Patent GB-984536-A (GB)
Title: Improvements in or relating to circuit arrangements for colour television receivers
IPC: H04N11/14
Filed: 1962-01-19
Abstract: 984,536. Colour television receivers. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. Jan. 19, 1962 [Jan. 24, 1961], No. 2054/62. Heading H4F. In a circuit arrangement for converting a signal comprising a luminance component and sub-carrier component modulated with colour signals at different phase angles (e.g. the N.T.S.C. type of signal) into a dot-sequential signal suitable for direct application to a single-gun cathode-ray tube, the operations of effecting a luminance correction (i.e. the Y to M correction) and of remoulding the subcarrier into dotsequential form is effected by a single modulator circuit which is supplied with reference signals (regenerated in the conventional manner by means of synchronizing " bursts ") at both the sub-carrier frequency and double the subcarrier frequency. Fig. 1 shows a first embodiment in which the composite signal E (i.e. luminance and subcarrier components) is applied to the first grid of a modulator stage 11, a signal fs + 2fs (where fs equals the sub-carrier frequency) is applied from oscillator stage 1 to the third grid and an output is derived from the anode circuit via a low-pass filter 17 which eliminates signal components at twice the sub-carrier frequency and higher frequencies. The conversion is effected by the multiplication of the sub-carrier component by the signal fs + 2fs, but since the luminance component is also present in stage 11 it is also multiplied. The resulting unwanted luminance component at frequency 2fs is eliminated by filter 17, but the unwanted luminance component at frequency fs is eliminated by producing an antiphase component. This is effected in a second modulator stage 14 which receives the luminance component only on its first grid by way of low-pass filter 13, receives a signal - fs on its third grid in phase opposition to the signal fs on the third grid of stage 11, and has its anode connected in parallel with the anode of stage 11 to a common anode load. The resulting signal derived through filter 17 is of the correct dot-sequential form but has an incorrect amplitude relationship between the monochrome component and the sub-carrier component. This is corrected by giving the circuit a gain boost at sub-carrier frequency by including a tuned circuit 18 as part of the anode load and series tuned circuits 21 and 22 in shunt across the cathode resistors 19, 20. As an alternative, filter 17 may be given a suitable transmission characteristic. A delay circuit 12 in the feed path to the first grid of stage 11 equalizes the delay due to low-pass filter 13. Oscillator 1 is a conventional reaction oscillator operating in a class C mode and has tuned anode circuits 2 and 3 to derive signals at frequencies fs and 2fs. As an alternative, stage 11 may be operated in a class C mode so as to produce the harmonic 2fs in its grid circuit. An analysis of the operation of the circuit is given. In a modification, Fig. 3 (not shown), the gain phase characteristics of the circuit are chosen to produce an output signal which may be brought into the correct form simply by the addition of a luminance component of appropriate amplitude which is derived from the output of filter 13. A second embodiment, Fig. 4, utilizes a modulator stage comprised of a beam switching tube 28. The sub-carrier component is separated by band-pass filter 34 and applied via centre-tapped transformer 35 to the deflection plates 29, 30, whilst the signal f + 2fs is applied to the first grid. The output is derived from the difference of the anode currents by means of transformers 33 and is applied via low-pass filter 17, which eliminates components of a frequency equal to 2fs and above, to an adding circuit 37 where it is combined with an appropriate proportion of luminance signal derived via low-pass filter 13, potentiometer 38 and delay line 36. A third embodiment, Fig. 5, makes use of a modulator utilizing two centre-tapped transformers 41, 42 and two diodes 39, 40. The arrangement is otherwise similar to Fig. 4.