GB1268795A - Colour killer circuit - Google Patents

Abstract

1,268,795. Television. INTERNATIONAL STANDARD ELECTRIC CORP. 18 Sept., 1969 [25 Sept., 1968], No. 46054/69. Heading H4F. A colour killer circuit for blocking the colour channel of a PAL or "SECAM" (Registered Trade Mark) receiver when the colour sync. signal A, Fig. 2c or the line identification signal, falls below a predetermined level S, comprises a bi-stable circuit 1 to 14, Fig. 1, switched by the negative line fly-back pulses 19, to give a ¢ line frequency square wave output, Fig. 2a. The fly-back pulses are applied to transistor 12 via a gate 25 and a reference signal and a signal derived from the colour sync. signal or the line identification signal are applied in opposition to control the gate, such that reduction of the derived signal below the reference signal causes the gate to disconnect the line fly-back pulses from the transistor whereby it remains in one stable state and produces a signal for said blocking action. The reference signal is set to represent said predetermined level by means of variable resistance 23. A sinusoidal signal at ¢-line frequency and derived from said synchronization or identification signal, is applied at 32 and rectified and smoothed at 30, 29, 28 to produce said derived signal. R/C circuit 15/16 forms the average, Fig. 2b, of the bi-stable output signal, Fig. 2a, the decrease of the average being arranged to cause the channel blocking.