Patent GB-1175341-A (GB)
Title: Curve Tracer for Oscillographic Display
IPC: G09G1/10
Filed: 1968-01-25
Abstract: 1,175,341. Cathode-ray tube displays. SANDERS ASSOCIATES Inc. 25 Jan., 1968 [10 Feb., 1967], No. 4065/68. Heading H4T. In an arrangement for tracing symbols &c. on the screen of a cathode-ray tube by appropriate X and Y component deflections each component deflection is produced under the control of a waveform which is derived by summing complementary percentages between 0% and 100% of two time varying electrical signals the respective amplitudes of which are modified by, e.g. in the case of the X component, the values corresponding to the X co-ordinates of two points on the symbols. The time varying electrical voltages may be sweep voltages varying between the same maximum and minimum limits and the two points may be the initial and terminal points of a linear element of the symbol. The instantaneous algebraic sum of the two sweep voltages is therefore substantially constant so that the deflection varies linearly with time and by making the absolute values of the slopes of the sweep voltages inversely proportional to the lengths of the elements the latter are traced at a constant speed and therefore have uniform intensity. Alternatively, the lengths may be computed to derive a corresponding beam intensity control signal. A further advantage of the arrangement is that when the symbol comprises a number of elements joined together the "closure" (i.e. the joins) is complete. Additionally, the time varying voltages may be other than sweep waveforms (in this case the sum may not always be constant) i.e. two 90 degrees phase displaced sinusoidal waves may be utilized for the tracing of curves. Fig. 1, shows typical symbols which may be traced, i.e. the number 71, square 200, circle 202, ellipse 204 and portion of an engineering drawing, e.g. a fillet 206 between intersecting straight lines. All symbols are represented by digital information which specifies the X, Y co-ordinates of the start point with reference to the screen origin, "O", (Fig. 1) and then specify the co-ordinates of subsequent points with reference to the start point of the symbol. Thus considering the number "71" the coordinates X L , Y L  of the start point L of the numeral 7 are expressed with reference to the origin "O" and the subsequent points M and N are expressed with reference to point 7. Likewise the start point P of the numeral 1 is expressed with reference to "O" and the point Q with reference to the point P. In the described embodiment, which is concerned with tracing the number "71", the co-ordinate digital data together with control data are derived from a continuously up-dated computer and loaded into a recirculating memory 10 (Fig. 2), comprising an acoustical delay line 20 and a shift register 18 driven by pulses E c  (see also. Fig. 3) from a clock and distributer 22. The control data comprising B bytes signifying the start of vector tracing and H bytes signifying "stop" are read out of stages 18c and 18a whilst the bytes representing the co-ordinate data are read out from stages 18a and 18b of the register 18. The operation is as follows. In response to various timing pulses from circuit 22 (see also Fig. 3) the X and Y data are read out in pairs from stages 18a and 18b respectively. Considering the operation appertaining to the X co-ordinate, that for the Y co-ordinate being identical, the XL bytes in register section 18a are transferred via gate 24 to a buffer 28 and thence to a parallel adder 34 and to a further gate 40. From adder 34 the bytes are transferred via gate 56 to a buffer 52 and thence to a digital-to-analog converter 70. At this time gate 40 is opened to pass the X L  bytes to a register 42 so that when the # X M  bytes subsequently reach adder 34 the required algebraic addition is effected. The resulting contents of adder 34 are then transferred via gate 58 to buffer 54 and thence to digital-to-analog converter 72. In the same manner the # X N  bytes, after addition in adder 34 to the # X M  bytes, are transferred to digital-to-analog converter 70 and if the number contained further co-ordinate representative bytes these would be transferred alternately to converters 72 and 70 in cyclic fashion. The converters are supplied with reference voltages comprising oppositely phased sweep waveforms e 1 , e 2  (see also Fig. 3) derived from an Up-Down pulse counter 144, digital-to-analog converter 146 and inverter 147 so that, when converters 70 and 72 contain the X L  and X m  information the outputs comprise the corresponding analog voltages which vary from zero to a maximum in the ease of the X m  information. These two outputs are then summed in summing amplifier 82 and utilized to drive the X deflection winding 83 of the display tube S. Due to the sweep waveform e 2  being initially a maximum and e 1  zero the beam takes up the position L(X L ), Fig. 1, and as e 2  decreases and e 1  increases a linear deflection results and the beam reaches the position M(X M ). During this cycle, period 81 (Fig. 3), the beam is unblanked by a pulse supplied via amplifier 162. A blanking period 88 then follows and a fresh tracing cycle occurs during period 90 when the X m  information reaches converter 70. The B byte then signals a break, i.e. that the next symbol, the 1, is not joined to the last element and this symbol is then traced in the same manner.