DE2063953

1329109 Transistor circuits INTERNATIONAL BUSINESS MACHINES CORP 22 Dec 1970 [29 Dec 1969] 60729/70 Heading H3T [Also in Divisions G1 and G4] In a circuit, Fig. 1, for determining whether an electric pulse is of the form shown in Fig. 3 or is as in Fig. 3 but with the voltage V2-V3 approximately zero, the pulse is fed to a first circuit 194 which determines the maximum value V2 of the pulse and to a second circuit 196 which determines the minimum value V3. The maximum value signal is attenuated in unit 198, the resultant signal being compared at 200 with the minimum value. The comparator provides an output pulse if the attenuated signal is greater than V3, i.e., if V2-V3 is sufficiently large. The circuit may be used in the character recognition system described in Specification 1,320,959, scanning discs 121, 122 passing light from the character to photo-cell 184 which provides a series of pulses each similar to that in Fig. 3, to units 196, 198 via a unit 192 which clamps signal level V1 to zero. Clamp 192-The signal from the photocell is fed to terminal 210, Fig. 4. A timing pulse 216, Fig. 5A, precedes the pulse from the photocell and is applied to terminal 218, Fig. 4, to discharge capacitor 224. Emitter follower 228 senses the potential at the gate electrode of FET 226 and places output terminals 212, 214 just below zero. An immediately following timing pulse 232, Fig. 5A, applied to terminal 234 replaces the charge on capacitor 224 to bring the output terminals 212, 214 close to zero. Attenuating unit 198-This may be a preset potentiometer (302, Fig. 6, not shown) at the output of the peak detector if the character and the background are uniform, but if the character and/or background are non-uniform, giving a value V2, V3 which may vary from scan to scan, the attenuator may be self-adjusting to vary the threshold with which the minimum value V3 is compared. In one embodiment Fig. 9, a voltage is passed by potentiomenter 312 to a comparator 320. If, due to change in the character blackness or in the background this voltage falls below the value of the threshold derived via a tap on resistor chain 321-325, comparator 320 steps counter 338 which changes the tap on the resistor chain so as to decrease the threshold level relative to earth. Succeeding scans will adjust the attenuation similarly. In a second embodiment, Fig. 10, the voltage supplied by potentiometer 3121 is compared at 348 with the threshold level provided by differential amplifier 350. C