Patent GB-1486914-A (GB)
Title: Colour television system
IPC: H04N9/465
Filed: 1974-10-01
Abstract: 1486914 Colour television INDESIT INDUSTRIA ELETTRODOMESTICI ITALIANA SpA 1 Oct 1974 [5 Oct 1973 13 Nov 1973 7 Dec 1973 (2) 3 April 1974] 42510/74 Heading H4F A PAL colour television signal includes as a colour synchronizing and switching identification signal a colour burst inserted in only every second line blanking period. Additional information, such as sound in a foreign language, additional sound, test signals or facsimile signals may be transmitted during the intervening line blanking periods and during the field blanking periods. In a first embodiment of receiver, Fig. 2, for a PAL signal having colour bursts at the same phase as the B-Y modulation axis, the chrominance component is fed to synchronous demodulators 12, 13 and a burst gate 14 which separates the colour bursts which are fed to a reference subcarrier generator 18 and an amplitude detector 19. The reference subcarrier, at B-Y phase, from generator 18 is supplied directly to demodulator 13 and via a 90 degree phase shifter 21, a 180 degree phase shifter 22 and a PAL-switch 23, to demodulator 12. PAL- switch 23 is controlled by a switching voltage generator 16 receiving line synchronizing pulses 15 and is controlled in phase by the output of amplitude detector 19. In a second embodiment (Fig. 4, not shown) a gate (26) is included between burst gate 14 and amplitude detector 19 and is controlled by the output of switching voltage generator 16 so that if the switching voltage generator 16 is operating in the correct phase the colour bursts are blocked by this gate. In a third embodiment (Fig. 3, not shown) amplitude detector 19 is omitted and the phase of switching voltage generator 16 is controlled by the output of a burst gate (25) receiving the output of demodulator 13. In any of these embodiments a half line frequency tuned filter may be included between amplitude detector 19 (or burst gate 25) and generator 16. In a further embodiment of receiver, Fig. 5, also for a PAL signal having colour bursts of B-Y phase, the chrominance signal is fed via a colour killer switch 52 to demodulators 57, 58 and via a burst gate 61 to a reference subcarrier generator 62 which supplies an output via an amplitude limiter 63 to demodulator 58 and PAL-switch 66 which feeds demodulator 57 via +90 and - 90 degree phase shifters 67, 68. The output of reference subcarrier generator 62 is also fed to an amplitude detector 64 which supplies a low-pass filter 71 which controls colour killer switch 52 and a filter circuit 70 which effects a differentiation and supplies half line frequency pulses to control a switching voltage generator 60. In a further embodiment (Fig. 6, not shown) amplitude detector 64 receives the output signal from amplitude limiter 63 and supplies control signals directly to colour killer switch 52 and switching voltage generator 60. Burst gate 61 is then controlled to open only during every second line period by a control circuit (73) receiving the line synchronizing pulses and the output of switching voltage generator 60; if gate 61 operates out of phase it does not pass the colour bursts, subcarrier generator 62 stops and switching voltage generator 60 is reset by the output of amplitude detector 64. In modifications of this last embodiment amplitude limiter 63 is omitted and amplitude detector 64, via a low-pass filter, controls colour-killer switch 52 and an additional switch connected in the line synchronizing signal input to switching voltage generator 60 (Fig. 7, not shown) or in the connection from the switching voltage generator 60 to the control circuit (73) controlling burst gate 61; in the latter arrangement amplitude detector 64 also supplies a signal via a high-pass filter (91) to switching voltage generator 60 to reset it if burst gate 61 operates in the wrong phase. In the former arrangement (Fig. 7, not shown) if burst gate 61 operates in the wrong phase it does not pass the colour bursts, subcarrier generator 62 stops and amplitude detector 64 opens the additional switch (86) to stop switching voltage generator 60; burst gate 61 then opens during every line blanking period and subcarrier generator 62 and switching voltage generator 60 restart; if necessary this is repeated until burst gate 61 operates in the correct phase. In a further embodiment, Fig. 9, which is a modification of Fig. 5, the output of amplitude detector 64, via a low-pass filter 85, controls a switch 102 which passes a signal from an amplitude detector 101 only when colour killer switch 52 is opened by the absence of an output from subcarrier generator 62, this signal resetting switching voltage generator 60. In Fig. 9, 84 designates the PAL-switch and 80 a gain controlled amplifier. In a further embodiment (Fig. 10, not shown) the PAL-switch (119) is controlled by a switching voltage generator (121) phase-controlled by a phase comparator (125) receiving outputs from the switching voltage generator (121) and from a burst gate (116) connected to the output of the B-Y demodulator (111). In a further embodiment of receiver, Fig. 12, for a PAL signal having colour bursts at a phase of +135 degrees relative to the B-Y modulation axis, the chrominance component is fed to a demodulator 153 and, via a one-line delay line 151, to a demodulator 152 and a burst gate 156. The output of burst gate 156 controls a reference subcarrier generator 159 which produces a reference subcarrier in phase with the received bursts which is fed to PAL- switch 160 which has one output connected directly to demodulators 152, 153 and the other output connected to demodulators 152, 153 via a 90 degree phase shifter 161. Burst gate 158, half line frequency selective circuit 162 and amplitude squarer 163 control PAL-switch 160 so the during the lines containing the colour bursts a reference subcarrier of +225 degrees phase relative to B-Y is supplied to demodulators 152, 153, whereas during the intervening lines a reference subcarrier of +135 degrees relative to B-Y is supplied to demodulators 152, 153. Demodulator 152 then produces an output proportional to (R-Y) - (B-Y) and demodulator 153 produces an output proportional to (R-Y)+(B-Y).