Verfahren bei der maschinellen Zeichenerkennung zur Erkennung der Lage eines Aufzeichnungstraegers gegenueber dem Abtastfeld

1,252,584. Character recognition. INTERNATIONAL BUSINESS MACHINES CORP. 10 July, 1969 [15 July, 1968], No. 34806/69. Heading G4R. Document scanning apparatus comprises means for scanning a region of a document expected to include a calibration mark in accordance with a scan pattern, the position of the scan pattern being varied by a predetermined amount on detection of an error in the way in which a calibration mark is intercepted during a scanning operation. The document, which is to be read under computer control for character recognition purposes, bears an L-shaped calibration mark, the vertical and horizontal bars of the mark being 400 mils long each and the bar widths being at least 20 mils. The nominal position of the junction of the bars is specified, and the scan starts 200 mils to the left and 200 mils up from this point to perform 12 coincident 420 mil downward sweeps followed by 12 coincident 420 mil leftward sweeps (also starting from the same point). If the condition is satisfied that a 12 mil (or more) length of black is sensed in the same position in every sweep of each set of 12 but not at the beginning, the positions of these blacks on the downward and leftward sweeps are used to derive error voltages subsequently used to correct deflection voltages used in the subsequent scanning of fields for recognition. If the condition is not satisfied the scanning is repeated but with the starting point displaced 20 mils to the right and 20 mils down, if it is still not satisfied, the scanning is repeated again but with the starting point displaced 20 mils to the left and 20 mils up (from the first scanning start point). If it is not satisfied now, the document is rejected. If it is satisfied on the second or third scanning, the error voltages are derived and used as with a successful first scan. The position of the junction of the bars on a first (or master) document is set by the operator on potentiometers and whether this setting is accurate to within 40 mils (both vertically and horizontally) is automatically checked using scanning, a lamp being lit if it is. This provides the nominal position of the bar junction for subsequent use as above. The invention may be used as a modification of the systems of Specifications 1,077,094 and 1,252,583 which are referred to.