Patent GB-753451-A (GB)
Title: Improvements in and relating to cathode ray tube systems
IPC: H04N9/24
Filed: 1952-07-31
Abstract: 753,451. Colour television. PHILCO CORPORATION. July 31, 1952 [Aug. 4, 1951], No. 19395/52. Class 40 (3). In a colour television receiver employing a cathoderay tube with a screen formed of a repeating series of red, green and blue stripes and including indexing stripes providing signals indicating the instantaneous position of the beam, clearly defined indexing signals are produced by modulating the cathoderay beam with an oscillation lying outside the frequency range of the video signals and detecting the components produced in the indexing circuit by the oscillation. As described, the indexing signals are employed to effect proper timing of the application of video signals to the tube. Alternatively, the signals may be employed to control the linearity of the beam deflection as disclosed in Specification 753,448, [Group XXXV]. The tube screen is constructed as shown in Fig. 2 and comprises stripes 19-21 of red, green and blue phosphor material disposed at rightangles to the direction of line scan. The stripes are backed by a coating 22 of aluminium, magnesium or beryllium and over each green stripe is disposed an indexing stripe 23 formed of a material such as gold or magnesium oxide which has a secondary emission ratio detectably different from coating 22. Fig. 3 shows a first embodiment. The tube 10 is provided with normal deflection and supply potentials, and the indexing stripes are connected through a resistor 34 to the accelerating anode 14 so that, in scanning, potential variations are produced across the resistor indicating the rate of traversing the stripes. The nominal frequency of these variations is of the order of 7 Mc. (in the example shown) and is subject to a variation of plus and minus 10 per cent due to variation in the scanning linearity and stripe spacing. A carrier wave oscillator 33 operating at 38.5 Mc. is connected to the tube grid 12 and causes a corresponding oscillation modulated by the 7 Mc. indexing signal to appear across resistor 34. The difference frequency component at 31.5 Mc. is selected by filter 35. The carrier wave oscillator is also connected to a 3-phase generator 38, such as a delay line, which produces outputs at 38.5 Mc. displaced in phase by 120 degrees. These outputs control sampling gate stages 25, 26, 27 which receive video signals representing respectively red, green and blue aspects of the image to be reproduced. The gate outputs are combined and applied to a filter 42 having a pass-band centred about 38.5 Mc. Thus there appears at the output of filter 42 a 38.5 Mc. signal modulated in amplitude and phase in accordance with the time-multiplexing of the three video signals. This signal is then applied to a mixer 37 where it is heterodyned with the 31.5 Mc. signal from filter 35 to produce a difference frequency signal of 7 Mc. which is selected by filter 43 and applied to the tube control grid 12. In effect, the timemultiplexel colour signals are modulated on to a 7 Mc. carrier, the carrier, however, varying in frequency in accordance with the rate at which the cathode-ray beam scans the screen so that faithful colour reproduction results. Due to non-linearity in the control grid circuit, spurious components may be produced in the indexing signals at the difference frequency between the two grid signals. To eliminate these, an output proportional to the spurious components is developed across a cathode resistor 44 and applied through cancellation amplifier 45 with appropriate gain and phase to resistor 46 in the indexing signal circuit. Other methods of effecting cancellation are disclosed in Specification 753,444. As a further alternative, a tube employing two separate guns as disclosed in Specification 745,353 may be employed. A second embodiment, Fig. 4, operates with incoming signals of the type including a brightness component extending over the band 0-3 Mc. and upon which there is superimposed a 3.5 Mc. sub-carrier conveying the colour information. The tube grid is again supplied with a 38.5 Mc. signal from oscillator 33, in this arrangement via adding circuit 47, and the 31.5 Mc. component from across resistor 34 is selected by filter 35. The incoming signal is separated into the brightness component by filter 49 and the sub-carrier component by filter 50. The normal 3.5 Mc. colour synchronizing bursts appearing on the trailing edge of the horizontal blanking pulses are separated by circuits 51, 52 and synchronize a 3.5 Mc. oscillator 53. The oscillator output is heterodyned with the output from filter 35 in mixer 54 and the sum frequency of 3.5 Mc. is applied to a balanced mixer 55 where it is heterodyned with the 3.5 Mc. sub-carrier from filter 50. The difference frequency component of 31.5 Mc. which is now phase and amplitude-modulated by the colour signal is then heterodyned with the 38.5 Mc. from oscillator 33 in mixer 56 to produce a difference frequency signal of 7 Mc. which is selected by filter 57. This 7 Mc. signal which conveys the colour information by phase and amplitude modulation, but also varies in frequency in accordance with the rate at which the cathode-ray beam scans the screen, is now applied to the tube grid through adding circuit 47 where it is combined with the brightness signal from filter 49 and the 38.5 Mc. oscillation. In yet a third embodiment, Fig. 1 (not shown), the 31.5 Mc. component across resistor 34 is selected by a filter and heterodyned with the original 38.5 Mc. oscillator to derive the 7 Mc. indexing signal varying in accordance with the scanning rate. This signal is then applied directly to control the operation of a three-phase generator actuating sampling stages operating on incoming red, green and blue video signal trains. As an alternative to heterodyning the 31.5 Mc. component, the whole of the output from across resistor 34 may be taken and detected by a conventional amplitude detector to derive the 7 Mc. indexing signal. Specifications 753,441, 753, 442 and 753,447 also are referred to.