508 Chapter 16
It is necessary to control the position of the lens, in relation both to the disc surface and
to the recorded spiral sequence of surface lumps, to a high degree of accuracy. This is
done by high-speed closed-loop servo-mechanism systems, in which the vertical and
lateral position of the whole optical readout assembly is precisely adjusted by electro-
mechanical actuators, which are caused to operate in a manner that is very similar to the
voice coil in a moving coil loudspeaker.
Two alternative arrangements are used for positioning the optical readout assembly, of
which the older layout employs a sled-type arrangement that moves the whole unit in a
rectilinear manner across the active face of the disc. This maintains the correct angular
position of the head, in relation to the recorded track, necessary when a “ three-beam ”
track position detector is used. Recent CD replay systems more commonly employ a
single-beam lateral/vertical error detection system. Since this is insensitive to the angular
relationship between the track and the head, it allows a simple pivoted arm structure to
be substituted for the rectilinear-motion sled arrangement. This pivoted arm layout is
less expensive to produce, is less sensitive to mechanical shocks, and allows more rapid
scanning of the disc surface when searching for tracks.
Some degree of immunity from readout errors due to scratches and dust on the active
surface of the disc is provided by the optical characteristics of the lens, which has a
suffi ciently large aperture and short focal length that the surface of the disc is out of focus
when the lens is accurately focused on the plane of the buried mirror layer.
16.3.2 Electronic Characteristics
The electronic replay system follows a path closely similar to that used in the encoding
of the original recorded signal, although in reverse order, and is shown schematically in
Figure 16.9. The major differences between record and replay paths are those such as
“ oversampling, ” “ digital fi ltering, ” and “ noise shaping ” intended to improve the accuracy
of, and reduce the noise level inherent in, the digital-to-analogue transformation.
Referring to Figure 16.9 , the RF electrical output of the disc replay photocell, after
amplifi cation, is fed to a simple signal detection system, which mutes the signal chain in
the absence of a received signal, to ensure intertrack silence. If a signal is present, it is
then fed to the EFM decoder stage where the interface and “ joining ” bits are removed,
and the signal is passed as a group of 8-bit symbols to the CIRC error correction circuit,
which permits a very high level of signal restoration.