Patent GB-1212584-A (GB)
Title: Improvements in cathode ray tube displays
IPC: G01S7/04
Filed: 1968-03-14
Abstract: 1,212,584. Cathode-ray tube displays. INTERNATIONAL STANDARD ELECTRIC CORP. 14 March, 1968 [20 March, 1967], No. 12400/68. Heading H4T. In a cathode-ray tube display arrangement in which individual parts of the display are identified by " tag " symbols (e.g. in a radar display) the " tags " being produced repetitively under the control of tracing signals supplied from a symbol generator 2 (Fig. 1) via conductor 3 to the beam deflection means of a long persistence tube 1 together with bright-up pulses supplied via conductor 13 to a beam intensity control electrode via an AND gate G2, the response of a light pen 4, of the kind which produces an output only in response to changes in light intensity above a given threshold level, when it is located close to but not accurately positioned with respect to a symbol " tag " is enhanced by utilizing the output of the light pen to interrupt some of the bright-up pulses so that a larger change in light intensity between the excited symbols and the after-glow symbols is produced due to the longer time available for the after-glow image to decay. During normal operation and when pen 4 is located adjacent the tube screen with switch 5 open, the pen output on line 8 changes the condition of a toggle 7 (which is of the type which reverses its condition when it receives inputs on both O and I) such that a positive potential output occurs on line 9. This is supplied as one input to an NOR gate G1 which has as a second input, over lead 11, the output of an oscillator 10 operating at between 2 and 5 c.p.s. In this condition of toggle 7 AND gate G2 is open and a number of bright-up pulses from symbol generator 2 pass to the tube intensity control electrode (synchronously with the tracing signals on conductor 3) during positive half-cycles of oscillator 10. When a particular " tag " symbol has been selected the switch 5 is closed which causes toggle 7 to reverse its condition by the pulse supplied over conductor 6, conductor 9 goes negative and the negative half-cycle of the output of oscillator 10 blocks gate G2. A few of the bright-up pulses are thus removed and the light intensity of the " tag " symbol falls to a low value. On the occurrence of the next positive half-cycle of the oscillator output G1 opens and G2 is unblocked and bright-up pulses are again supplied over conductor 13. The " tag " symbol is thus again produced with full intensity and the resulting enhanced output of the pen 4 due to the large change in illumination between the newly excited symbol and its after glow image produces another output from the pen which, if the pen is accurately positioned with respect to the symbol is of sufficient intensity, to be effective over lead 8 to reset the toggle 7 to the " O " condition. This results in conductor 9 going positive to lock gate G1 in the open condition so that gate G2 is maintained open and a " flicker " free " tag " symbol is produced. If the pen is not accurately positioned the light intensity will be insufficient to reset toggle 7 and the " flicker" continues to be reproduced. When the pen is accurately positioned, comparison of the pen output signal (in a temporal manner) with the signals supplied by the symbol generator enables the symbol to be identified.