Patent GB-939432-A (GB)
Title: Signal detection circuit
IPC: G06V30/10
Filed: 1962-04-16
Abstract: 939,432. Automatic character reading. NATIONAL CASH REGISTER CO. April 16, 1962 [July 31, 1961], No. 14584/62. Class 106 (1). In a character reading system two characters are scanned to produce a complex signal from which character information signals are derived by sampling the complex signal at the instants the character signals occur to derive a maximum amplitude signal a fixed proportion of which is used as a threshold level in a clipping circuit. The characters are projected in image on to a rotating drum having four scanning holes 22a-22d, Fig. 2, each associated with a photo-multiplier tube. The signal from one tube is shown at c in Fig. 2. It increases generally in the middle of the scan because of increased illumination. The tube signal is rendered complex by noise some of which comes from particles in the paper and some from splattering of ink. The invention is concerned to eliminate the noise signals and produce clean pulses for each character stroke crossed by the scan. The four photo-tube signals a-d are applied to normalizing amplifiers 110a-110d, Fig. 1, which modify the signals to make the average black to white difference constant for each lead. The signals are then bottom-clipped to remove paper noise in circuits 115a-115d. For this purpose there are connected to each lead oppositely poled diodes which pass the black and white signals respectively to capacitor stores. The white level is smoothed by a resistor-capacitor circuit and applied to a potential divider, the black signal being applied to the other end. The centre tap is arranged to give 15% of the black/white signal difference which voltage is used as a threshold level for clipping circuits in each channel. The resultant signals, shown at c 1 , Fig. 2, are applied to further clipping circuits for top clipping. The clipping level is derived as shown in Fig. 6, each lead a 1 -d 1  being applied through diode OR gate to a transistor T 5  which acts as a sampling gate being opened by pulse signals A-D appearing at the output of the circuit. These output pulses are shown at C, Fig. 2, as square waves occurring at the stroke positions of the sensed characters. They are timed to sample the a 1 -d 1  signals at their maximum values, a pulse on any of the output leads serving to gate the maximum signal on any of the input leads into a capacitance store C 4 . Signal on C 4  is smoothed by resistor R 5  and capacitor C 5  and emitter-follower T 6  applies the smoothed signal to potential divider 118. Centre point 119 gives 50% of the maximum signal value at any instant so that clipping takes place on the dotted line in Fig. 2, c 1 . The output is as shown at c 2 . Defective black signals as at 27 are restored by differentiating the signals at 130a-130d and applying them to pulse generators 135a-135d giving fixed length square pulses C. Delays are included to cause the square pulse to start at the instant the corresponding input peak reaches its maximum. As shown in Fig. 8, the circuits 130 and 135 eliminate variations arising from non-standard width or blackness of the character strokes.