Abstract: Apparatus for developing digital signals that respectively represent a plurality of different angular positions of the crankshaft of an internal combustion engine. The crankshaft drives two slotted wheels that are each associated with a sensor. Two pulse trains are developed. The first pulse train is a series of equally angularly spaced pulses. The second pulse train is comprised of a plurality of angularly spaced window pulses of different widths. Pulse falling edges of the first pulse train are counted over the duration of the windows of the second pulse train to develop digital signals that represent angular positions of the crankshaft. The system prevents edge counting until a valid edge of window pulse is detected.