Patent GB-948731-A (GB)
Title: Improvements relating to interpolating or extrapolating devices
Inventor: WHITE ERIC LAWRENCE CASLING
IPC: G06G7/30
Filed: 1959-05-01
Abstract: 948,731. Electric analog computers; electric selective signalling systems. ELECTRIC & MUSICAL INDUSTRIES Ltd. May 2, 1960 [ May 1, 1959], No. 14852/59. Addition to 796,994. Headings G4G and G4H. A modification of the interpolating device claimed in claim 1 of the parent Specification in which the transformers are so arranged that the total ampere turns of the individual transformers are substantially zero whereby the device is adapted to respond to alternating current signals. This is achieved by means of a feedback system connected to the transformer. The embodiment of Fig. 1 generates a quadratic interpolating function and input currents of amplitudes corresponding to the ordinates y 1 , y 2 , y 3  of points determining the quadratic curve are applied to the transformer windings 4, 6, 5. Any voltage set up across a further transformer winding 10 is applied to a 90 degree phase shift amplifier 12, the output of which is connected to another transformer winding 11, the amplifier gain and phase shift being such that the current in winding 11 will reduce the voltage in winding 10 practically to zero, and the total ampere turns of windings 4, 5, 6, 10, 11 which are wound in a common core will also practically be zero. A variation of x in interval x,to x 3  is represented by the movement of ganged selectors 7, 8 and 13 over tappings of linearly tapped auto-transformer 9 and a non- linearly tapped auto-transformer 14 respectively, and the output of amplifier 15 will vary to represent values of y for the particular value of x represented at any instant by the contacts engaged by the selectors. Under the condition that the total ampere turns of windings 4, 5, 6, 10, 11 are practically zero the amplitude of the current fed by winding 11 to selector 13 represents y 2  - y 1 +y 3 /2 which can be shown, for a quadratic function, to equal k<SP>2</SP>.d<SP>2</SP>y/dx<SP>2</SP> where k = x 2 -x 1  = x 3 -x 2  and the current flowing into amplifier 15 from the mid point of 9 varies in linear steps with movement of the selectors 7, 8 to represent the ordinates of successive points on chord AC while the current flowing into amplifier 15 from selector 13 varies in quadratic steps to represent the "lift" from chord AC to curve ABC at the value of x presented by the contact engaged by 13 so that the output of the amplifier 15 represents the ordinate of the respective point on curve ABC. The invention is applicable to other interpolation functions for example one generated by an interpolating device described in Application 865,453 which generates a general conic function and a particular embodiment is described (Fig. 3 (Fig. 3, not shown) employing an interpolating function of the above form but for the specific case when k = 2, when the function represents a parabola. A digital to analogue converter for supplying input signals to an interpolating system for decimal code signals comprises a series of windings 31 1 - 31 r  on a common core, there being one winding for each digital order of the code. Selectors 32 1  to 32 r  are set automatically or manually. The converter is particularly characterized by the provision of two further winding 33, 34 on the core, the voltage across the latter being used as an input to feedback amplifier 35 feeding current to the output winding 33 the amplifier introducing a 90-degree phase shift and being of such a gain that its input is reduced practically to zero and does not therefore load the converter circuit and introduce any errors due to such loading. Specification 865,453 is referred to.