Patent GB-1076621-A (GB)
Title: A colour television image reproducing system
IPC: H04N9/26
Filed: 1964-09-24
Abstract: 1,076,621. Colour television; transistor stepwave generators. YAOU ELECTRIC CO. Ltd. Sept. 24, 1964 [Feb. 17, 1964 (9)], No. 38968/64. Headings H3T and H4F. To reproduce N.T.S.C-type colour signals, using a single gun cathode-ray tube having a line phosphor screen and post deflection colourswitching grids, a blocking oscillator is arranged to generate a waveform having a frequency of one-third of the horizontal scanning frequency fh and this waveform is used to produce a stepped waveform for application to the colourswitching grids. From video detector 3 and video frequency amplifier 4, Fig. 1, the luminance signal component of the received N.T.S.C. signal is applied to the cathode of the CRT and the synchronizing signal and chrominance signal are separated and derived in separator 8 and band-pass amplifier 12 respectively from the composite signal. The colour synchronizing signal is amplified in stage 13 in synchronism with an oscillating signal having a frequency <SP>1</SP>/ 3 fh derived in blocking oscillator 20 which is fed with the horizontal deflection pulses from horizontal deflection output circuit 11. A crystal oscillator 16 arranged to oscillate at a frequency shifted <SP>1</SP>/ 3 fh from the colour subcarrier frequency fec, i.e. at a frequency of fscŒfh/3 is held in a certain phase relationship to the colour synchronizing signal from amplifier 13 by means of phase detector 14 and reactance circuit 15. A sub-carrier of the linesequential type is formed in a phase modulator and amplitude limiter 17 from the output of the crystal oscillator 16 and the output of a sawtooth wave-shaping circuit 21 which is adapted to form a sawtooth wave having the horizontal scanning frequency from the output of deflection circuit 11. Demodulator circuit 18 produces a line-sequential colour difference signal, by phase detecting the chrominance signal from amplifier 12 with the line-sequential type subcarrier (the phase of which changes 120 degrees in successive lines) from circuit 17, and applies the colour difference signals via amplifier 19 to the first grid G 1  of the tube. Since the output of the fsc+fh/3 crystal oscillator gains in phase successively by 120 degrees relative to fsc during each horizontal scanning period, the phase is conversely adjusted to lose in proportion to the slope of the horizontal period sawtooth wave so that the oscillator phase is maximum at 120 degrees thereby maintaining a constant phase difference with relation to fsc during each horizontal scanning period. The stepped-waveform voltage which is applied to the colour switching grids of the tube via switching circuit 23 is produced in a matrix circuit 22 by adding a sawtooth wave of horizontal scanning frequency, from integrating circuit 27 fed with the output from horizontal deflection circuit 11, and a sawtooth wave of one-third the horizontal scanning frequency from integrator 26 fed with the output from blocking oscillator 20. To compensate for the reduction factor of each chrominance signal the stepped waveform from matrix circuit 22 is passed to shaping circuits 24 and 25 to effect automatic control of the output level of the bandpass amplifier 12, and to be applied to the screen grid G 2  of the tube respectively. In a second embodiment the stepped-waveform voltage may be produced by means of a storage counter consisting of one transistor TR 2  and three diodes D 1 , D 2  and D 3 , Fig. 6, together with a blocking oscillator. Negative horizontal pulses H are applied at point (a) and for each pulse condenser C 2  is charged through diode D 1  and capacitor C 1 , and during the scanning period in the absence of such pulses capacitor C 1  discharges while the potential at point (b) of capacitor C 2  is held constant. The potential at point (b) thus has a steppedwaveform, but as the third horizontal pulse is applied, the blocking oscillator of transistor TR 1  operates so that a positive pulse is applied across diode D 3  to point (b) completely discharging capacitor C 2 . The output is thus a stepped-waveform of period 3H as shown. To synchronize the crystal oscillating frequency fscŒfh/3 with the colour synchronizing signal (burst), the colour synchronizing signal is applied to a phase detector every third cycle of horizontal scanning. Accordingly, a gate pulse which may be derived from a blocking oscillator having a cycle corresponding to three horizontal scanning cycles is applied to the colour synchronizing signal amplifier 13, Fig. 1.