Patent GB-959015-A (GB)
Title: Improvements relating to character reading apparatus
Inventor: INGHAM WILLIAM ELLIS
IPC: G06K9/00
Filed: 1959-09-19
Abstract: 959, 015. Automatic character reading. ELECTRIC & MUSICAL INDUSTRIES Ltd. Sept. 19, 1960 [Sept. 19, 1959], No. 31977/59. Heading G4R. In a character reading apparatus the character is sensed to derive a signal having transitions from one state to another and means for indicating that a recognition is uncertain if a transition occurs outside one of a number of predetermined time intervals after a preceding transition. The characters are specially shaped being composed of rectangular elements fitting into five vertical strips as-described in Specification 927,082. They are printed in magnetic ink and sensed by being A. C..magnetised and moved under a row of reading heads. Each head produces either no signal or an A. C. voltage, the transitions indicating the points at which the head crossed on to the black parts of the character. The first strip of each character is a black strip. Clock pulses are generated at a frequency proportional to the speed of movement of the character, the number of pulses occurring between transitions of the signal being counted. The clock pulses are applied at terminal 1 and the two-state signals from the reading head at terminal 2. Eighty clock pulses are generated during the sensing of the character there being 16 pulses to each strip of the character. The clock pulses are applied through an And gate 3 to a four-stage counter 4 counting up to 16, the number of pulses in each strip. The other input to gate 3 is from a flip-flop 8 which is set with flip-flop 11 by the character signals on terminal 2 via a "begin" element 9. This may be a differentiator responsive to a signal transition from white to black. An "end" element 10 responsive to a transition from black to white resets flip-flop 11 via an inhibit gate 37. This gate is controlled by a flipflop 38 which is set by the first transition by the output of flip-flop 8. The gate 37 prevents a spurious black signal preceding a character from causing the entry of a white signal as the first signal. The outputs of elements 9 and 10 are also applied to an Or gate 12 which passes a signal at any transition, via a delay 13 and an inhibit gate 14 to And gates 15 and 16 receiving other inputs from opposite sides of flip-flop 11. The flip-flop 11 serves as an input store to a shift register 17 which has five stages, one for each strip of the character. When a black signal reaches the final stage of the register 17 an output via element 18 resets the flip-flop 8 and sets a monostable device 19 which resets after about 200 microseconds. On resetting an element 20 produces a resetting signal for the circuit, the delay giving time for the stored signals to be read out to recognition circuitry 21. The stages of counter 4 are gated at 22 and 23 which respond to counts of twelve and four respectively giving the bounds of acceptable signal lengths between transitions. The output of gate 22 is applied as a shift pulse to the register 17. The gates 15 and 16 are applied to switches 28 and 29, manually set to cause the delayed transition pulse from gate 12 to reset the counters to variable zero positions, the switch 28 relating to the white interval and switch 29 to the black. This allows compensation to be made for recurrent variations of the black or white strip or group of strips whereby they are greater or less than the ideal width of 16 clock pulses, or a multiple of such widths. The output of the "maximum count" gate 23 sets error flip-flop 25. Operation. The front edge of a character sets the flip-flop 8 and allows clock pulses to pass into the counter. Assuming that the counter is reset to zero the strip is interpreted as an acceptable strip if the signal, black or white, persists for a count of from 12 to 20 pulses. When the counter reaches a count of twelve the gate 22 sends a pulse to the shift register to shift the first black signal into the register 17. When a transition occurs the reset signal from gate 12, delayed at 13 by a quarter clock pulse spacing resets the counter via gate 16. If no transition occurs the gate 22 produces an output each time the counter has a count of twelve and the signal stored in flip-flop 11 is entered in register 17. A transition occurring outside the range of 12-20 or, in the next cycle, 28-36 and so on in following cycles causes flip-flop 27 to be set to reject the character, the recognition being inhibited over lead 30. A rejection may be indicated by audible or visible means and the document concerned passed to a reject bin. If the printing is such that the black strips tend to be broader than the white the switch 28 is set to cause a transition signal from gate 12 to reset the counter to a value of minus two so that it has to count 14 clock pulses before a black signal is resistered.