In certain situations it is necessary to detect a slight change in a continuous wave signal and to be able to specifically determine the exact position of the change. This is especially necessary in situations where a continuously changing signal (with respect to time) is generated and where it is necessary to determine, at a particular instant of time, any change in the signal from a previous corresponding instant of time. Where the signal is periodic in nature and substantially repeatable between cycles it is possible to establish a memory data base of signal levels as a function of time. Comparisons can then be made on successive cycle against the known, or precalculated data base. Changes between the data base and the new signal result give a positive signal change indication.