Patent GB-1273611-A (GB)
Title: Electronic colour correction circuit
Inventor: HUBOI ROBERT WALDEMAR
IPC: H04N1/60
Filed: 1969-06-26
Abstract: 1,273,611. Colour correction circuit. EASTMAN KODAK CO. 26 June, 1969 [28 June, 1968], No. 32326/69. Heading H4F. Relates to a colour correction circuit forming part of computer 6, Fig. 2, of a system in which an original 1 to be printed is scanned by a flying spot scanner 2 associated with a photomultiplier 5 and a rotating drum 4 having successive red, green and blue filters, the connected signals being displayed on a cathode-ray tube 2 associated with a second rotating colour filter drum 4. The circuit includes a matrix having adjustable coefficients which permits the correction of each colour to be a function of all three colours so that the correction is matched to that of the printer (not shown). The circuit may also be used where the balance of a colour film must be matched to the sensitivity of a colour television camera. In the prior art, Fig. 1, the video signal from photomultiplier 5 is fed to a "white" level gate 10 which is switched on when the spot of scanner 2 is swept to the edge of original 1, giving a reference level for calibration purposes. Capacitor 11 stores this level so that its potential is always available at the collector of transistor 76 and at one of the inputs of differential amplifier 78. Timing pulses for the red, blue and green colour filters are applied on lines 13, 14 and 15 respectively, for example the red timing pulse turns on transistors 70 and 16. Thus the voltage at the base of transistor 76 and therefore the collector voltage thereof is determined by the setting of logarithmic potentiometer 46 during the time a red filter is positioned in front of scanners 2 and 7. When mode switch 84 is in the calibrate position, differential amplifier 78 gives an output which is the difference between the stored white level as modified at the collector of transistor 76 by the logarithmic potentiometer 46, and the instantaneous video signal. The output of amplifier 78 is fed through gate 80 to an amplifier consisting of transistors 38 and 39 which regulate the voltage on the dynodes of the photomultiplier 5. Gate 80 is triggered by the white level signal but stays open throughout the entire correction cycles whereas gate 10 is only on during the period when the border of the original is scanned. Transistors 16, 17 and 18 are turned on with transistors 70, 72 and 74 respectively, therefore the input to the differential amplifier 78 is governed by the setting of potentiometer 46 whilst the level capacitor 40 is being charged through the transistor 16 i.e. during the red period capacitor 40 charges to the level of the output of amplifier 78 when gate 80 is open. When switch 84 is in its automatic position amplifier 78 receives information through transistor 20, transistors 21, 22, 23 being gated by the same colour timing pulses which are fed on lines 13, 14 and 15. Therefore the entire video signal of a particular colour is integrated by its respective capacitor, e.g. 102 for red. Thus in the automatic mode the integrated density of each colour is set to a particular level. This arrangement does not take into account the effect one colour may have on another colour in the printer due to the dyes or inks not being pure colours. To adapt the circuit of Fig. 1 to any automatic printing system the circuit of Fig. 3 is added to Fig. 1, capacitors 40, 42 and 44 being the same capacitors in both Figs. and having a charge related to the desired response of the photomultiplier. Whenever a red timing pulse turns on transistor 16, the collector thereof is shorted to ground and the voltage across capacitor 40 is coupled to the output of transistor 38 and thence to the dynodes. In addition the output of the dynode amplifier is coupled back to an input of differential amplifier 51 so that the desired red density is provided at the output of amplifier 51 at all times. When transistor 16 is not conducting the collector voltage thereof comprises the colour density signal being fed from gate 80 minus the charge of capacitor 40 i.e. the red density. The other terminal of amplifier 51 will have the density of the colour then being computed. The net result at the output of amplifier 51 is the difference which is the red density. The red, green and blue densities are mixed in any proportion by matrices M 1 , M 2  and M 3  and the resultant as it effects the red density is supplied by gate 91, the green and blue correction functions being supplied by gates 92 and 93, and all three sequentially occurring correction functions are combined in amplifier 64 to form a single output. The output of amplifier 64 is fed back to amplifier 78 along with the colour function of the white level signal from the collector of transistor 76 and the instantaneous video signal from photomultiplier 5 via transistor 20.