A pseudorandom encoder is an electro-mechanical device used for measuring distance and finding position. It is different from the conventional absolute and incremental encoder in its disk pattern, which gives it aspects of the other two designs.
The disk of a pseudorandom encoder has an evenly spaced notch design just as an incremental encoder to measure distance. A second track of code is used to find the position just like an absolute encoder; however, an absolute encoder tells position from reading from multiple parallel tracks while a pseudorandom encoder uses one single track. This aspect allows pseudorandom rotary encoders to have smaller diameters and larger through-holes. To know its position, the encoder must first move a certain distance to read the code. This distance is generally small, about 1-2 degrees for rotary encoders and 1 mm for linear encoders, though this idiosyncrasy does hinder its usefulness in some applications such as track-balls since position is not immediately known when the system starts.