Patent GB-1220993-A (GB)
Title: Colour killer and automatic chroma control circuits
Inventor: CARPENTER DAVID HARRY
IPC: H04N9/71
Filed: 1968-03-06
Abstract: 1,220,993. Colour television. RCA CORPORATION. 6 March, 1968 [6 March, 1967], No. 10867/68. Heading H4F. In a colour television receiver signals controlling an automatic chroma control (A.C.C.) circuit and a colour killer circuit are derived from a common terminal point. Referring to Fig. 1, periodic colour synchronizing signals, specifically colour "burst" signals, in the chrominance signals from a driver amplifier 24 are separated out by means of a gated amplifier 32 the output circuit of which includes a filter 36 having a crystal 114 resonant at the burst frequency (i.e. the chrominance sub-carrier frequency) and the C.W. wave thus produced across capacitor 116 is supplied to the control grid of the sub-carrier oscillator 40 to synchronize the operation thereof. When no burst signals are present oscillator 40 which operates (when not synchronized) at a frequency slightly above the sub-carrier frequency, produces a negative voltage at its control grid 120 by rectification of the positive half-cycles of its own oscillations. When bursts are present and the oscillator is synchronized the C.W. wave from capacitor 116 adds in phase to the existing oscillations at grid 120 and the resulting negative D.C. voltage produced by grid rectification is increased by an amount proportional to the amplitude of the C.W. wave which is itself proportional to the amplitude of the separated bursts and thus proportional to the amplitude of the chrominance signals at the output of driver amplifier 24. This voltage which may be employed for A.C.C. purposes is further increased (approximately doubled) by adding to it the voltage produced across a capacitor 152 by rectification, by means of diode 150, of the negative half-cycles of the C.W. wave at the grid 120 to provide a voltage for control of the colour killer circuit. The A.C.C. operation is effected by controlling the grid 52 of the chrominance amplifier 30 in accordance with the voltage across capacitor 152. The absolute value of this voltage, is far too negative to be employed directly and it is first necessary to translate it to a more positive range. This is done by supplying it to a resistor 156 which with resistor 158 forms a potential divider with the remote end of 158 connected to a positive potential source, specifically, the cathode 100 of the gated amplifier 32. The voltage at point 159 is then suitable for A.C.C. operation and this point is connected to grid 52 via resistor 48 and portion of inductor 46. The colour killer operation is effected by supplying to the grid 52 (of the chrominance amplifier 30) a negative voltage of such value that amplifier 30 is cut-off when the amplitude of the colour synchronizing bursts fall below a predetermined value. The range of the voltage across capacitor 152 is suitable for this purpose but again its absolute negative value is such that it would produce cut-off under all conditions and to overcome this it is translated to a more positive range by means of the potential divider formed by resistors 172, 174 and the potentiometer 178 fed with the positive potential at the cathode 100 of gated amplifier 32. The resulting voltage at point 180 is supplied to the grid of a valve 162 (in a colour killer circuit 42) the anode of which in addition to being supplied from a positive voltage source via a large resistor 186 is also supplied with the line fly-back pulse 90 via an integrating resistor 182 and a potential divider comprising capacitors 194, 196. The operation of the circuit is stated to be as follows. As the voltage across capacitor 152 becomes less negative with decreasing amplitude of the colour synchronizing bursts valve 162 commences to conduct at a point determined by the setting of tap 176 on potentiometer 178. The voltage at the anode 160 is therefore reduced and by the rectifying action of the anode on the pulse 90 (the peak amplitude of which applied to the anode 160 must exceed the potential supplied to resistor 186) the voltage at anode 160 becomes negative with respect to ground. Diode 168 then conducts and the voltage at anode 160 is supplied to the grid 52 to cut-off amplifier 30.