GB1288990A - - Google Patents

Abstract

1288990 Colour television PHILIPS ELECTRONIZ & ASSOCIATED INDUSTRIES Ltd 16 Dec 1970 [19 Dec 1969] 59703/70 Heading H4F In a PAL switch circuit arrangement in which a direct voltage is produced in response to erroneous phase operation of the PAL switch, this voltage being supplied to the switch to stop it, a delayed direct voltage is derived from the switch when in its stopped condition which is supplied to the circuit arrangement in a sense such as to suppress the stopping action of the switch. Thus the switch always restarts after which both direct voltages disappear until an incorrect switching phase again occurs. This circuit arrangement prevents the switch being permanently stopped when a continuous direct voltage is produced due to an erroneous reference oscillator-burst signal phase relationship occurring when the switch is stopped. As shown, a reference subcarrier signal is supplied to terminal 9, and received colour synchronizing (swinging burst) signals in opposite phases are supplied to terminals 1 and 2. The phase of the reference signal is changed in synchronism with the change in phase of the swinging burst by means of the PAL switch which operates at halfline frequency. If the switch is operating in the correct phase a negative voltage appears across output capacitor 11 and transistor 13 is cut-off. The PAL switch, which may be externally synchronized, comprises transistors 14 and 15 which form a multivibrator, their bases and collectors being crossconnected in a supply network 16 so that a square-wave is produced. If transistor 13 is turned-on by a positive direct base voltage, transistor 14 is cut-off and the multivibrator stops. As a result the phase of the switch is corrected, the positive input voltage to transistor 13 disappears and the multivibrator continues to operate in the correct phase. However if the phase relation between the burst signals and the reference signal differs from the desired value a positive direct voltage may appear across capacitor 11 even when the multivibrator is stopped, and this voltage will drive transistor 13 continuously thus permanently stopping the multivibrator 14, 15, 16. To overcome this, the emitters of transistors 14 and 15 are coupled with each other via large capacitor 19 and to earth via individual resistors 20, 21. In normal operation with transistors 14 and 15 alternately conducting no voltage appears across capacitor 19. This situation is not changed in any way if the switch is stopped for a short time for phase correction. But if the switch is stopped for any length of time capacitor 19 charges causing a negative baseemitter voltage to be applied to transistor 13 so that it is cut-off and the multivibrator is restarted.