Patent GB-655975-A (GB)
Title: Improvements in or relating to arrangements for displaying letters and numerals on acathode ray tube
Inventor: CLACK GEORGE THOMAS
IPC: G09G1/02
Filed: 1947-12-19
Abstract: 655,975. Electric indicating systems. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd. Dec. 19, 1947, No. 33637. [Class 40 (i)] [Also in Group XL (c)] In apparatus for displaying letters or numerals on the screen of a cathode-ray tube, a plurality of pairs of voltages are developed from a basic sinusoidal voltage source such that each pair if applied to the C.R.T. deflection plates produces a characteristic diagram whose outline is common to a group of the letters or numerals to be displayed. Electronic means are provided whereby the waveforms may be mutually phase displaced or distorted, or one or more portions of the resultant trace may be blacked out, to display any selected letter or numeral. Fig. 1 shows that letters and numerals have characteristic outlines falling into distinct classes, A, V, U, M, W, H have a triangulated outline, E, F, T, I, L, J a rectangular, O, C, G, Q, D a circular, and S, Z, N, B, R, P, K, X, Y and numerals 2 to 9 a figureof-eight outline. The basic sinusoidal voltage source may be the mains. A triangulated outline results from the application to the deflection plates of the basic sinusoid and its full-wave rectified waveform; a square outline from two phase displaced peak clipped fullwave rectified sinusoids of the same frequency, a circular outline from two sinusoids of the same frequency phase displaced by 180 degrees and a figure-of-eight from the basic sinusoid and a double frequency sinusoid, phase displaced by 180 degrees. Letter A is formed (Fig. 2) by applying the unaltered basic sinusoid and a +ve bias to the horizontal deflection plates of the C.R.T., and to the vertical plates a waveform 90 degrees out of phase with the basic sinusoid and derived there. from by full-wave rectification, and alternate peak clipping which may be produced in the circuit of Fig. 34, in which diodes D7, D8 rectify the basic sinusoid. Diodes D5, D6 develop an adjustable +ve cathode bias for diode D11 which clips the peaks of the halfcycles passing through D8. Full and clipped half-cycles are superimposed to trace A. H is formed similarly, but the full half-cycle apex is clipped by an appropriately phased blackout pulse on the C.R.T. grid (Figs. 3, 4, not shown). M is formed (Figs. 5, 6) by combining a saw-tooth horizontal deflection waveform H with a negative-going vertical deflection waveform V in which the amplitudes of alternate peaks differ. Blackout pulses G may be applied to the C.R.T. grid at the beginning and end of the trace. The vertical waveform may be derived from the circuit of Fig. 38, in which the grid of pentode VI is positively biassed by the rectifying diodes D21, D22, and the grid of pentode V2 is biassed positively by the rectifier diode D23, from the basic sinusoidal voltage. The anodes are D.C. fed through a common load resistance R10 across which the waveform is developed. V2 conducts only on alternate half-cycles and similarly varies the anode voltage of V1 so that the amplified full-wave rectified output waveform differs in amplitude on alternate half-cycles. U, V, W are formed by analogous methods. O is formed by applying the basic sinusoid in quadrature to the horizontal and vertical plates, and becomes C by applying a phased blackout pulse G to the tube grid (Figs. 7, 8, not shown). Q and G are formed by superimposing on the horizontal waveform a phased " pip " (Figs. 9, 10, 11, not shown) which may be generated by the circuit of Fig. 36 wherein the basic sinusoid is full-wave rectified by diodes D16, D17, and the wave-form across resistance R7 is applied through R8 and R9 to diode D18. Condenser C1 shunts R8 and is charged on the +ve peaks, and discharged when the applied voltage falls to zero. A steady small +ve voltage is developed across D18, with negative "pips" at the zero points at which C1 discharges. D is formed (Figs. 12, 13, not shown), by applying the basic sinusoid to the vertical plates 90 degrees out of phase with a half-wave rectified waveform on the horizontal plates. E is formed (Figs. 15, 16) by a saw-tooth vertical deflection waveform synchronous with the basic sinusoid combined with a horizontal deflection waveform derived from full-wave rectification and pulse clipping of the basic sinusoid, which may be generated by the circuit of Fig. 33 in which the basic sinusoid is full-wave rectified by diodes D1, D2 while diode D3, positively biasses the cathode of diode D4 to clip the waveform peaks across R2 in accordance with the bias. F, T are formed analogously by applying blackout pulses to remove the appropriate legs of the E. J and L are formed (Figs. 18, 19), by trapezoidal horizontal and vertical deflection voltages in quadrature, and derivable from the basic sinusoid by the circuit of Fig. 35 in which it is peak clipped by the diodes D14, D15, which are anode and cathode biassed from the smoothed output of the full-wave rectifying diodes D12, D13. B, P, R, K are formed (Figs. 20, 21, 22, 23, not shown), by vertical deflection by the basic sinusoid and horizontal deflection by a half-wave rectified double frequency waveform in quadrature therewith. Phased blackout pulses distinguish the appropriate letters. X and S are formed (Figs. 24, 25, not shown), by blacking out the appropriate parts of the figure-of-eight trace produced by the basic sinusoid on the vertical deflection plates and a double frequency sinusoid on the horizontal plates. Z and N are formed (Figs. 26, 27, 28, not shown), by blacking out the appropriate parts of the trace produced by deflection in one direction by a trapezoidal waveform of the basic frequency and in the other by a sinusoid of twice basic frequency. Y is formed (Fig. 30) by the basic sinusoid on the vertical plates and a double frequency sinusoid with alternate cycles suppressed on the horizontal plates, the apex of the trace being blacked out. The horizontal waveform may be derived from the circuit of Fig. 37 in which diodes D19, D20 conduct during +ve half-cycles of the basic frequency fN, to suppress alternate cycles of the double basic frequency 2fN; or from the circuit of Fig. 39, in which diodes D24, D25 alternately conduct, on successive half-cycles of the basic sinusoid to bias the suppressors of pentodes V3 or V4 to cut off. The valves have a common anode load, V3 is a dummy and V4 amplifies a double basic frequency sinusoid with suppression of alternate cycles. Numerals 2 to 9 are formed by blacking out the appropriate parts of a figure-of-eight outline by means analogous to the above. In the circuit of Fig. 40 the appropriate character waveforms are applicable to the C.R.T. by switches A, B, C, D, E, &c. which also apply positive pulses to the grid of the normally cutoff valve V5, which conducts to increase momentarily the potential of condenser C2 for each letter, biassing V6 to increase incrementally the shift voltage on the horizontal plates for each letter so that a plurality of characters may be successively displayed on a long persistance C.R.T. screen. " Line " and " space " keys LB, SB, respectively return the trace to its initial position, and shift one space without displaying a character. Diodes may be replaced throughout by metal rectifiers. Characters may be selected by coded line signals, or the deflecting waveforms may be transmitted by line or radio. The device is applicable to the identification of aircraft, by triggering an airborne transmitter from a ground emitted radio signal to transmit waveforms appropriate to display the aircraft identification at the ground station. Specification 582,758 is referred to. Reference has been directed by the Comptroller to Specification 624,089.