Abstract: A system for detecting and reproducing variable-area-type optically recorded audio signals by cyclically detecting substantially only the position of a boundary of the sound track. In one embodiment, the boundary is detected by imaging the boundary onto an array of photosensors and scanning the array to provide an output signal containing a transition corresponding in the time domain to the position of the boundary. The transition signal is filtered and sliced to provide electrical interpolation to improve the resolution substantially over the special resolution of the photosensitive elements of the array. The system may also include a multitrack capability with means for minimizing the effect of film weave.