Patent GB-1015806-A (GB)
Title: Improvements in or relating to television signal correcting circuit arrangements
Inventor: PAY DONALD ALEXANDER
IPC: H04N5/202
Filed: 1963-04-01
Abstract: 1,015,806. Electric analogue calculating. MARCONI CO. Ltd. March 18, 1964 [April 1, 1963], No. 12750/63 Heading G4G. [Also in Division H4] A gamma correction circuit comprising a first unit having a linear voltage characteristic of constant slope, one or more further units having voltage characteristics comprising two portions of constant slope but opposed sign, and means for combining the outputs of the units; each characteristic extending from " black " to " white " with the characteristic of each further unit starting and finishing at the same ordinal value and such further units being adjustable that the junction point of the characteristics is adjustable to a predetermined " grey " level, so that the combined characteristic of the circuit comprises plural successive regions adjoining at predetermined onset points at which the slope changes discretely, the position of the onset points being adjustable by varying the grey level of corresponding such further units. An input signal 1, e.g. a positive going sawtooth (Fig. 2) is capacitance coupled to the base of a transistor 3 where it is clamped by a transistor 6 receiving rectangular pulses over terminal 8 to a level set by potentiometer 27. The resultant signal is passed via transistor 4 to the base of transistor 5 feeding a pair of double series diode clipper circuits 9, 10 and 11, 12 connected to the sliders of potentiometers 13, 14 between earth and the emitter of transistor 15. The collector of transistor 4 is connected to the base of transistor 16 whose emitter is connected over Zener diode 19 and a series resistance to the base of transistor 15 and also over adjustable resistance 17 to the slider of a potentiometer 18 energized between negative supply and earth. The video signal at the emitter of transistor 4 connected to negative supply over Zener diode 20 is applied to the base of transistor 21 whose collector is connected to positive supply over parallel potentiometers 22, 23, which develops gamma connected signals, and whose emitter is returned to earth. The sliders of the potentiometers are connected to the collectors of transistors 24, 25 whose bases are connected to the respective junctions of diodes 9, 10 and 11, 12 and whose emitters are returned to earth. Resistance 17 and potentiometer 18 are adjustable to equalize the signals at the emitters of transistors 5 and 15 and to make them zero for peak white, and in each of the clipper circuits 9, 10 and 11, 12 the diode with the most negative cathode terminal will conduct so that the signal at the junction of 9, 10 will be as shown in line (b) of Fig. 3, in which input signals from black to white are shown as abscissµ and corrected output signals as ordinates; the point Z being determined by the adjustment of potentiometer 13. The signal at the emitter of transistor 4 is a linear characteristic as shown in line (a), and the signals are combined by transistors 21, 24 and potentiometer 22 to an effective summed signal at 26 shown by line (c) having a first region of B to X and a second region of X to W of lesser slope; the proportion of signal (b) being varied by adjustment of potentiometer 22. A further signal derived from the junction of diodes 11, 12 similar to that shown in line (b) with a junction point determined by adjustment of potentiometer 14 is combined with the output signal by transistor 25 and potentiometer 23, whose adjustment determines the proportion thereof which is added, so that the characteristic of the output signal has three regions with a junction point between the first and second adjustable by potentiometer 13 and one between the second and third adjustable by potentiometer 14, the potentiometers respectively varying the slopes of the second and third regions. Potentiometers 22, 23 vary the relative gains between the successive regions. In a modification (Fig. 4, not shown), transistor 3 may be omitted.