Patent GB-945803-A (GB)
Title: Improvements in or relating to circuit arrangements for beam-indexing colour television display tubes
IPC: H04N9/24
Filed: 1961-01-17
Abstract: 945,803. Television. PHILIPS ELECTRICAL INDUSTRIES Ltd. Jan. 17, 1961 [Jan. 20, 1960], No. 1891/61. Heading H4F. In a colour television receiver utilizing a beam indexing cathode-ray display tube 1 (Figs. 1, 2 and 3) in which index signals are produced by the release of secondary electrons from indexing strips forming part of the tube screen 2 when the latter is scanned by an electron beam, phase variations in the index signals due to differences in the transmit times of the secondary electrons from different parts of the screen to the wall electrode 3 are compensated by introducing equal and opposite phase variations into the beam modulation of the tube. Because of the geometry of the tube structure the required compensating phase variations must follow a parabolic law having horizontal and vertical scanning frequency components and in accordance with a feature of the invention the compensating phase variations are produced by the output of an adder 14 in which are combined parabolic waves 19, 20, of horizontal and vertical scanning frequency respectively, derived by integrating in 15 and 16 the horizontal and vertical deflection saw-tooth waves 17 and 18 respectively. In the embodiment of Fig. 1, which utilizes a tube having two beams controlled by respective modulating electrodes 4, 5, the output of adder 14 is supplied to a phase control circuit 13 to vary the phase of oscillations from an oscillator 11 which modulates the beam (pilot beam) in electrode 4 to produce the required index signals which, in known manner, are employed together with the chrominance signals (input to mixer M 1 ) the chrominance sub-carrier derived from the " burst " signals 22 (input to mixer M 2 ) and the luminance signal Y (input to adder 12) to provide a colour signal for application to beam modulating electrode 4. Instead of applying the compensating phase variations to the signal which modulates the pilot beam the compensation may be effected in the chrominance sub-carrier by supplying the output of adder 14<SP>1</SP> (similar to adder 14) to the device 21 in which the chrominance sub-carrier is derived from the " bursts " 22. Fig. 2 shows an alternative arrangement (also utilizing a twobeam tube) in which the compensating variations are introduced into the chrominance subcarrier in device 21 and combined in mixer M 2  with the local oscillator signal which modulates the pilot beam via electrode 4. In the embodiment of Fig. 3 (which utilizes a singlebeam tube) the compensated chrominance subcarrier from device 21 is combined in mixer M 2  with the combined chrominance signals (line 34) and index signals (line 33) from mixer M 1  and after the addition of the luminence signals Y in adder 12 are supplied to the beam-modulating electrode 23 via a gate 31 which is controlled by pulses of indexing frequency derived from a delay device 30 such that during the scanning of index strips the signals from 12 are interrupted and electrode 23 is brought to a fixed potential. In this embodiment a gate 24, in a loop circuit including a trigger 25, phase detector 26, oscillator 27 and variable reactance 28, is opened by the output of oscillator 27 only during the periods that the beam is scanning the index strips thus ensuring that the index signals are free from video information. Also, to ensure synchronism of oscillator 27, gate 24 is also opened for short periods at the beginning of each line by pulses at line frequency supplied via line 29. The phase control circuit 13 (Fig. 1), is described with reference to Fig. 4 (not shown), and the device 21 (Fig. 1) in which the chrominance subcarrier is derived from the " burst " frequency may have the form described with reference to Figs. 6 and 7 (not shown). Specification 945,802 is referred to.