Optical Storage - Upgrading and Repairing PCs - page 528

• 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).
Figure 11.4. Areas on a CD (side view).
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.
Figure 11.4 shows these areas in actual relative scale as they appear on a disc.
Officially, the spiral track of a standard CD starts with the 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 the outer part of the track is being read, 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 11.1 shows some of the basic information about the two main CD capacities, which are 74 and
80 minutes. 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 the track spacing within the
limitations of the original specification. A poorly performing or worn-out drive can have trouble
reading the 80-minute discs.
The spiral track is divided into sectors that are stored at the rate of 75 sectors per second. On a disc
that can hold a total of 74 minutes of information, that results in a maximum of 333,000 sectors. Each
sector is then divided into 98 individual frames of information. Each frame contains 33 bytes: 24