Patent GB-1193576-A (GB)
Title: Clipping Circuits.
Inventor: ROSS JOHN D
IPC: H04N9/72
Filed: 1968-07-31
Abstract: 1,193,576. Clipping circuits for colour television. CENTRAL DYNAMICS Ltd. 31 July, 1968, No. 36585/68. Heading H4F. A circuit is provided for clipping a composite signal having first and second components, in which the first component is clipped whenever it exceeds a certain level but the second component is not clipped when it exceeds the same level. As applied to colour television signals, the first component is a luminance signal while the second component is a chrominance signal, and the invention resides in separating the chrominance signal from the composite signal and applying it to means for desensitizing the clipping circuit whenever the chrominance signal exceeds the clipping level. As shown in Fig. 5 the chrominance signal is selected by a filter 22<SP>1</SP> which includes capacitive and resistive means for maintaining the phase and amplitude of the chrominance signal. The filter 22<SP>1</SP> preferably comprises a band pass filter with a superimposed high pass capability to eliminate distortion. Gating pulses from a source 20 synchronized with the blanking period of the television signal are supplied to a gate 12<SP>1</SP> with the selected chrominance signal, resulting in removal of the synchronization pulses and colour bursts from the blanking period thereof. The composite television signal passes through a clamping circuit 31 which fixes the back porch at the blanking level, and then by the addition of the gating pulses from source 20 to the clamped composite signal in an adder 29 the levels of the synchronization pulses and colour bursts are depressed below the black clipping level so that the output from the black clipping circuit 18 during the blanking interval is always at the black clipping level, which is set at the blanking level. The chrominance signal from the gating circuit 12<SP>1</SP> is also applied to the black clipping circuit 18, e.g. as shown in Fig. 2, and is added directly to the clipping level of this circuit as established by a variable resistance 48. The clipping level is thus effectively raised by the magnitude of the chrominance signal and this ensures that the chrominance component of the composite signal is not clipped when it exceeds the clipping level established for the luminance signal. Transistors 38, 40 of the clipping circuit act as a difference amplifier in which, when the input signal excursions to the base of transistor 38 are more negative than the base potential of transistor 40, established at the desired clipping level by resistance 48 and the superimposed chrominance signal, the change in biasing of the transistors 40, 44 causes the emitter potential of the transistor 40, and thus the output level, to equal the clipping level. The chrominance signal may similarly be applied to the white clipping circuit 16, Fig. 5, but this is not generally necessary. In a modification (see Fig. 3, not shown) the gating circuit (12) is situated before the chrominance filter (22), which comprises only a bandpass component. In yet another embodiment the adder 29, Fig. 5, is omitted, and the blanking interval of the composite signal is clamped at the blanking level before the signal is applied to the gating circuit.