A rotary encoder post-processing apparatus for use with a rotary encoder of the kind comprising: an annular track comprising an alternating pattern of at least two different encoding regions; a first detector at a first fixed position which is arranged to produce a first alternating output signal as the track of encoding regions rotates about its axis past the detector, the state of the output signal depending on which of the two different encoding regions is facing the first detector; a second detector at a second fixed position which is offset from the position of the first detector and is arranged to produce a second alternating output signal as the track of encoding regions rotates about its axis past the detector, the state of the output signal depending on which of the two different encoding regions is facing the second detector; in which the combined values of the two output signals are capable of assuming a plurality of unique states with the states changing from one to another as the encoder rotates, in which the post processing apparatus comprises: a memory which stores compensation values, each value being associated with a respective one of the possible state changes that can occur when the combined values of the two output signals change upon movement of the encoder track, and in which each compensation value is indicative of an average error in the position at which the corresponding change of state occurs, and a processing means which in use generates a position signal that is updated when there is a change in the combined states of the output signals of the two detectors, the position signal being corrected by an amount indicated by the compensation value associated with the latest change in state an annular track comprising an alternating pattern of at least two different encoding regions; a first detector at a first fixed position which is arranged to produce a first alternating output signal as the track of encoding regions rotates about its axis past the detector, the state of the output signal depending on which of the two different encoding regions is facing the first detector; a second detector at a second fixed position which is offset from the position of the first detector and is arranged to produce a second alternating output signal as the track of encoding regions rotates about its axis past the detector, the state of the output signal depending on which of the two different encoding regions is facing the second detector; in which the combined values of the two output signals are capable of assuming a plurality of unique states with the states changing from one to another as the encoder rotates, in which the post processing apparatus comprises: a memory which stores compensation values, each value being associated with a respective one of the possible state changes that can occur when the combined values of the two output signals change upon movement of the encoder track, and in which each compensation value is indicative of an average error in the position at which the corresponding change of state occurs, and a processing means which in use generates a position signal that is updated when there is a change in the combined states of the output signals of the two detectors, the position signal being corrected by an amount indicated by the compensation value associated with the latest change in state a memory which stores compensation values, each value being associated with a respective one of the possible state changes that can occur when the combined values of the two output signals change upon movement of the encoder track, and in which each compensation value is indicative of an average error in the position at which the corresponding change of state occurs, and a processing means which in use generates a position signal that is updated when there is a change in the combined states of the output signals of the two detectors, the position signal being corrected by an amount indicated by the compensation value associated with the latest change in state