Handbook for Sound Engineers

Optical Disc Formats for Audio Reproduction and Recording 1139

would be shortened and astigmatism from the cylin-
drical lens would cause the reflected laser spot to be
flattened and rotated to one side. This would cause more
light to fall on two (opposite) pairs of photodiodes than
on the other pairs. This generates a voltage interpreted
by the servo system as a command to pull the lens down
from the disc. This provides the correct focal path
length where astigmatism would not affect the beam.
Hence, it would have a round shape, and an equal
amount of light would fall on each part of the
four-quadrant photodiode, providing a neutral signal to
the servo system. When the disc is too far from the lens,
the laser spot rotates in the opposite direction, gener-
ating a voltage that pushes the lens upward. In practice,
the process in this servo loop is a dynamic one, with the
objective lens moving in constant accord with disc devi-
ations to provide a correct focal path length.

In three-beam pickups, the two secondary beams are
used for auto-tracking. The central beam spot covers the
pit track while the two tracking beams are aligned
above and below and to either side of the center beam.
When the beam is tracking the disc properly, part of
each tracking beam is aligned on the pit edge; the other

part covers the mirrored land between pit tracks. The

main beam strikes a four-quadrant photodiode, and the

two tracking beams strike two separate photodiodes

mounted on either side of the main photodiode.

If the three spots drift to either side of the pit track,

the amount of light reflected from the tracking beams

varies. There is less average light intensity reflected by

the tracking beam that encounters more pit area and

greater reflected light intensity from the tracking beam

that encounters less pit area. The relative output volt-

ages from the two tracking photodiodes thus form a

tracking correction signal, as shown in Fig. 30-8. Oper-

ating similarly to the signal used in the auto-focus servo

loop, this tracking signal forms a control voltage for the

auto-tracking servo mechanism. For example, when the

pickup’s objective lens drifts to the right of the pit track,

the right tracking beam encounters more reflective land

and its reflected intensity is greater. When this brighter

spot strikes the right tracking photodiode, a voltage

greater than that on the left photodiode is generated.

This voltage shift causes the servo system to move the

pickup to the left, toward the pit track center. Likewise,

the opposite occurs when the pickup drifts to the left. In

this dynamic process the servo system continually

moves the pickup to compensate for track deviations.

In addition to auto-focus and auto-tracking, a CD

pickup uses other motor systems to move the pickup

across the disc surface in response to user commands.

For example, the pickup must search rapidly across the

disc as it reads data, or jump from one track to another.

These functions are handled using control signals

derived from the auto-tracking and auto-focus circuits;

however, separate motors are used to move the pickup

itself. Three beam pickups are mounted on a sled that

moves across the disc surface. In many designs, linear

motors move the pickup and position it to within

capture range of the auto-tracking circuit, which takes

control when the selected disc location is found. A

spindle motor is used to rotate the disc with constant

linear velocity. Thus the player must vary the disc speed

Figure 30-6. Three-beam optical pickup showing diffraction
grating, objective lens, and photodiode.

Figure 30-7. Astigmatism used in auto-focusing.

Monitor diode

Laser diode

Grating

PBS

Collimator

lens

QWP

Collective lens

Cylindrical lens

Disc

Objective

lens

Photodiode

Spot

lens

PBS

Obj. lens Disc

Cylindrical

lens

Photodiode

faceplate

Cylindrical

lens

Photodiode In focus

Disc near

Disc far

Figure 30-8. Auto-tracking correction signal.

Disc too far Disc too near

Disc

S 1 deviation

S 2 +

AF signal

0