What are the differences between single-spot and three-spot pickups?
A. single-spot and three-spot pickups generate the track following signal differently. The single-spot design uses the difference in intensity between the left and right halves of the returning beam. The three-spot design uses the difference in intensity of the two separate “outboard” spots generated for this purpose. The single-spot pickup tracks the center of the track, whereas the three-spot pickup tracks the edges of the track. There may also be other differences, depending on the implementation, which can vary considerably. For instance, single-spot pickups typically used a half-silvered mirror to separate the incident and reflected beams, whereas the three-spot pickup typically uses a polarizing beamsplitter. Also, single-spot pickups use a galvanometer-type positioning mechanism, but the three-spot must use a linear sled which is typically driven by gears.
