Patent GB-1290595-A (GB)
IPC: G06V30/10
Filed: 1970-04-17
Abstract: 1290595 Transistor pulse and switching circuits BURROUGHS CORP 17 April 1970 [22 April 1969] 18561/70 Heading H3T [Also in Division G4] In a pattern recognition system, the scanner output is thresholded using a threshold level provided as a function of maximum and minimum scanner output, the threshold level being corrected in response to the thresholder output so as to render the thresholder more responsive to changes in the scanner output when the scanner output is of a nature to cause the thresholder to produce an output pulse. A flying-spot scanner scanning a black character to be recognized provides an input via an amplifier and contrast stabilizer to a maximum detector, minimum detector, comparator and threshold correction circuit. The maximum detector (Fig. 7, not shown) compares the input in a differential amplifier (91, 92) with the voltage across a capacitor (93) supplied via an FET (101) which prevents uncontrolled current leakage from the capacitor, and if the input exceeds (i.e. is more white than) the capacitor voltage, the capacitor is charged up further until they are equal. The capacitor voltage at the differential amplifier is supplied to the comparator mentioned previously. The capacitor voltage gradually decays (i.e. becomes more black) via a resistor (105). The minimum detector (Fig. 8, not shown) is generally similar, mutatis mutandis, except that the maximum detector output, supplied via the comparator and a potentiometer (123), gradually charges the capacitor (118) to a white level from which it is discharged under the control of the differential amplifier (110, 111) if the input falls below the capacitor voltage. In the comparator (Fig. 5) the signals from the maximum and minimum detectors are combined using diodes 55, 56 and potentiometer 60 to provide a threshold which is compared with the input in differential amplifier 51, 55 and if the input is below (i.e. more black than) the threshold, a signal via a transistor 54 to a discrimination circuit (Fig. 3) sets a tunnel diode 30 to one of two stable states. The tunnel diode is reset to the other by each of a series of clock pulses. The voltage across the tunnel diode is supplied to an output amplifier feeding recognition logic, viz a pulse for each black clock period. Production of such a pulse is prevented if there is no reliable dark area information, by the connection shown using diodes 57 (Fig. 5) which then enables a shunt transistor 52 in the differential amplifier 51, 55. Each clock period, a flip-flop (71) in a gap correction circuit (Fig. 6, not shown) is set to 1 or 0 according as there is an output pulse from the output amplifier or not respectively. In the threshold correction circuit (Fig. 4, not shown), when the input falls below (i.e. blacker than) a very black level (44) as determined by a differential amplifier (41, 43) a flip-flop (47) is set to lower the threshold in the comparator (Fig. 5) so that narrow white areas between strong black areas are not missed. Each clock pulse is ANDed (63, 60) with the inverted set (i.e. effectively the reset) state of the flip-flop (71) in the gap correction circuit, to reset the flip-flop (47) in the threshold correction circuit so that this occurs if there was no pulse from the output amplifier in the preceding clock period. When the flip-flop (71) in the gap correction circuit is set, it shifts the load line of the tunnel diode (30) in the discrimination circuit (Fig. 3) to an area of the characteristic with only one stable state (tracking mode). During a search mode of the system, the mode of the tunnel diode having two stable states is disabled with a "search disable " signal A " blanking pulse " inhibits the setting of the tunnel diode mentioned previously, during the blanking periods of the CRT in the flying-spot scanner. .