive only to light of one colour emitted by the screen such that the sense of the control signal is independent of the brightness of the picture so that when the scanning beam impinges on stripes of one colour the corresponding colour signal is being applied to the tube, the photo-cells being actuated sequentially by an output from the reference oscillator. The photo-cells should be sensitive enough to respond to the light emitted at black level. Preferably three pairs of photo-cells are employed responsive respectively to red, blue and green light. First type, controlling sampling circuits, Fig. 1 and Fig. 5 (not shown). As shown in Fig. 1 simultaneous red, blue and green signals from a receiver 27 are applied to the grid of the tube 10 sequentially by a sampling circuit 34 controlled by phase-displaced gating pulses derived from the reference oscillator 38 operating at the stripe recurrence frequency, e.g. 7 Mc/s., and phase displaced actuating pulses of waveform 54 also derived from the oscillator 38 are applied respectively to three pairs 17, 18, 19 of reciprocally connected photo-cells responsive respectively to red, blue and green light such that one cell of each pair is actuated during the positive portion of an applied pulse 54 and the other cell is actuated during the succeeding negative portion. The pulse outputs, Fig. 2 (not shown), are smoothed in a circuit 53, 55 to give a D.C. error signal, Fig. 3 (not shown), which is applied through a narrow band amplifier 57 and phase control circuit 42 coupled to the oscillator 38 to effect the required control. In a modification, Fig. 5 (not shown), phase-displaced sinusoidal outputs from the oscillator 38 are applied to the sampling circuit 34 and to actuate the photo-cells, each pair of separate photo-cells being replaced by a single dual-element cell; a luminance signal is also applied to the cathode of the tube. First type; controlling scanning circuit, Fig. 6 (not shown). In this arrangement the local oscillator 38 controls the photo-cells and the instantaneous speed of the scanning beam, the colour sampling being independent of the local oscillator. The two cells of each pair of photocells are connected in the same sense and are actuated in push-pull by a sinusoidal output from the oscillator 38, the difference of the two smoothed outputs being applied to the circuit 42 controlling the phase of the oscillator. Second type, Fig. 7. This is described as applied to a system producing a received signal as shown in the graph 107 comprising a lowfrequency luminance component and a chrominance component comprising a phase-modulated 3.58 Mc/s. colour sub-carrier. The luminance component is separated by a low-pass filter 109 and applied to the cathode of the tube 10 and the chrominance component is separated by a high-pass filter 108 and converted to a corresponding 7 Mc/s. signal for application to the grid of the tube 10 by heterodyning in a mixer 116 with a 10.58 Mc/s. signal obtained by heterodyning in a mixer 113 an output from the 7 Mc/s. reference oscillator 38 with a C.W. 3.58 Mc/s. signal applied over a lead 114 and derived from a 3.58 Mc/s. burst signal incorporated with the received horizontal sync. pulses. The reference oscillator 38 is controlled by the photo-cell unit 17 as in the embodiments described above.