| 摘要 |
1,198,206. Television.R.C.A. CORPORATION. 29 May, 1968 [29 May, 1967], No. 25642/68. Heading H4F. An automatic chroma control (ACC) circuit for a colour TV receiver comprises means for deriving D.C. voltage components from the input and output of a colour-synchronizing- burst gate amplifier and means for controlling the gain of a chrominance amplifier in response to a predetermined combination of the said voltage components. The voltage component from the output of the burst gate amplifier is in practice derived at the grid of a phase locked colour reference oscillator to which the burst signals are applied via a narrow band filter which incidently removes any noise signals. Noise voltages present at the input to the burst gate amplifier, however, are rectified by the grid-cathode diode and produce a negative bias voltage which reduces the gain of the burst gate amplifier. The resulting reduction in the amplitude of the burst voltage applied to the oscillator causes the ACC circuit to increase the gain of the chrominance amplifier and produces an undesired increase in the signals applied to the colour demodulator. This error is minimized by applying the noise generated D.C. voltage component at the input to the burst gate amplifier to the ACC circuit to offset the variation in the voltage derived at the output of the burst gate oscillator. In a first embodiment, Fig. 1, a closed loop ACC arrangement identical to that described in Specification 1,198,207 in which a D.C. voltage component appearing at the grid of a colour reference oscillator 18 is applied via resistor 132 to an ACC circuit 28 comprising a currentcontrolled amplifier the output of which is applied as bias to a chrominance amplifier 12, is modified by connecting the earthy-end of the grid-leak resistor 94 of the burst gate amplifier 14 to the input of the current controlled amplifier. Grid-leak resistor 49 may be selected in value so that the increase in current flowing through it from an increased negative D.C. voltage component at the grid of the burst gate amplifier 14 due to noise compensates for the reduction in the current in grid-leak resistor 132 due to the resulting decrease in the amplitude of the burst signals applied to the oscillator. A second embodiment, Fig. 2 (not shown), comprises an open loop ACC arrangement in which the chrominance amplifier (12) is outside the ACC loop. The grid-leaks of the burst gate amplifier (14) and the reference oscillator (18) are connected directly to ground and the D.C. voltage components at the grids of these valves are applied to the input of a voltage amplifier which provides a bias voltage to control the gain of the chrominance amplifier (12). |
