Sidebar 2: Laser Pickups
In a three-beam laser system (fig.1, left), the first and third beams are directed at the disc ahead of and behind the signal beam, at opposite edges of the pit track. Their reflections are picked up by photo cells that are physically separated from the main-signal pickup cell. When both tracking cells "see" the same proportion of the total track width, their equal-but-antiphase outputs cancel. When they are displaced to either side of the signal track, their outputs are of different amplitude, cancellation is incomplete, and the resulting + or – offset voltage is fed to the head servo to get it back on center-track. Focus-correction information in the three-beam system is derived from the distribution of the main signal beam between the quadrants of a third photocell, which also receives the signal data. Perfect focus throws a circular beam; imperfect focus throws an elliptical beam in either of two directions depending on the direction of the focus error (lens too close or too far from the disc surface).
In a single-beam system (fig.1, right), tracking, focus, and signal information are all gleaned from the shape and size of a single reflected laser beam thrown onto a pair of side-by-side dual photocells. Tracking errors shift the beam toward one side pair or the other. Focus errors narrow or spread the beam between the two pairs, depending on the direction of the error.—J. Gordon Holt
In a single-beam system (fig.1, right), tracking, focus, and signal information are all gleaned from the shape and size of a single reflected laser beam thrown onto a pair of side-by-side dual photocells. Tracking errors shift the beam toward one side pair or the other. Focus errors narrow or spread the beam between the two pairs, depending on the direction of the error.—J. Gordon Holt
