Apparatus using information about the extent of errors in sensed data for performing as a control function at least one of adjusting the position of a magnetic head to improve alignment relative to a track and selecting from two or more data signals a data signal having the least amount of errors is shown. The apparatus uses information about the extent of errors to perform a control function for reproducing the data stored in predetermined storage locations in a storage media. The apparatus positions a transducer for sensing from predetermined storage locations stored data containing at least one constraint. The transducer generates a first signal representative of the data containing at least one constraint stored in the sensed data and any errors introduced into the sensed data during the sensing. An input device, preferably in the form of a detector is responsive to the first signal for generating a control signal containing information about the extent of errors in the sensed data and for extracting a data signal. The control signal is used for performing the above control functions. A method for using information about the extent of errors for a control function to improve the extracted data stored at the predetermined storage locations is also shown.