I/O Magic MagicSpin - Manual (Page 6)

MagicSpin Installation Guide
Page 6
Technical background
CD-ROM Technology
CD-ROM stands for Compact Disc Read Only Memory. A compact disc format used to hold
text, graphics and hi-fi stereo sound. It's like an audio CD, but uses a different track format for
data. The audio CD player cannot play CD-ROMs, but CD-ROM players usually play audio CDs
and have output jacks for a headphone or amplified speakers.
CD-ROMs hold 650MB of data, which is equivalent to about 250,000 pages of text or 20,000
medium-resolution images.
A CD-ROM drive (player, reader) connects to a controller card, which is plugged into one of the
computer's expansion slots. Earlier drives used a proprietary interface and came with their own
card, requiring a free expansion slot in the computer. Today, most CD-ROMs for PCs use the
SCSI or EIDE interface and can be installed without taking up an extra slot.
The first CD-ROM drives transferred data at 150KB per second. The speed then doubled to
300KB, then quad speed at 600KB. Today, The MagicSpin CD-ROM is more than 40 times the
original transfer rate.
CD-ROM Construction and manufacturing
Compact disks start as round wafers made from a polycarbonate substrate, measuring 120 mm
(about 4.75 inches) in diameter and about 1.2 mm in thickness, which is less than 1/20th of an
inch. These blanks are made into production CDs using a process not dissimilar to how old vinyl
records were made (which is somewhat fitting, if you think about it.)
The first step in the creation of a CD is the production of a master. The data to be recorded on the
disk (either audio or computer data, there are many different formats) is created as an image of
ones and zeros. The image is etched into the master CD using a relatively high-power laser (much
more powerful than the one you would find in a regular CD player) using special data encoding
techniques that use microscopic pits to represent the data. The master CD is then used to create
duplicate master stamps.
The actual CDs are produced by pressing them with the master stamp. This creates a duplicate of
the original master, with pits in the correct places to represent the data. After stamping, the entire
disk is coated with a thin layer of aluminum (which is what makes the disk shine, and is what the
laser reflects off when the disk is read) and then another thin layer of plastic. Then, the printed
label is applied to the disk.
Many people don't realize that the data surface of the CD is actually the top of the disk. The
media layer is directly under the CD label, and the player reads the CD from the bottom by
focusing the laser through the 1.2 mm thickness of the CD's substrate. This is one reason why the
bottom of the disk can have small scratches without impeding the use of the disk; they create an
obstacle that the laser must look through, but they don't actually damage the data layer. On the
other hand, scratches on the top of the disk can actually remove strips of the reflective aluminum
coating, leaving the disk immediately unusable.