GB1150713A - Improvements in or relating to Brightness Control Circuit Arrangements - Google Patents

Abstract

1,150,713. Colour television. TELEFUNKEN PATENTVERWERTUNGS G.m.b.H. 12 May, 1966 [22 May, 1965], No. 21235/66. Heading H4F. The brightness of a television image is controlled by varying the bias applied from a potentiometer network R1, R2, R3 to the cathode of the final video amplifier 7 (or to the emitter electrode when a transistor is employed), the potentiometer network being so arranged that it presents a substantially constant negative feedback resistance whereby variation of brightness is not accompanied by a variation of image contrast. To this end R1 is made approximately equal to R3 and R2 is made small compared with R or R3. The resistance to earth from the slider of R2 is then equal with the slider at each end of its adjustment and only increases slightly at intermediate positions. The contrast is controlled by potentiometer 6 in the output of the preceding amplifier 2. If the circuit is rearranged so the signal from amplifiers 2 has a negative D.C. component, a negative bias is provided for amplifier 7 by returning resistor R1 to a negative supply. Some variation of negative feed-back may be introduced by making R 1 >R 2 + R 3 so as to compensate for gradation distortion due to the characteristic curve of amplifier 7. Fig. 3 (not shown) illustrates a modified arrangement for use where the bias requirements are such that (R1) cannot be made approximately equal to (R3). The circuit includes a further resistor (R4) connected in shunt with R2 and the smaller of resistors (R1) and (R3). Thus the solid line connection shows the case where (R1)<(R3). The parallel value of (R4) and (R3) is made approximately equal to (R1). (R2) is made small compared with the smaller of (R1) and (R3). Fig. 4 (not shown) illustrates another arrangement in which the potentiometer (R5), (R6) and (R7) include a series variable resistor. (R6) is made large compared with (R5) and small compared with (R7).