1134 Chapter 30
about 0.6μm wide. A photograph of a pit surface, taken
with a scanning electron microscope, is shown in
Fig.30-2. The track pitch, the distance between succes-
sive tracks, is 1.6μm; the track pitch acts as a diffrac-
tion grating, producing a rainbow of colors. There is a
maximum of 20,188 revolutions across the disc’s stan-
dard data surface width of 35.5 mm.
The linear dimensions of a track are the same at the
beginning of a spiral as at the end. This means that a CD
rotates with a constant linear velocity (CLV), a condition
in which a uniform relative velocity is maintained
between the data spiral and the pickup. To accomplish
this, the rotation speed of a disc varies depending on the
radial position of the pickup. Because each outer track
revolution contains more pits than each inner track revo-
lution, the disc must be slowed as it plays outward to
maintain a constant rate of data. In particular, the disc
rotates at a speed of about 500 rpm when the pickup is
reading the inner circumference, and as the pickup moves
outward, the rotational speed gradually decreases to
about 200 rpm. A constant linear velocity is maintained
through a CLV servo system; the player reads frame
synchronization from the stored data and varies the disc
speed to maintain a constant data rate. The CD standard
permits a maximum of 74 minutes, 33 seconds of audio
playing time on a disc. However, by reducing parameters
such as track pitch and linear velocity, it is possible to
manufacture discs with over 80 minutes of music.
The fact that the disc data surface is physically sepa-
rated from the reading side of the substrate provides a
significant asset. Damage and dust on the outer surface
do not lie in the focal plane of the reading laser beam
and hence their effect is minimized.
The polycarbonate substrate has refractive index of
1.55; the velocity of light slows from 3 × 10^5 kilome-
ters/second (km/s), to 1.9 × 10^5 km/s. Because of the
bending from the refractive index and thickness of the
substrate, and the numerical aperture (NA) of 0.45 of
the laser pickup’s lens, the diameter of the laser spot is
reduced from approximately 800μm on the disc surface
to approximately 1 μm at the pit surface. The laser
beam is thus focused to a point larger than a pit width.
The reflective data pit surface, known as land, causes
almost 90% of the laser light to be reflected back into
the optical pickup. When viewed from the laser’s under-
side perspective, the pits appear as bumps. The height
of each bump is between 0.11 and 0.13μm (110 and
130 nm.) This dimension is slightly smaller than the
laser beam’s wavelength in air of 780 nm (some players
use 790 nm). Inside the polycarbonate substrate, the
laser’s wavelength is about 500 m. The height of the
bumps is thus approximately one-quarter of the laser’s
wavelength in the substrate.
There is a phase difference between the part of the
beam reflected from the bump, and the part reflected
from the surrounding land. The phase difference causes
destructive interference in the reflected beam. In theory,
when the beam strikes an area between pits virtually all
of its light is reflected, and when it strikes a pit virtually
all of the light returning to the pickup is canceled, hence
virtually none is reflected. In practice, the laser spot is
larger than required for complete cancellation between
pit and land reflections, and pits are made slightly shal-
lower than a quarter wavelength; this yields a better
tracking signal, among other things. Typically the pres-
ence of a bump reduces reflective power by about 25%.
In any case, the data surface varies the intensity of the
reflected laser beam. Thus the data physically encoded
on the disc can be recovered by the laser, and converted
to an electrical signal using a photodiode.30.2.2 Data EncodingThe audio program played from a CD is the culmination
of a data transformation that takes place during master
encoding and that undergoes decoding each time the
disc is played. Various media are used to hold master
recordings. Originally, many CDs were mastered from
data recorded on ¾ inch U-matic videotape cassettes
using a digital audio processor. In many cases, Exabyte
8 mm data tapes are used to hold the master recording.
For audio mastering, the DDP (Disk Description Proto-
col) file format may be used to hold Red Book and PQFigure 30-2. Scanning electron microscope photograph of
the CD data surface. Courtesy University of Miami.