Patent GB-1408696-A (GB)
Title: Signal transition detection circuit
IPC: G06K7/00
Filed: 1973-05-23
Abstract: 1408696 Optical bar reading system RCA CORPORATION 23 May 1973 [1 June 1972] 24531/73 Heading G4M [Also in Division H3] A signal transition detector comprises means for determining the peak value of the signal and developing a fraction of the difference between this and a second value, means for delaying the signal and then detecting its crossing of the fraction value and means responsive to the crossing for resetting the peak value determining means. Fig. 1 shows an arrangement in which a label 12 printed with light and dark bars is optically scanned, some bars, due to printing variations, being less light and some less dark as represented by single and double cross hatched areas respectively. The resulting signals have their base lines levelled in a D.C. restoring clamp circuit 28 and are fed via a halfbit delay 30 to one input of a comparator 66, the other input is supplied through a feed-back peak detector 32 the output level of which is divided by two at 50 so that the output of the comparator switches each time the signal passes through the half-peak level. Feedback via 67 and 68 around the comparator provides hysteria to prevent jitter. The arrangement resets the capacitor at the end of each bit to the then existing white level so that any change in white level during the bit is corrected. The resetting is effected by a clock generator comprising two one-shot circuits 86, 88 the first providing a relatively small delay and the second a delay sufficient to make the total delay up to a bit period, i.e. 800 ns. The first transition detected by transition detector 70 or 72 provides, via OR gate 82, a pulse having a negative leading edge which resets both one shots and a positive pulse which triggers the first. After its delay it triggers the second which in turn triggers a further delay 90 which may be a CR circuit so that the OR circuit is again activated to restart the cycle, the delay in 90 being such that the total slightly exceeds the bit period. Thus if a transition occurs at the end of a bit period the clock is reset and restarted in the correct phase. If however there are a plurality of successive light or successive dark bars the clock will continue in approximately correct phase. Fig. 2 shows, at 1, the D.C. restored signal and at 2 the output of the peak rectifier. It will be seen that the rectifier follows the signal until the latter part of the bit where, for example Figs. 103, a clock pulse with a 100 ns delay is produced which is passed via a further delay circuit 92 the output of which momentarily operates a switch 90 to reset capacitor 38 of the peak rectifier to a zero level. During the final part of the bit period the capacitor rapidly recharges, in the case of a light bar to the white level, e.g. 111 or in the case of a dark bar to the next white level, e.g. as shown immediately following 111. Curves 3 and 4 of Fig. 2 show respectively half the peak level and the half bit delayed signal and curve 5 shows the resulting output signal. Fig. 3 shows an arrangement in which the theshold is derived from both the positive white and negative black peak levels. These levels are reset to the level at the end of the bit in response to white to black and black to white transition detectors respectively.