GB1205164A - Improvements in gamma correction circuits - Google Patents

Abstract

1,205,164. Gamma correction circuit. COMPAGNIE FRANCAISE THOMSON HOUSTON-HOTCHKISS BRANDT. 4 June, 1968 [1 June, 1967; 20 Dec., 1967], No. 26592/68. Heading H4F. An adjustable gamma correction circuit for a television transmitter-receiver assembly comprises a transistor current-generator 1, a gamma correction transistor 2, a differential amplifier 3, a phase reversal transistor 4 and a variable-gain amplifier and combining circuit 5. The video signal to be corrected is fed from input terminal E through the current generator 1 to the gamma correction transistor which produces two outputs, one being a first signal altered according to a fixed law of the form: (output) = = (input)γ from the use of the transistor 21 as a semiconductor junction, γ being a constant = 0À4, and the second being simply an unaltered signal whose amplitude is equal to the first due to the use of the transistor 21 as a linear amplifier. The first, altered, signal is fed via the differential amplifier 3 together with the second, unaltered, signal and an output proportional to their difference is fed to the combining circuit, and the second, unaltered, signal is via the phase reversing transistor 4 to the combining circuit 5, whence an output signal is fed to terminal S of the form: (output) = A[(input)<SP>0À4</SP>- (input)] + (input), where A is the gain of the combining circuit. The D.C. component of the video signal fed to the current generator 1 is clamped by a level blocking or clamping voltage applied at terminal 11, and the output from the current generator is blocked for signals corresponding to black and open to other signal values. The gain A of combining circuit 5 is adjustable between 0 and 1 by potentiometer 53 which may be distant from the circuit. To eliminate background noise and parasitic signals the collector of the current generator transistor 12 may be connected to a source of negative pulses fed in concomitance with the line blanking periods. Further to reduce noise a low-pass filter of lower bandwidth than the television assembly may be connected in series with the first output from the gamma correction transistor, compensation in the second output being provided by a delay line. A source of constant amplitude negative pulses may be connected to either the emitter or the base of the gamma correction transistor by means of diodes in series opposition, Figs. 6 and 7, not shown, to cause a shift in amplitude of the characteristic curve of the semi-conductor junction and match it more closely to the correction curve desired.