Audio Engineering

Compact Disc 507

refl ecting layer is deformed in the recording process to produce a sequence of oblong
humps along the spiral path of the recorded track (actually formed by making pits on
the reverse side of the disc prior to metallization). Because of the shallow depth of focus
of the lens, due to its large effective numerical aperture ( f /0.5) and the characteristics
of the laser light focused on the refl ecting surface, these deformations of the surface
greatly diminish the intensity of the incident light refl ected to the receiver photocell, in
comparison with that from the fi at mirror-like surface of the undeformed disc. This
causes the intensity of the light reaching the photocell to fl uctuate as the disc rotates
and causes the generation of the high-speed sequence of electrical ‘ 0’s and ‘ 1’s required
to reproduce the digitally encoded signal.

The signals representing ‘ 1’s are generated by a photocell output level transition, either
up or down, while ‘ 0’s are generated electronically within the system by the presence of a
timing impulse that is not coincident with a received ‘ 1 ’ signal. This confers the valuable
feature that the system defaults to a ‘ 0 ’ if a data transition is not read, and such random
errors can be corrected with ease in the replay system.

Objective lens

assembly

Laser diode

Wedge lens

Photocell array

Beam

splitter

mirror

Compact disc

Figure 16.8 : Single-beam optical readout system.