Abstract: A slowly varying bias signal is added to one input of a Wave Form Transition Sequence Detector, as more fully described in a related application cited herein, to provide a differential output therefrom which is directly proportional to the time between transitions occurring on two input wave forms. The slowly varying bias modulates the level of one input wave form, and thereby varies the time required for the Wave Form Transition Detector to detect a transition occuring thereon. By symmetrically varying the response time of the Wave Form Transition Sequence Detector to one input wave form in the neighborhood of the occurrence of a transition on the second input waveform, the average differential output of the Wave Form Transition Sequence Detector over a cycle of the slowly varying bias level will be proportional to the time between transitions occurring on the two input wave forms.