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[ The PC Guide | Systems and Components Reference Guide | CD-ROM Drives | Compact Disk Media ]

Media Construction and Manufacture

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 (see the encoding section for details on this). 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.

CDs are fairly hardy but are far from indestructible. They are reasonably solid but overly flexing them can make them unreadable. They are not too sensitive to heat but will warp if left to bake in direct sunlight on a hot summer's day. CD media should always be cared for properly. The use of caddies or jewel cases will protect them; in general, the less handling, the better.

Next: Data Encoding and Decoding

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