| 摘要 |
1,195,451. Television automatic gain control. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 31 Jan., 1968 [2 Feb., 1967], No. 4950/68. Heading H3T. In a television A.G.C. circuit video signals with the sync. pulses negative going are fed to an emitter-follower 2 the output of which is connected to a diode gate 10, to the anode of which line flyback pulses are applied from terminal 35, via resistors 20, 11. The diode 10 is thereby gated to conduct during the backporch periods, the pulses appearing at the anode being fed over a two-stage D.C. amplifier comprising transistors 21, 25 and a smoothing capacitor 29 to the gain control signal output terminal 32. To the output of emitter-follower 2 is also coupled sync. separator 5, the output of sync. pulses from which, on line 6, are also applied via a potentiometer 7, 8 to the base of a clamping transistor 9. The collector-emitter path of this is connected across the join of resistors 11, 20 and earth, and in this way the transistor 9 prevents the line flyback pulses operating the diode gate 10 during the sync. pulse periods. Video signal is also fed to output terminal 33 from the emitter follower 2 via capacitor 15, the D.C. level being restored by means of a clamp circuit comprising diodes 17, 18 connected in series. The clamping level is determined by means of a potentiometer 13, and clamping is performed by means of line flyback pulses obtained via resistor 19 from the join of resistors 11, 20. In an alternative arrangement (Fig. 4, not shown) the clamping diodes are replaced by the emitter-collector path of a transistor (40), the clamping pulses being applied to its base and also to the black-level clamp, via differentiating circuits which are provided to narrow the clamping pulses: this renders the arrangement suitable for use with a synchronization system using phase comparison, in which a phase angle may occur between the sync. pulses and line flyback pulses. |
