Patent GB-781913-A (GB)
Title: Improvements in and relating to cathode-ray tube apparatus
IPC: H04N9/24
Filed: 1954-01-06
Abstract: 781,913. Colour television. PHILCO CORPORATION. Jan. 6, 1954 [Jan. 6, 1953; Sept. 25, 1953], No. 381/54. Class 40 (3). [Also in Group XL (c)] Relates to a colour television receiver employing a cathode-ray tube 12 with a screen 24 comprised of a repeating sequence of red, green and blue vertical phosphor strips and in which indexing regions are provided between each group of three strips to provide a signal indicating the instantaneous position of the cathoderay beam on the screen. An oscillator 62 modulates the tube beam at a frequency of 31.5 Mc/s., and the indexing output comprises this 31.5 Mc/s. signal modulated in amplitude at the frequency of traversing the indexing regions. This frequency is 7 Mc/s. in the embodiment described, but is subject to variation in dependence upon the scanning linearity of the horizontal scanning generator 32 and non- uniformities in the spacing of the colour phosphors and indexing regions. The upper sideband of the indexing signal at 38.5 Mc/s. is selected by a narrow-band amplifier 64 and provides information to control the application of colour signals to modulate the cathode-ray beam in synchronism with the beam impingement on the phosphors. Owing to the selective nature of amplifier 64, a phase-shift is imparted to the indexing signal variable as a function of its frequency, and such phase variations produce errors in the synchronization of the colour signal. In accordance with the invention, this effect is avoided by applying the output from amplifier 64 to angle modulate the 31 Mc/s. signal from oscillator 62. In the embodiment illustrated in Fig. 1, the 38.5 Mc/s. indexing signal is applied to a frequency-responsive detector 68 producing an output of amplitude and polarity dependent upon departures of the signal from 38.5 Mc/s. and this output is applied to control a reactance stage 66 determining the frequency of oscillator 62. The arrangement operates to maintain the frequency of the 38.5 Mc/s. upper side-band of the indexing signal substantially constant by varying the frequency of the 31.5 Mc/s. input signal. In effect the indexing information is transferred to oscillator 62 and its output is then employed to control the colour synchronization. In a modification of this embodiment, Fig. 4 (not shown), the 38.5 Mc/s. upper-side band is maintained absolutely constant in frequency by comparing it with a 38.5 Mc/s. reference signal in a phase comparator circuit, the output from which controls the reactance stage acting on the 31.5 Mc/s. oscillator. In a second embodiment, Fig. 3 (not shown), the 38.5 Mc/s. signal is allowed to vary and is employed directly to control the colour synchronization, but the phase errors imparted by the side-band amplifier 64 are compensated by applying the 38.5 Mc/s. to a frequency sensitive detector the output of which controls a phase modifying circuit connected between the 31.5 Mc/s. oscillator and the cathode-ray tube grid. The phase compensation applied may be sufficient not only to overcome the effect of amplifier 64 but also errors introduced in subsequent stages. The invention is described as applied to receivers utilizing a signal made up of a 0-3.5 Mc/s. luminance signal and a 3.89 Mc/s. sub-carrier modulated in phase and amplitude in accordance with the hue and saturation of the image. As shown in Fig. 1, the luminance signal is selected by filter 74 and applied directly to the cathode-ray tube via adder 78, whilst the sub-carrier component is selected by filter 76 and applied to a mixer 86. The normal synchronizing bursts at the subcarrier frequency are separated at 80 to synchronize at 3.89 Mc/s. oscillator 82. The output from oscillator 62 is mixed with the 3.89 Mc/s. from oscillator 82 in mixer 84 to produce a difference signal of 27.61 Mc/s. and this signal is then mixed with the sub-carrier signal in mixer 86 to produce a sum signal of 31.5 Mc/s. This signal carries the colour modulation, but by reason of its derivation from oscillator 62 varies in frequency in accordance with the indexing information and hence ensures the accurate synchronization of the colour signal with scanning of the cathode-ray tube screen. The signal is reduced to 7 Mc/s. in mixer 88 and combined with the luminance signal in adder 78. In certain circumstances it is necessary to modify the colour characteristics of the incoming signals in order to conform to the colour actually produced by the cathode-ray tube phosphors. This is effected by synchronously detecting the signal at a particular phase in mixer 90 and adding the detection products in proper relative amounts to the original signal in adder 78. Specifications 745,353, 753,451, 781,911, [Group XL (a)], and U.S.A. Specification 2,372,210 are referred to.