Abstract: In a data storage and retrieval system employing a magnetic disk as the storage medium, a monophase encoder is coupled, typically, to an electric seek motor for optoelectronically producing an output representative of the position of a data transducer with respect to a multiplicity of concentric data tracks on the disk. Electrically connected to the monophase encoder is a triple detector circuit for deriving from the encoder output a transducer position signal representative of the transducer position on the disk, an actual transducer speed signal representative of the transducer speed relative to the disk, and a track boundary signal representative of boundaries between the data tracks on the disk.