Patent GB-1299409-A (GB)
Title: Improvements relating to beamed-intensity control systems for cathode-ray tubes
Inventor: ENGLISH BRIAN BOX
IPC: G01R13/26
Filed: 1970-05-09
Abstract: 1299409 Cathode-ray tube circuits FERRANTI Ltd 3 May 1971 [9 May 1970] 22540/70 Heading H4T In a cathode-ray tube display system arranged for beam deflection by incremental amounts in X and Y co-ordinate directions the effective brightness of the beam is reduced when the direction of the trace is not parallel to either co-ordinate direction. The invention is described in connection with a computer controlled display in which traces parallel to the co-ordinate directions are produced by a succession of X or a succession of Y incremented deflection dx, dy, whereas traces making an angle with these directions are produced by incremental deflections alternately in the X and Y directions. The latter traces increments which have a length 2 times dx (or dy), in the case of a 45 degree trace, are drawn in time 2dt so that the effective brightness is #2 times that of a trace parallel to either co-ordinate direction and the present invention provides logic circuitry which in response to the occurrence of deflection signals alternately for the X and Y directions, reduces the effective brightness of the beam to approximately 1/#2 of the value it has when X or Y deflection signals occur successively. Thus all traces are produced with equal brightness and since the intensity of the beam varies along the radius of the beam spot the increase in brightness at the peripheral regions due to the longer trace times is prevented and all traces are also produced with substantially equal width. When sloping traces are to be of an intermittent nature, e.g. chain-dotted, the logic circuitry controls the "dotting" circuit such that the variation with slope of the lengths of the elements are minimized. Fig. 1, shows the described embodiment in which a computer 12 is programmed to supply incremented deflections comprising, in respect of each X and Y co-ordinate, twenty steps, defined by a twenty-pulse counter clock 14 together with a steady X or Y signal throughout the Y or X incremental counts respectively. In operation, a preliminary resetting operation is effected by the computer supplying a steady X signal to AND gate 15 followed by a steady Y signal to a bi-stable stage 13 and to AND gate 16 each for the duration of twenty clock pulse and results, in bi-stable stage 13 being left in the set state Q=1 and NQ (not Q)=0 and a bi-stable stage 21 in the reset state NQ=1. In this condition AND gate 22 is conditioned to pass bright-up pulses BU over line 23 to the intensity control electrode 24 of the tube 11. Stage 13 is switched to the set state (or left in this state) by the 17th clock pulse if a Y signal is present at the S (= signal) input or switched to the Reset state (or left in this state) if a Y signal is not present at the S input. Similarly stage 21 is switched to the set state (or left in this state) by the 3rd clock pulse if an X or Y signal is present at the S input and switched to the Reset state by each 9th clock pulse. Thus, if after the preliminary resetting (the last signal of which was a Y signal) the next signal is also a Y signal stage 21 remains in the reset state and the bright-up pulse BU is effective during the whole of the twenty clock period whilst twenty steps of the incremental deflection signal dy are applied to the Y-plates of the tube. If however, the Y signal is followed by an X signal (indicating that a sloping trace was to be produced) the operation is such that stage 21 is switched to the set state by the 3rd clock pulse and remains in this state until reset by the 9th clock pulse. Thus gate 22 is prevented from passing the bright-up pulse BU for six clock period and so the effective brightness is reduced to 14/20 ( = approximately 1/#2) of the value it would have were the Y signal followed by yet another Y signal or if an X signal were followed by another X signal. The effective brightness of the beam may alternatively be controlled by dimming instead of interrupting it and instead of controlling it over a single period a number of periods may be utilized. When the traces are to be of an interrupted nature a logic circuit 27 (Fig. 2) responsive to the count in a counter 26 determines the nature of the interruptions, e.g. the mark/space ratio and when the NQ output becomes 0 between the 3rd and 9th clock pulses the supply of clock pulses to the counter is interrupted via AND gate 31 so that the time which the counter requires to reach the count at which logic circuit 27 operates to produce the interruptions is increased thus increasing the durations of the mark periods to compensate for their tendency to shorten with slope.