Patent GB-1255759-A (GB)
Title: Electrical signal handling apparatus
IPC: G06V10/10
Filed: 1970-06-16
Abstract: 1,255,759. Thresholding circuits. INTERNATIONAL BUSINESS MACHINES CORP. 16 June, 1970 [15 July, 1969], No. 29115/70. Heading H3T. [Also in Division G4] Electrical signal handling apparatus comprises a logarithmic converter for producing logarithm values of two input signals, a differential amplifier circuit operating on the logarithm values and producing an output difference signal, and level control means responsive to the output difference signal for varying the operational level of the differential amplifier circuit so as to control the output difference signal. In a character recognition system or mark reader, a spot of light from a CRT 101 (Fig. 1) is sensed by photomultipliers 106, 107 on the CRT screen and on a document 105 on to which it is imaged by mirrors 102, 104 and a lens 103. The photomultiplier outputs 201, 202 are converted to logarithmic form 203, 204 and differenced in a differential amplifier 220, the difference signal being thresholded in a video processor 108 to produce a two-level output 109, 302. Level control 300 follows the difference signal when the two-level output is zero (white) to control the operating point (without materially affecting the gain) of differential amplifier 220. Fig. 2 shows diodes D21, D22 producing the logs 203, 204 of the photomultiplier outputs 201, 202, for comparison using differential transistor pair T21, T22 the common mode current of which is controlled by the level control output 303 at transistor T23, the resistors R21, R22 being much greater than the emitterbase resistances of transistors T21, T22. The difference signal is obtained at 205, 301 via a level-changing transistor T24, the bias of which comes from a voltage divider R25, R26 and filter capacitor C21. Fig. 3 shows level control unit 300, in which the difference signal 301 is compared with a nominal reference VREF in a differential pair T31, T32, the result being compared with a negative bias voltage - 6V in a differential pair T33, T34 to control charging and discharging of a capacitor C31. Large excursions from the bias voltage - 6V are prevented by a bipolar diode clipper D33, and a low-pass filtering action is provided by resistors R32-R34. The capacitor C31 voltage is provided via the emitter follower arrangement shown to the output 303. If the output 302 of the video processor 108 is positive (black), a diode D31 is back-biased so a diode D32 and transistor T37 conduct to cut-off T31, T33 so that the capacitor C31 does not follow the difference signal 301. The level control could alternatively be done using a second differential amplifier in Fig. 2 or an additional input to the differential pair T21, T22. It is said that the video processor 108 may be as in any of Specifications 1,032,902 (in which the threshold is determined from sums of tap outputs on delays associated with the input and the output of the threshold device), 1,097,298 (involving different thresholds for marks and spaces, derived using retention of smallest mark and largest space voltages), 1,119,017 (in which the threshold is derived using sums of tap outputs on a delay and a peak-to-peak mean value). Differential amplifier 220 could be followed by an electronic antilog converter.