Patent GB-919239-A (GB)
Title: Improvements in or relating to cathode ray tube display systems
Inventor: JOHNSTON JAMES STEWART
IPC: G09G1/18
Filed: 1960-06-27
Abstract: 919,239. Cathode-ray tube circuits; pulse radar; transistor bi-stable circuits. DECCA Ltd. June 21, 1961 [June 27, 1960], No. 22479/60. Classes 40 (6) and 40 (7). [Also in Groups XIX and XXXVIII] A display system comprises a cathode-ray tube (C.R.T.) with orthogonal deflector means, a sine-cosine signal generator providing outputs in binary digital form representative of the sine and cosine of the angular direction of a trace to appear on the display, two digital integrators arranged to integrate respectively the sine and cosine outputs, and means which apply the integrated outputs to the deflector means to provide respective orthogonal deflection signals for the beam of the C.R.T. The invention is described as applied to a pulse radar P.P.I. display, which may be off-centred and of adjustable scale, including the provision of " inter-scan " markers. The display tube may be a conventional C.R.T. with orthogonal beam-deflection coils or plates. Or the display tube may comprise a phosphor-carrying screen, an anode arranged on the screen, a cathode adapted to emit electrons on to the whole area of the screen and electron beam directing means between the cathode and the screen including two sets of parallel conductors, one set being orthogonal to the other and each set having apertures therein such that electrons will pass through an aperture in a conductor only if that conductor is maintained at a suitable potential with respect to the cathode; using such a tube the position of the electron beam is controlled by the digital integrators. Beam-brightening pulses may be derived to afford a rectangular co-ordinate grid on the display. Scanning aerial 12, Fig. 1, is driven by a shaft 15 coupled to a sine-cosine signal generator 16, which may be a mechanical or optical digitizer, preferably producing outputs in Gray code, fed to digital time-base (d.t.b.) units 17, 18 which produce the X and Y deflection signals for C.R.T. 19. The start of the time-base scans is controlled from pulse timer 14 which feeds pulses at the radar p.r.f. to an Eccles-Jordan bi-stable unit 20, the latter being re-set by a timing flip-flop 21 to produce gate pulses of predetermined duration to control the digital time-bases. Each of the d.t.b. units comprises a unit for adding, at each pulse from a clock pulse generator, the sine (or cosine) signal in digital form to itself to produce an accumulated total having a maximum value corresponding to the required step change in position of the C.R.T. beam; this adding unit gives an output each time the accumulated total gives a carry pulse from the digit of greatest significance and the carry pulses are fed to a further accumulator where they are added to the co-ordinate value of the origin of the scan. One adding unit is provided for each of the two orthogonal deflection directions. Each adding unit output is fed to an associated bi-directional counter, so that each signal from a carry pulse in the first adding unit accumulator effects a unit change in the bi-directional counter to give a total increasing or decreasing at a rate proportional to the sine or cosine input and also to the clock pulse repetition rate. The outputs of the counters are thence fed into digital-to-analogue converters. Adding and accumulator unit, Fig. 2.-A clock pulse generator 30 comprises a crystalcontrolled oscillator and pulse shaping circuit, and has a pulse rate to give e.g. 300 steps for effecting deflection from the centre to the edge of the C.R.T. For operation on different range scales means may be provided for selecting appropriate pulse rates. The sine (or cosine) is fed in binary 10-digit form-if the sine-cosine signal generator output is in a Gray code a code converter is incorporated-to adders 32 which are connected to bi-stable units 33 forming the accumulator, the units being so inter-connected that on receipt of each clock pulse the input signals are addedto the accumulated total and, as soon as a carry pulse is produced at the stage of greatest significance, it is fed as an output pulse on line 34 into a further adding and accumulator unit 35, comprising a bi-directional counter, having an accumulated total which is increased (or decreased) by a unit step for each carry pulse. The position of the origin of each display trace is controlled by a setting control unit 37. Bi-directional counter, Figs. 3 and 4.-The various stages of the counter each comprise a bi-stable device 40, AND gates 41, 42 and OR gate 43. The direction of count is determined by applying a signal to one or other of two lines 44, 45. The input signal to be counted is applied through a lead 46 directly to the first device 40; if either of first AND gates 41, 42 has simultaneous input signals from first device 40 and from one of leads 44, 45, a signal will be applied to the following OR gate 43 and thence via a rectifier 49 to device 40 of the next stage. If as a result of a certain input pulse the nth bi-stable device has to change state it cannot do so until all the (n - 1) preceding devices have changed state; to avoid this delay, rectifiers 47 are provided so that, for an increasing count, a particular bi-stable device will change state when all the preceding stages are in a " 1 " state, and for a decreasing count, when all the preceding stages are in a " 0 " state. The counter may be arranged to count a Gray code or binary coded decimal number. Before any count begins the initial value must be inserted and a re-set circuit is therefore provided for each stage. The complete circuit for a single stage is depicted in Fig. 4. The stage is re-set by applying a signal to the base of one or other of transistors 50, 51, depending on the re-set condition required, this signal, on application of a re-set pulse to the emitters of the transistors, being fed through the one of rectifiers 52 to the collector of one of transistors 53, 54 interconnected to form a bi-stable circuit. A signal is routed to one or other of rectifiers 55, 56 (forming part of two AND gates) according to whether the count it to be up or down. The input signal to be counted is fed through rectifier 57. Diodes 59 form an OR gate. As an alternative to the adder-accumulator 31, Fig. 1, the code-converter shown in Fig. 5 may be utilized to produce from a regular clock pulse frequency a pulse rate whose average frequency is proportional to a digital number representing the sine (or cosine) input signal; the counter may operate to count in a Gray code. 800,000 p.p.s. are fed via line 60 into a distributer comprising a diode matrix 61 and a series of bi-stable units 62, and are counted such that outputs can appear on leads 63 at repetition frequencies of 400,000 &c. p.p.s. AND gates 64 decide which of these pulse rates are fed to OR gate 65. Inputs 66 to the AND gates are the various digits of the number to be converted. Code and digital-to-analogue conversion. Fig. 6.-Clock pulse generator 70 is arranged to produce pulses at a rate corresponding to radar distance steps of 1/10 mile; these pulses are fed to a distributer 71 of the kind described with reference to Fig. 5. The distributer feeds mixers 72, 73 which may be the AND/OR gate combination 64, 65 of Fig. 5. The outputs of the mixers are fed to counters 75, 76 which are as described with reference to Figs. 3 and 4. The origin for each display scan is controlled by applying X and Y shift signals in digital form to leads 78 of re-set circuits 79 which feed in the appropriate co-ordinate signal to corresponding counters 75, 76 on application of a re-set pulse on a lead 80 from an Eccles-Jordan circuit 81 which is controlled by distributer 71 to provide a range scan gate pulse starting at the beginning of each scan and terminating at the end thereof. To insert inter-scans of further displays, which may begin from a different origin on the C.R.T. screen, inhibit gates 82 control the pulses fed from mixers 72, 75 to counters 75, 76. The output signals in digital form from the counters are converted to analogue deflection form by summing amplifiers 85 each having a feed-back resistor 86 and a number of input resistors 87 corresponding to the various digits of the input signal from the corresponding counter, the resistors being proportioned according to the significance of the various digits. The final output signals may be arranged to control directly the magnitude of the currents through C.R.T. deflection coils. With the alternative form of C.R.T. the outputs of counters 75, 76 may be applied to the tube. Distributer 71 is used to effect frequency division of the clock pulses to produce radar trigger pulses on lead 88. Inter-scan marker traces are generated by switching the inputs to mixers 72, 73 in Fig. 6 from the aerial-controlled generator to alternative control members and switching the origin co-ordinates applied to re-set circuits 79. A number of different inter-scan marker traces may be provided, and a marker may be built up of separate traces during different inter-scan periods. Range markers are provided by using beam-brightening pulses obtained from counters 75, 76; and a frequency-dividing circuit may be provided on the input to the distributer 71 or the inputs to the counters so that brightening pulses at any desired spacing are obtained.