CD-R / CD-RW
One drawback of a standard CD-ROM drive is that it is unable to write data to the disk. It is this one sticking point that has kept the CD-ROM from all but replacing floppy disk drives as a standard for removable media a PC. In response to this, the Philips Corporation defined a standard for a CD Recorder, known as a CD-R, and a CD Re-Writeable known as a CD-RW. These standards were published in Part II and Part III of Phillips so-called "orange book" in 1990.
One of the biggest differences between a standard CD-ROM and a CD-R is that of the media used to record data.
In a standard CD-ROM disk, a glass master CD is made by the manufacturer with the pits and lands etched into it. This master is then used to stamp copies into blank plastic CDs.
A thin layer of aluminum is placed over the stamped side of the disk. This is what makes the bottom of the CD reflective. Then another thin layer of plastic is placed over the aluminum layer. Finally, any markings or labels are stamped on top of this final layer of plastic.
The CD-R is constructed in a similar way, but instead of stamping predefined pits and lands, a single blank track is stamped into a special layer of polycarbonate plastic.
The grooved side of the plastic is then coated with a special dye that has a green or blue tint. On top of that, a thin layer of gold is used for reflecting the laser beam.
There is then a layer of lacquer and a final layer of polymer plastic to protect it. A special dye layer is designed to absorb light at a specific frequency.
This absorption of light energy creates a mark, usually by distorting the polycarbonate underneath it, which is read by any read head in any CD-ROM as a 1. This technology is also known as Write Once Read Many, or WORM.
The CD-RW disk has a slight variation on the WORM technology. Instead of making the pits in the disk permanent, they can be returned to their original state by the write head laser.
Additionally, instead of using a layer of gold that is coated with special dye, a layer of metal alloy (usually silver, indium, antimony, and tellurium) is embedded into the layer of plastic with the pre-stamped groove.
In its original state, this layer has a rigid, polycrystalline structure. When the writing laser is focused on this layer, the heat melts the crystals into a non-crystalline state.
These areas reflect less light than areas that were not struck by the writing laser, differentiating between a 1 bit and a 0 bit. To erase the data, the writing laser heats up any pits on the disk, causing them to return to their original crystalline state.
The process of the erasing and re-writing data to CD-RW disk, known as annealing, is not without its shortcomings. In its infancy CD-RW media had a tendency to have a memory, that is areas that were originally written as pits, would not always return to a perfectly flat crystalline state.
When reading the disk the data would sometimes be misinterpreted as a 1 instead of the 0. The technology has improved with time, but it is still not perfected.
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