Patent GB-753441-A (GB)
Title: Improvements in and relating to signal sampling systems
IPC: H04N9/24
Filed: 1951-09-14
Abstract: 753,441. Colour television. PHILCO CORPORATION. Sept. 14, 1951 [Sept. 15, 1950], No. 21648/51. Class 40 (3). [Also in Group XXXV] In a colour television receiver employing a cathode-ray tube with a screen formed of a repeating series of red, blue and green stripes (see Fig. 3A) which are scanned transversely of the stripes, incoming red, blue and green colour component signals in separate channels are sampled sequentially and connected to the intensity-controlling electrode of the tube in step with the scanning of the stripes whereby signals corresponding to each particular colour modulate the beam as the correspondingly coloured stripes are traversed. In a first embodiment the signal sampling is controlled by an oscillator the frequency of which is automatically controlled by photo-cells viewing the tube screen to maintain the sampling in stop with the scanning. In a second embodiment special electrodes are incorporated in the screen of the cathode-ray tube and provide signals indicating the instantaneous position of the beam to control the sampling. Fig. 7 shows the circuit of the first embodiment. Figs. 2 and 8 show waveforms throughout the circuit. Red, blue and green component video signals A, B and C are applied to sampling valves 60, 61, 62, the valves having a common anode load 71 feeding the control grid 7 of cathode-ray tube 4 through a phase inverting stage 110. The sampling valves are rendered effective in sequence by means of three 120 degrees-phasedisplaced oscillations D, E, F generated by an oscillator 75. The oscillator is of the form described in Specification 639,980, [Group XXXV], and comprises a Hartley circuit connected between the cathode 76, control grid 77 and screen grid 78. The blue sampling oscillation is derived directly from the tuned circuit 81, whilst the 120 degrees phase-advanced and 120 degrees phase-retarded oscillations for the red and green samplers are derived from coupled circuits 100 and 98. The oscillations are applied to the control grids of the sampling valves through RC networks 102, 96, 104 whereby the valves are biased to become conductive only at the peaks of the oscillations as indicated by the levels X in Fig. 2. The individual sampled video signals are shown in curves G, H and I and the combined signal across the common anode load 71 is shown in curve J. The oscillator anode circuit includes a quadrature resonant circuit 79 coupled to the tuned circuit 81 whereby the frequency may be varied by a control voltage applied to grid 80. In accordance with the invention, the screen of the cathode-ray tube is observed by three photocells 28, 29, 30 associated with red, blue and green filters 31, 32, 33. As the beam scans across the coloured stripes, the cells generate trains of indexing pulses L, M, N which indicate the instaneous position of the beam. If the sampling oscillator is operating at the correct frequency the sampled video pulses coincide with the corresponding indexing pulses. To .maintain the oscillator frequency correct irrespective of any irregularity in the spacing of the coloured stripes and non-linearity in scan, the indexing pulses are compared with the sampling oscillations to derive a control signal for application to grid 80 of the oscillator. Each photo-cell feeds a band-pass amplifier tuned to a narrow band of frequencies disposed about the average repetition frequency of the indexing pulses, the resulting output oscillations, curve K, being adjusted in phase by circuits 127, 128, 129 so that each oscillation is passing from positive to negative as the corresponding indexing pulse is at its peak. Each indexing oscillation, together with the corresponding sampling oscillation, is applied to a phase comparison valve 131, 132, 133, the sampling oscillation being applied to the control grid through a R.C. circuit, such as 140, 142, so that the valve conducts to produce an anode pulse S, Fig. 8, at the peak of the sampling oscillation. The amplitude of the pulse depends on the phase of the indexing oscillation i applied to the second control grid, the pulse amplitude increasing when the sampling oscillation occurs early, Fig. 8B, and decreasing when the sampling oscillation occurs late, Fig. 8C. The common anode circuit 136, 137 has a time constant corresponding to about six cycles of the sampling oscillator whereby the anode potential is determined by the average of the signals from the three comparator valves integrated over several colour groups. The anodes are connected to potential divider 150, 151, the tapping point extending through line 93 to grid 80 of the sampling oscillator. To ensure photocell pulses are produced during black-level modulation by the video signals, the cathoderay tube is biased to have a residual beam of low intensity and the resulting illumination is screened out by a filter 38. Due to the blanking of the beam during the retrace intervals the control system is rendered ineffective and may fail to regain control until the beam has scanned several stripes following the blanking interval. The resulting region of imperfect colour rendition along one edge of the screen may be masked. The tube screen may be constructed with phosphor stripes which fluoresce with different colours or may have a white phosphor employed in conjunction with a striped colour filter. The embodiment shown in Fig. 7 may be modified to operate with a single photo-cell observing, for example, only the green stripes. In the second embodiment, Figs. 5 and 6B, the cathode-ray tube is formed with red, blue, and green phosphor stripes 40, 41, 42, Fig. 6B, but preceding each group is a space occupied by an electrode 47. The surface of the phosphor stripes facing the scanning beam is coated with a thin film of aluminium and the electrodes 47, which are deposited on the other side of the film, are formed of a material, such as gold, having a secondary emission ratio substantially greater than that of aluminium. The aluminium film connects to a lead 48, Fig. 5, and is held at a potential slightly less than the final anode 50. As the beam traverses the screen, the secondary emission current to anode 50 increases sharply each time the beam crosses an electrode and a pulse is applied through circuit 51, 52 to amplifier 54. The amplifier output pulses are applied to a delay line 53 from which pulses are derived at tappings 55, 56, 57 to operate the signal sampling stages 20, 21, 22. The respective tappings on the line are selected to provide delays equal to the time the scanning beam takes to travel from the electrode to the centre of the corresponding colour stripe. To ensure the production of adequate pulses, the video signals are arranged for negative modulation so that the beam returns to full intensity in the absence of signal. Specification 753,442 also is referred to.