Patent GB-1018634-A (GB)
Title: Electronic servo apparatus
IPC: G06V30/144
Filed: 1964-08-21
Abstract: 1,018,634. Automatic character reading. INTERNATIONALE BUSINESS MACHINES CORPORATION. Aug. 21, 1964 [Sept. 3, 1963], No. 34201/64. Addition to 996,509. Heading G4R. [Also in Division G1] In a flying spot scanner as described in the parent Specification for following the outline of a character in a series of part circles moving in steps round the character outline, there are provided means adapted when the search circle fails to intersect the character within a given interval, to increase the diameter of the search circle. A break in a line which allows the scanning spot to pass through will normally be treated as the end of a line and the search circle will follow the outline of the other side of the line. To prevent this the means according to the invention respond when the normal large search circle has failed to produce a black signal for 270 degrees, that is, from point E, Fig. 4A, to point F, to disable the video output circuit for 180 degrees. This prevents the scanning stepping along the underside of the line and causes the centre of the circle to remain fixed at point H, the circle is enlarged so that if the reason for the non-appearance of a black signal is a gap, the line beyond the gap is cut at I and the scan continues to step along the upper side of the line as before. If, on the other hand, it is the end of the line the enlarged circle will fail to cut the character again until point J and this will initiate the return scanning operation as in normal operation, the search circle reverting to normal size. The scanner 210, Fig. 1, is controlled by signals obtained from oscillator 200 via phase shifters 201, 202 which produce cosine and sine waveforms, via attenuators 203, 205 having three gain factors and integrators 204, 206 which produce cosine and sine signals, the former being inverted at 207 to obtain the cosine signal. These signals are applied via summing amplifiers to the deflection plates. The light reflected from the character produces signals from photo-cell 214 which are applied to a clipper 217 to obtain black or white signals. A black signal passing through AND gate 218 sets single-shot 219 which remains set for 180 degrees of the oscillator signal. During this time the single-shot output passes via OR gate 266 to switch the attenuators to the lowest gain so that the search circle performs 180 degrees small diameter path within the character line. The output of the clipper 217 is also taken via AND gate 218. If the search circle reaches the end of the line or passes through a gap the " black fail " circuit 250 responds. This consists of a capacitor normally charged by a black signal from single-shots 219 or 268 and discharged via a resistor at such a rate that the absence of a black signal for 270 degrees will cause a threshold circuit to provide an output to single-shot 251 which fires at point F and remains active until point H. The output is inverted at 254 to disable gate 255 but at the end of the single-shot pulse the gate passes a signal from the circuit 250 to increase the gain of attenuators 203, 205 to provide the largest search circle. While single-shot 251 is active (from point F to point H) gate 218 is blocked via inverter 252 so that no black signal can reach the single-shot 219. The scanning spot therefore passes through the line via point G which is coincident with point E. At point H the larger search circle begins and cuts the line beyond the gap at I or at J if it is a line end. In either case normal scanning is resumed, singleshot 219 again firing and disabling the " black fail " circuit 250 so that the search circle returns to normal size. To improve the ability to bridge a flaw or gap the sensitivity of the circuit is increased in the region where such an occurrence may be expected. For this purpose the output of single-shot 251 is taken to single-shot 257 which fires at the end of the input pulse, i.e. at point H. The time of single-shot 257 is 45 degrees, at which time a second single-shot 258 fires for 90 degrees. This passes a signal to the clipper 217 to reduce its threshold level. The black level is thereby reduced over a 90 degrees arc of the search circle centred about the point of expected interception with the character. The extra large search circle may also be used to cross the line of a closed character such as " 8 " or " 0 " to perform an interior scan. For this purpose a gate 261 enabled by a signal from an external logic circuit sets a flip-flop 262 which is reset by a further input from the logic circuit when the interior scan is finished. When flipflop 262 is set, flip-flop 263 sets to enable gate 264. The next black signal therefore fires singleshot 265 having a time of 270 degrees and connected via OR gate 266 to the small circle control of the attenuators. The small circle is therefore traced for 270 degrees (from N to P in Fig. 5A). The trailing edge of the 270 degrees pulse enables AND gate 267 to set flipflop 269 and to fire single-shot 268 having a time of 180 degrees. The output of flip-flop 269 passes via OR gate to cause the attenuators 203, 205 to trace a circle of the extra large diameter. So as to pass through the line, the video output being disabled during this period by the signal from single-shot 268 which disables gate 218 via inverter 259. At point S the character is cut again and the scanning proceeds normally along the inner side of the line. Background compensation.-The clipper 217 compensates for the colour of the background by storing a representative signal on capacitor C1, Fig. 3. The positive white signal passes through diode D1 to charge capacitor C1 and resistors R2 and R3 provide a path for a slow discharge occupying three or four periods of oscillator 200. Since the beam spends about 180 degrees of its time on the white background the charge in capacitor C1 is a measure of its reflectivity. A predetermined fraction of the stored voltage is applied via emitter follower T1 to the lower side of capacitor C2 which is charged at the upper end by black signals through diode D2 and resistor R4. The time constant is short so that the capacitor becomes fully charged during the black part of the scan. When a black edge is encountered the charging current through resistor R4 applies a negative voltage to the top of resistor R5 producing an output from the top of the potential divider formed by R5 and R6, R7. During the initial search for a character the clipping level is raised to prevent a scanning operation being initiated by spurious specks &c. This is effected by a signal or terminal 217 which makes transistor T3 conduct to apply negative voltage through resistor R9 to the white level signal from divider R2, R3. The effective black level is made more negative, that is, nearer the black signal level. The threshold is lowered for greater sensitivity by a signal on lead 277 which causes transistor T4 to conduct and cuts out resistor R7 so that a greater proportion of the charging signal appears on the output line 276.