Hardware Reference
In-Depth Information
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
are74and80minutes.TheCDstandardoriginallywascreatedaroundthe74-minutedisc;
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 bytes are audio data, 1 byte contains subcode informa-
tion, and 8 bytes are used for parity/ECC (error correction code) information. Table 11.2
shows the sector, frame, and audio data calculations.
Table 11.2 CD Sector, Frame, and Audio Data Information
 
Search WWH ::




Custom Search