Patent GB-948821-A (GB)
Title: Apparatus for comparing intervals of time
IPC: G06V10/10
Filed: 1960-06-16
Abstract: 948,821. Automatic character recognition. INTERNATIONAL BUSINESS MACHINES CORPORATION. June 16, 1960 [June 29, 1959], No. 21141/60. Heading G4R. Intervals of time between a datum pulse and a variable pulse are registered in an apparatus having a plurality of time-analogue voltage generators each associated with one cycle of a number of cycles, switching means switching the input pulses in each cycle into the corresponding generator, means for storing the time analogue voltage for a cycle and means for regenerating an interval pulse. The apparatus is described as applied to a character recognition system Fig. 2 in which a character on a moving card 41 is projected by lens 47 on to a revolving slotted disc 53 dissecting the image in vertical scans. A photo-cell 57 produces corresponding signals which after amplification, clipping and inversion are used to reset a trigger 72 previously set at the beginning of the scan by a timing pulse T1 read from a magnetic drum 65 revolving with the disc 53. The trigger 72 therefore produces pulses Vg whose duration represents the distance from the beginning of the scan to the intersection with the tops of the character. A delay 71 produces other timing pulses T2. The purpose of the apparatus is to determine the slope of the upper contour of the character e.g. a "6" as shown in Fig. 5. The distance X1 from the beginning of scan a which is a time value is converted into a voltage and stored until the next scan when it is compared with the time X2 taken in the following scan. The interval pulses Vg, one for each scan are applied to three gates 81, 83, 85, Fig. 3a which are enabled in succession by outputs from a chain of three triggers 73, 75, 77, and distributed to three charge and storage circuits 87, 89, 91. The pulse Vg when present in circuit 87 (say) causes normally conducting valve 93 to turn off and allows condenser 102 to charge from a +250 v source. The duration of the pulse Vg is therefore linearly converted into a voltage on condenser 102. A cathode follower 105 produces a corresponding output signal. Timing signals from the trigger chain are used to gate out these signals in succession through OR gate 113 on to line 115 and the condenser 102 is then discharged by a corresponding discharge circuit 117, 119, 121. The amplitude signal in line 115 is converted back to a timed pulse signal in a circuit comprising a condenser 157 which, when valve 153 is cut off by a T1 pulse at the beginning of a scan, charges at a linear rate to produce a saw-tooth voltage. The valve 143 acts as a threshold circuit conducting when the sawtooth voltage from condenser 157 on the grid exceeds the voltage on the cathode from line 115. The output from the anode is differentiated at 163, 165, squared and amplified in inverters 171, 173, and applied as a timed pulse on line 177 to triggers 179 and 181 Fig. 3b. The former turns on and the latter turns off on receipt of this signal. The interval pulse Vg from the next scan is also applied to the triggers turning 179 off and 181 on. If the pulse on line 177 arrives after the end of pulse Vg, trigger 179 remains on whereas if it arrives before the end of pulse Vg, trigger 181 remains on. The triggers are interconnected through gates 183, 185 and inverters 187, 189 to ensure that only one is on at any time. If trigger 179 is on a negative slope is indicated and a positive slope is indicated by trigger 181 being turned on. The output from the operative trigger is a duration pulse the length of which indicates the difference in time between intersections with the upper edge of the character on adjacent scans, that is, it indicates the slope. The polarity of the slope is stored on triggers 191, 193 and the magnitude is converted to an amplitude signal on condenser 205 by charging it for the duration of the pulse. The amplitude signal is applied to a pair of threshold devices 211, 213 responding to a gentle slope and a steep slope respectively. The outputs are stored on triggers 233,235. The signals from triggers 191, 193, 233 and 235 are combined in five ways in AND gates 237, 239, 241,243 and 245. These give indications that the slope is gentle and positive, gentle and negative, steep and positive, steep and negative or zero respectively. Successive gate signals indicate the slope of the upper edge of the character for each adjacent pair of scans. Signals representing the slope at predetermined scans may be used with other feature signals in character recognition circuitry.