Patent GB-1416380-A (GB)
Title: Device scope
IPC: G01R13/22
Filed: 1973-06-08
Abstract: 1416380 Cathode-ray tube circuits; television SOC LANNIONNAISE D'ELECTRONIQUE SLE-CITEREL 8 June 1973 [9 June 1972] 27440/73 Headings H4F and H4T The trace brightness of a cathode-ray tube is stabilized by means of a control loop comprising a photo-sensitive device viewing the tube screen and controlling the magnitude of a unidirectional voltage supplied to the intensity control electrode of the tube together with a video signal representative of an image to be displayed. The invention is described in connection with a monochromatic oscilloscope used for displaying an image for recording by a photographic camera, the image being produced in response to video signals derived from a television camera carried by a meteorological satellite. Alternatively, the signals may be derived from a video tape recorder. As shown in Figs. 1 and 3, a " regulation process " is initiated, periodically, by the application of a pulse a to input K of a monostable flip-flop S which produces a pulse b, of about 100 msec. duration, this pulse being employed to open switches W1 and W2 thereby interrupting the beam deflection input to terminal X from time-base B and the video input U to the brightness control electrode Z via a summing amplifier A3. Additionally, this pulse forms one " conditioning " input to AND gate P1 and via a differentiating circuit D and terminal T2 of a bi-stable flip-flop M, sets the latter to the " 1 " state and resets a counter N1 to zero. When the beam deflection input is interrupted the beam takes-up the position X = 0 ; Y = 0 (see Fig. 2a) and is " viewed " by a phototransistor Q1 the output of which, together with the output of a second photo-transistor Q2, which is screened from the tube, form two inputs to a differential amplifier A1. Although the characteristics of the two photo-transistors are matched, amplifier A1 does not fully eliminate differences in the " rest " currents and to achieve this a further comparator amplifier A2 is utilized, an adjustable reference voltage VO from potentiometer R11 and a low-pass filter F being included in the J4 input. The output of amplifier A2 is then effective via the " Q " terminal of the flip-flop M as a second " conditioning " input to AND gate P1 and pulses from a clock generator H then pass to the counter N1. As the latter counts-up a D/A converter N2 supplies a rising stepped voltage to the Z input of the oscilloscope via amplifier A3 which thus results in the output of photo-transistor Q1, likewise increasing in steps so that the input to amplifier A2 increases as shown by waveform C (Fig. 3) until it reaches the reference voltage VO. At this point, time t1, AND gate P1 becomes " de-conditioned " by the loss of input from the Q terminal of bistable M and the reference voltage VO, which is " memorized " by counter N1, remain effective. When pulse b from monostable S terminates, time t2, switches W1 and W2 close, the scan commences and the video signal input U is superimposed on the reference voltage VO as a beam intensity control signal VZ, waveform e (Fig. 3). If the " equilibrium " condition is not reached before counter N1 has reached full count by the time t2 the corresponding voltage step output from D/A converter N2 is employed as the " best adjustment " and further counting of clock pulses is prevented by employing the output of a gate P2 which decodes the full count to set flip-flop M to its " 0 " state via terminal Z2. Instead of employing digital counting to produce the rising stepped voltage an analogue arrangement involving the charging of a capacitor may be employed (Fig. 4, not shown). Fig. 2b shows a plan view of the arrangement of the oscilloscope R, the photographic camera G and the photo-transistors Q1, Q2 covered by a screening hood C.