GB1345708A - Amplitude quantizing arrangements - Google Patents

Abstract

1345708 Analogue to digital converters EMI Ltd 14 May 1971 [22 May 1970] 24861/70 Heading G4H In an analogue to digital converter for converting brightness levels of video signals to digital signals "1" and "0" which are either above or below a threshold level, the threshold level is iteratively computed until an optimum level is obtained. Video brightness signals are applied from a pick-up tube 1 to a quantizer 2 and to gates 3-5. Initially, only the gate 3 is open so that the brightness signals for the first frame are fed via OR gates 9, 10 to accumulators 11, 12, each of which forms an average of its received signals. A circuit 13 adds the signals from the accumulators and halves the sum so that a circuit 6 receives and stores a mean brightness threshold signal. During subsequent frames the gate 3 is closed and brightness signals above the mean brightness open the gate 5, whereas brightness signals below the mean brightness open the gate 4. Thus the circuits 11-13 compute a new threshold level and a comparator 14 compares the previous threshold level with the new threshold level. If the difference in levels is less than a predetermined amount, an output from the comparator opens gates 17, 18.