CD-Audio and CD-ROM were established in the early 1980s as the “new thing” to replace he venerable analog long-playing record. This digital format was very quickly accepted as the standard because it provided compact size, high fidelity and tremendous durability. But soon the consumer, accustomed to recording LPs onto magnetic tapes, demanded the same ability in the new compact disc format. New technology made that possible a few years later with CD-R (recordable CDs) and then CD-RW (rewritable CDs).
CD-Audio and CD-ROM feature a spiral of pits on a molded plastic substrate over-coated with a reflective aluminum alloy.
To make a recordable disc, you start with a plastic substrate that has blank grooves rather than a predefined pattern of pits. The blank grooves can keep the drive on track before the data is written. In addition to a layer of metal, the media includes a thin layer of dye. Pulsing at high power, the laser in the drive can ablaze or “burn” marks in the dye. Read back with the laser at the normal, lower read power, those marks look like pits to the detectors in the drive. Because the high laser power permanently changes the dye, this format can be written only once.
For addition rewritable capability (CD-RW), a thin layer of so-called phase-change metal replaces the dye layer. That material requires two extra “dielectric,” or glassy, layers for protection. The drive employs a high-power laser to write amorphous marks in the metal layer, an intermediate-power level to write amorphous marks in the metal layer, an intermediate-power level to write crystalline marks and a low-power level to read the recorded data. To the drive, the crystalline areas appear bright and the amorphous areas appear dark. As a result, the disc can be read in the same manner as a CD-ROM. The crystalline-to-amorphous transition is reversible. CD-RW media can thus be rewritten approximately 1,000 times. In both CD-R and CD-RW, the difference between bright and dark regions is not as stark as that in CD-ROM technology. But newer drives tend to be able to read the various formats.
