Patent GB-1134646-A (GB)
Title: Colour television receiver
IPC: H04N9/47
Filed: 1967-10-19
Abstract: 1,134,646. Colour television receivers. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 19 Oct., 1967 [22 Oct., 1966], No. 47653/67. Heading H4F. Relates to a colour signal identification circuit arrangement for a colour television receiver for displaying a colour television signal, the nature of which alternates from transmitted line to line, for example in the PAL or SECAM (Trade Mark) system, in which a " swinging " colour subcarrier burst signal is transmitted during each line blanking period which alternates its phase (PAL-system) or its frequency (SECAM- system) from line to line. The separated receiver colour information signal at output 4 of circuit 1, Fig. 1, is fed to an identification signal generator 15 which in response to the frequency or phase of the burst produces an alternating voltage of frequency equal to half line frequency (the identification signal) which is fed via signal processing circuit 50 to a switching signal generator 28 (a bi-stable multivibrator) which generates a switching signal which actuates an identification switch 11 so that a colour information channel is chosen for each line which matches the colour information appearing during that line. Processing circuit 50 includes a phase detector 35 which compares the phases of the identification signal, fed to its input 33 via signal path 18, and the switching signal from generator 28, to produce a control signal output which influences the identification signal applied to generator 28. The resulting influence on the generator 28 being less when the colour signal identification is correct than when it is incorrect. Received sync. signals are fed to a line time-base generator 19, the outputs of which supply a gate signal to detector 15, and voltage pulses to signal path 18, generator 28 and display tube circuits 9. Although the control signal (V#) which is applied to input 51 of signal path 18 may be applied to any point on the signal path 18 it is preferably applied to a point located between output 32 and generator 28. The magnitude of the control signal may be adjusted so that the identification signal is completely cut-off from generator 28 if the phase relationship is correct so that the generator 28 is then responsive only to the line flyback pulse applied at its input 27. If the phase relationship is not correct, the control signal need not result in any diminution of the effect of the identification signal, which in conjunction with the line flyback pulse restores the correct phase of generator 28. Line flyback pulses which may be applied at input 26 may be superimposed on the peaks of the identification signal, thus further reducing the influence of noise interference on the operation of switch 11. In Fig. 2, which shows the processing circuit 50, if no identification signal appears at input 17, i.e. no colour information in the signal to be processed by the receiver, only a pulsatory signal voltage from the line time-base applied at input 26 is applied to the control grid of tube 69. The anode current and thus the anode voltage of tube 69 are then pulsed and these pulses are rectified in a colour killer detector 109, the resulting rectified voltage cutting-off the colour circuits at the display tube. When the signal contains colour information, input 17 receives identification signal Vid of half line frequency which is applified by tube 55 and filtered by circuit 63, 65 tuned to half line frequency (which reduces interference signals) to form signal V<SP>1</SP>id which is fed to the input 33 of phase detector 35 where its phase is compared with the phase of the switching signal Vsch at the output of generator 28. When diodes 85 and 87 are rendered conducting by the switching signal, signal V<SP>1</SP>id is passed to the junction of resistors 89, 91 and smoothing capacitor 93. This capacitor is thus charged to a negative direct voltage V# which is applied via resistor 77 to the control grid of tube 69. The internal resistance of phase detector 35 is made high so that with capacitor 93 it forms a time constant exceeding 10 line periods. Voltages V<SP>1</SP>id, V# and the pulses applied at input 26 are applied simultaneously to the grid of tube 69 and are denoted by Vg. Capacitor 67 and resistor 77 produce a phase shift in signal V<SP>1</SP>id so that the line time-base pulses coincide with the peaks of this signal. Diode 75 fixes the peaks of signal Vg at V# and thus signal Vg does not exceed the cut-off voltage of tube 69 so that no signal is produced across the anode circuit of tube 69. As a result generator 28 is not affected by the identification signal and detector 109 does not produce a cut-off voltage. If the receiver receives a colour signal and the multivibrator 28 switches in such a phase that the switch 11 assumes a wrong position, signal Vsch and signal V<SP>1</SP>id at the inputs of phase detector 35 produce a positive voltage at capacitor 93. Tube 69 thus conducts and its anode current is pulsed, the pulses which are at half line frequency appear just at those instants when they can trigger multivibrator 28 into the correct phase. Detector 109 receives a signal voltage and its output voltage continues to cut-off the colour signal as long as the multivibrator 28 does not operate in the correct phase. Any interferences in the signal V<SP>1</SP>id appearing at the control grid of tube 69 do not affect the correcting operations. Switch 83 is provided to cut-off the colour channel.