A system for converting mechanical movement of a body under detection to a digital signal comprises a synchro generator having a primary and a secondary windings movable relative to each other, one of the windings capable of being coupled to said body, an exciting circuit for providing multiple-phase exciting currents to the primary winding at a frequency higher than the frequency corresponding to the maximum speed of the body, and a phase-locked-loop control circuit responsive to the output of the secondary winding for generating a frequency-multiplied digital signal at a frequency equal to the product of the frequency of an AC signal induced in the secondary winding and a predetermined factor.