Patent GB-904019-A (GB)
Title: Improvements in circuit arrangements for the formation of a signal from a plurality of other signals
IPC: H03G3/06
Filed: 1958-08-25
Abstract: 904,019. Colour television. PHILIPS ELECTRICAL INDUSTRIES Ltd. Aug. 25, 1958 [Aug. 27, 1957], No. 27191/58. Class 40 (3). [Also in Group XXXVI] In order to provide a voltage output signal which is linearly dependent on a plurality of forming voltage signals, a circuit is described which comprises resistances with displaceable tappings, said forming signals-being fed to the ends of said resistances, the voltage signal from a tapping being applied to the control electrode of a first amplifier, the sum of said forming signals being applied to the control electrode of second amplifier the desired voltage corresponding to the sum of the two amplifier outputs, the coefficients of the output signal being so variable between 0 and 1 by means of said tappings that the sum of the coefficients is equal to 1 irrespective of the positions of the said tappings. In the embodiment shown (Fig. 1) applicable to a three-colour television system, the invention is applied to the forming of a correction signal with the aid of a brightness signal V ij ( = αV R  + #V g +#V B ). which produces an expression for correction signal V H  linearly dependent on V y , V g  and V b  so that the coefficients of said expression may be varied separately while their sum always remains equal to one. Three tapped resistors 12, 13, 23 are connected together in a triangle to the apices of which are fed V y , V g , Vb, and the tappings of which are connected in turn to the grid of valve 16, the brightness signal being applied to the grid of another valve 17, the cathodes being connected by the tapped resistor 45, the valves 16, 17 thus acting as an interpolator. The valves may be replaced by transistors. The required output, the correction signal V<SP>1</SP> h , is taken from the tapping of resistor 45 and is proved in the Specification to be equal to #{V g (1 - γ 3 ) + V 3 γ 3 } + (1 - #) {αV R +#V g +#V b } when switch 20 is connected to point c, similar expressions but in γ 1 , γ 2  being obtained when the switch is connected to points a, b respectively, where γ 1 , γ 2 , γ 3  are tappings on resistances 12, 13, 23 and # a tapping on resistor 45. It is stated that with a neutral colour, the correction signal remains equal to one obtained when the coefficients cannot be varied, and that a bright white signal is not corrected at all, since the sum of the coefficients is always equal to one. Hue correction is achieved by variation of the γ values and saturation correction by that of the # value. In another embodiment (Fig. 3, not shown), the switch is replaced by a rotatable contact slidable on the tapped resistors. In the embodiment of Fig. 4, the brightness signal V ij =k(αVr+#V g +#V b ) (where k is a constant) is fed to the grid of valve 17 from a tapping 31 on a resistor 30 in parallel with the cathode resistor 29 of a valve 28, signals V r , V g , Vb being fed to the grids of valves 28, 27, 26 respectively. Valves 22, 24, 25 replace the voltage sources of Fig. 1 and have K 1 V b , K 1 V g  and K 1 V r  respectively fed to their grids, resistors 23, 13, 12 constituting the tapping resistors as before. In a further embodiment, the number of signals is increased, and consequentially the number of tapped resistors and switch contacts (Figs. 5a and 5b, not shown), the mathematical proof being extended to m signals. Specification 904,018 is referred to.