CHAPTER 13

694 Chapter 13 Optical Storage
■ Program (data) area. This area of the disc starts at a radius of 25mm from the center.
■ Lead-out. The lead-out marks the end of the program (data) area or the end of the recording session
on a multisession disc. No actual data is written in the lead-out; it is simply a marker. The
first lead-out on a disc (or the only one if it is a single session or Disk At Once recording) is
6,750 sectors long (1.5 minutes if measured in time, or about 13.8MB worth of data). If the disc
is a multisession disc, any subsequent lead-outs are 2,250 sectors long (0.5 minutes in time, or
about 4.6MB worth of data
The hub clamp, lead-in, program, and lead-out areas are found on all CDs, whereas only recordable
CDs (such as CD-Rs and CD-RWs) have the additional power calibration area and program memory
area at the start of the disc.
The center hole in a CD is 15mm in diameter, which means it has a radius of 7.5mm from the center
of the disc. From the edge of the center hole to a point at a radius of 20.5mm is the HCA. That is followed
by the PCA, which starts at a radius of 20.5mm from the center. The PCA is followed by the
PMA, which starts at a radius of 22.35mm, and then the lead-in area, which starts at a radius of
23mm from the center of the disc. The program (data) area of the disc starts at a radius of 25mm from
the center, and that is followed by the lead-out area at 58mm. The disc track officially ends at
58.5mm, which is followed by a 1.5mm buffer to the edge of the disc. Figure 13.4 shows these areas
in actual relative scale as they appear on a disc.
Figure 13.4 Areas on a CD (side view).
Officially, the spiral track of a standard CD-DA or CD-ROM disc starts with this lead-in area and ends
at the finish of the lead-out area, which is 58.5mm from the center of the disc, or 1.5mm from the
outer edge. This single spiral track is about 5.77 kilometers or 3.59 miles long. An interesting fact is
that in a 56x CAV (constant angular velocity) drive, when reading the outer part of the track, the data
moves at an actual speed of 162.8 miles per hour (262km/h) past the laser. What is more amazing is
that even when the data is traveling at that speed, the laser pickup can accurately read bits (pit/land
transitions) spaced as little as only 0.9 microns or 35.4 millionths of an inch apart!
Table 13.1 shows some of the basic information about the two main CD capacities, which are 74- and
80-minute. The CD standard originally was created around the 74-minute disc; the 80-minute versions
were added later and basically stretch the standard by tightening up the track spacing a little bit.