Technical background, CD-ROM Technology

MagicSpin Installation Guide

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.

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