To derive direction information, the lines on the disk are read out by two different photo-elements that
"look" at the disk pattern with a mechanical shift of 1/4 the pitch of a line pair between them. This shift is
realized with a reticle or mask that restricts the view of the photo-element to the desired part of the disk
lines. As the disk rotates, the two photo-elements generate signals that are shifted 90 out of phase from
each other. These are commonly called the quadrature QEPA and QEPB signals. The clockwise direction
for most encoders is defined as the QEPA channel going positive before the QEPB channel and vise
versa as shown in Figure 2