Patent GB-979718-A (GB)
Title: Improvements in and relating to electric signal apparatus
IPC: G06V30/194
Filed: 1961-10-31
Abstract: 979, 718. Automatic character reading. PHILCO CORPORATION. Oct. 31, 1961 [Oct. 31, 1960], No. 39009/61. Heading G4R. [Also in Division H4] In character recognition apparatus the twostate signals derived from scanning the character in a raster are applied to a tapped delay line and there is a controllable coupling for each tapping to give a high or low signal coupling to an output circuit and a source of intermittently generated control signals, each coupling control means being responsive to the control signals and the polarity of a respective one of the tapping points to set the coupling according to the polarity of the signal at the tapping at the time of occurrence of the control signal. The character 21, Fig. 2 is scanned on a raster of seven vertical lines 18 by a flying spot scanner 16 Fig. 1 and the reflected light is received in a photo-cell 22 the output of which is amplified at 34 and applied in positive and negative form to the inputs of a delay line 32. Tappings 42, 44 on the positive and negative sides are applied through a pair of switching networks 46, 52 to an output line 48. Switches in corresponding positions in the two networks are ganged together to ensure that only one tap in each pair is connected at any time. There is one pair of switching networks and one output line 48 for each character to be recognised or alternatively the amplifier 34 and delay line unit within the dash line 10 may be repeated for each character to be recognised. In the scanning of the character "M" Fig. 2 the wave-form obtained, Fig. 2A, has "O" value for the white parts and value "1" for the black parts. Positive and negative forms of this wave-forms are applied to the delay line and tappings. The tappings connected to the line 48 produce a resultant signal as shown in Fig. 3 as the positive and negative signals balance or reinforce each other. The switching networks are arranged so that at the time when the waveform Fig. 2A is properly registered in the delay line a maximum signal 84 is obtained. Threshold device 60, Fig. 1 detects signals in excess of the level 86. The switches consist of diodes which are biased off or not under the control of transistor triggers one for each pair of tappings. The triggers are set for the recognition of a character by closing instantaneously a switch 56. This causes a voltage generator 54 to produce timed control pulses on leads 144 and 146. The effect of the control pulses is that each trigger is first unset and then reset according to the voltages standing on the two tappings over which it has control at the instant a trigger signal is applied by circuit 12. The arrangement is such that the positive of the two leads is always selected for connection to the output line 48. In subsequent scans of the same character the line 48 will have a sharp positive maximum 84 when the derived waveform is properly located in the delay line 32. Defective characters:- The characters 21 may be repeatedly printed, for example, on letters 20. The printing surface may become damaged so that it no longer prints the part below the line 242, Fig. 2. The circuit may be arranged to adjust itself to this defect. The waveform produced by the scanner is changed by the defect from Fig. 7A to Fig. 7B, the final black pulse 26 which should have lasted four units now last only two units. The peak signal on the output line 48 is less than the proper maximum but sufficient to cause the threshold device 232 Fig. 6 to give an output. The delay line 32 is shown as two chains of four inductance stages interconnected by capacitances. The tapping points 42a, 42b, 42c and 42d on the upper line have switching diodes 128a-128d and tapping points 44a-44d on the lower line have switching diodes 134a-134d. Corresponding pairs of diodes 128a-134a are alternatively biased off by a control trigger 138a. The triggers controlling the A and B stages are connected for setting purposes to stages C and D through diodes 164a, 166a and 164b and 166b. The recognition signal from 232 passes to a delay 234 which delays it by two units of time (equivalent to two stages of the delay line) and then applies it to the generator 54 which provides resetting pulses on leads 144, 146. The delay of two units allows the signal, Fig. 7B, to have moved on by two stages. The control triggers 138a, 138b are therefore reset by the signals at the C and D stage tappings. These are the same signals which were at the A and B stages when the waveform was properly positioned in the delay line. The triggers are accordingly reset continually to accommodate minor or gradual changes in the characters to allow defective characters to be recognised. The threshold device may be designed to give an actuating signal to the generator 54 only if the response signal is appreciably lower than its ideal level. For economy some of the sections of the delay line may have fixed or manually controlled tap connections. Secret communication:- The circuit may be used for the transmission of binary data in a coded form. The waveform of Fig. 2A may represent the binary "1" and another waveform the binary "0". Corresponding recognition circuits are provided and messages are transmitted by an appropriate succession of the two waveforms. As described above the waveforms representing "1" and "0" can be continuously changed in a random manner to provide further security against unauthorised reception. The circuits automatically adapt themselves to the changes. The contribution of each tap may be weighted, if desired by varying the capacitance coupling 132a, 136a &c. Fig. 6. Inductance or resistive couplings may be used instead.