| 摘要 |
1451917 Electrical encoder INTERNATIONAL BUSINESS MACHINES CORP 5 Dec 1973 [26 Dec 1972] 56409/73 Heading G4R A circuit for providing a digital signal 18 when an analog input 14 exceeds a threshold comprises means 21-28 for comparing the input with fixed threshold levels and means 40, 50, 60, 70, 80 for selecting one of these levels to be the threshold in dependence on the results of previous comparisons. Input 14 is derived from the scanning 11, 13 of a document 12, and the output 18 indicates the presence of a "black" or a "white" bit on the scanned document. Signal 14 is compared at 21- 27 with (increasing) voltages V1-V7 from a resistor chain 28 which provide levels 1-7 expressed as percentages of absolute black, levels 1 and 4 (10%, 24.3%) corresponding to light and dark (highcontrast) printing. The outputs from 21-27 are stored (under the control of clockpulses 17') in latches 30 from which the signals corresponding to levels 1-4 are fed to gates 41-44). The outputs from 30 also pass to circuits 70, 80 which cause counters 52, 62 to be in- or decremented by clockpulses 17". The outputs from 52, 62 are decoded and fed back to 70, 80. The most significant output bits 51, 61 of each counter are fed back to control gates 41-44: they form a Gray code sequence 00-10-11-01 which effectively identifies the output threshold with levels 1 to 4 in turn. In effect, counters 52, 62 record the numbers of certain sets of the levels which have already occurred and adjust the threshold accordingly. The decoder outputs co-operate with 70 and 80 to ensure that 52, 62 do not exceed their pre-set minimum and maximum counts, and that the level transitions occur only in the order 1-2-3-4-3-2-1. |
