A calculation module for calculating the position of a mechanical element, the calculation module being operable to receive an input signal, detect a first plurality of corresponding features in the input signal, calculate the spacing between successive features of the first plurality of corresponding features, generate a first numerical sequence in accordance with the spacing between successive features of the first plurality of corresponding features, detect a second plurality of corresponding features in the input signal, calculate the spacing between successive features of the first plurality of corresponding features, and generate a second numerical sequence in accordance with the spacing between successive features of the first plurality of corresponding features.