Patent GB-962288-A (GB)
Title: Circuit arrangement for controlling the gamma value of a television video signal
IPC: H04N5/20
Filed: 1962-05-11
Abstract: 962,288. Electric analogue calculating. FERNSEH G.m.b.H. May 11, 1962 [May 13, 1961], No. 18183/62. Heading G4G. A gamma control circuit for a television video signal comprises plural channels of differing gamma over which the signal is fed; each comprising an individual rectifier connected to a common junction point developing an output signal; means being provided for simultaneous variation of the bias potentials applied to all such rectifiers. In Fig. 1, a video signal 1 is capacitance coupled to the base of a transistor 4 across a known clamp circuit 5 and capacitance coupled to the base of transistor 24. The signal from the emitter load 6 of transistor 4 is applied to a distortion circuit comprising resistor 8, semi-conductor diode 9, Zener diode 11, and resistor 12 returned from the diode junction to negative reference source 22. Output signal 13 of gamma 0. 4 from the junction of 8, 9 is coupled by impedence converting transistor 14 to emitter load potentiometer 15 whose input is capacitance coupled to output junction point 18 over semi-conductor diode 17. The undistorted signal of gamma = 1. 0 from the emitter potentiometer 25 of transistor 24 applies video signal 1 over capacitance 26 to point 18 over semi-conductor diode 27 opposed to diode 17. The anode of 17 and the cathode of 27 are resistor coupled to earth, and adjustable bias is applicable to the diodes over resistor 29 from potentiometer 31 energized at 32, 33 so that at its terminal positions a respective channel is cut off and at intermediate positions gamma is controlled to a value intermediate between 0.4 and 1. 0. In a modification (Fig. 2, not shown) three channels are provided for video signals of different gamma values (0. 4, 1.0 and 2. 0) derived from impedance converters and capacitance coupled at equal amplitudes through semi-conductor diode rectifiers to a common point. The third channel contains two opposed diodes and the function is biased over a series resistor from a variable potentiometer supplied with voltages such that the effective gamma is controllable over the range 0. 4 through 1. 0 to 2.0. In Fig. 3, a video signal 1 is capacitance coupled to the base of transistor 52 biased by resistors 53, 54 from a positive reference source 55, and the video signal from the emitter load 56, 57 is capacitance coupled to the base of transistor 4 across transisterised black level clamp circuit 5 supplied from a negative reference source 64. The signal across the emitter load of 4 is parallel fed to distortion circuits comprising resistors 66, 71 with semi-conductor diodes 72, 73 and resistors 67, 68, 69, 70 with diode 74 to supply signals having gamma valves of 0. 4 and 2. 0 respectively to the bases of transistors 14, 78. A signal of gamma 1. 0 is derived from potentiometer 56 in the emitter load of transistor 52 and capacitance coupled over semi-conductor diode 83 to common point 18, which also receives signals from the fixed emitter resistance of transistor 14 and the emitter potentiometer 79 capacitance coupled over diodes 78, 82 respectively to point 18 which is fixedly biased from the reference sources over resistors 84<SP>1</SP>, 85<SP>1</SP>. The anodes of diodes 78, 82, 83 are resistance coupled to the ends and centre point of potentiometer 91 whose centre tap is resistance connected to a positive reference source while its ends are similarly resistance connected to a negative reference source. Diodes 82 and 78 respectively conduct when the slider is at the ends of the potentiometer and diode 83 when it is central so that the potentiometer is adjustable to give the output signal a gamma value varying from 0. 4 through 1. 0 to 2. 0 with constant amplitude.