shaft encoders are optical devices that provide two logical outputs called A and B. Conceptually, the shaft is attached to two discs. Each disc has alternating transparent and opaque segments, 256 in all. There is a light source on one side of the disc and a light detector on the other side. When a transparent segment of the disc lies between the source and detector the corresponding output is 1, and when an opaque sector lies between the source and detector the corresponding output is 0. Thus the output alternates between 1 and 0 as the shaft is turned.

The sectors of the two discs are not lined up. Instead, one is shifted by half a segment angle with respect to the other. (This shift is not exact; it may vary from a quarter of the segment angle to three quarters of the segment angle, but in any particular encoder it is the same for all segments.) Because of the relationship between the two discs, the output signals are shifted with respect to one another: