Self-adaptive, all-or-nothing converter of an analog image analysis signal

Claim 1

A self-adaptive all-or-nothing converter for converting an analog image analysis signal into a two-valued signal, the converter comprising: a comparator for comparing the analog image analysis signal with a decision threshold to produce a two-valued output signal taking one value when the analog image analysis signal is above the decision threshold, and the other value when it is below the threshold; and correction means responsive to the modulation level of the analog image analysis signal to adjust the relative levels, as applied to the comparator, of the analog image analysis signal and the decision threshold, said correction means comprising a peak detector for detecting successive modulation peaks significant of the locally palest of the successive elements of the image as represented by the analog analysis signal, a store for storing the value of the most recently detected peak, first processing means for applying a slow decay to the stored peak value, second processing means for applying a rapid decay to the stored peak value, and a control circuit for controlling said second processing means, the control circuit being arranged, in operation to detect whenever the modulation signal has remained below the threshold for at least ten elementary points of the image represented by the analog image analysis signal, and then to cause the second processing means to apply said rapid decay to the stored peak value to bring said stored peak value rapidly to a value close the palest image level then present in the image analysis signal.