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CD Recording Media - Design and Durability

Let's take a look at how CD media are made and at what that means for their durability. There are darned few hard data available on many of these matters and even fewer reliable numbers, so apply a substantial fudge factor - and give me better numbers if you can find them. Please remember in what follows that the light used to read a CD is infrared and not visible to the human eye. What you see is not what the reader sees, so do not be misled. I am also neglecting here (out of ignorance, I must admit) the whole business of the grooves pressed into all discs and containing both positioning information and code for the type of medium.

To begin with, we should examine a pressed disc. Essentially, it begins with a fairly thick layer of tough, clear plastic. The top surface of the plastic has been inscribed with pits and is covered with a very thin layer of metal. On top of that is a thin layer of that plastic, then the graphics which are silk-screened on the very top. Going back to that inscribed top of the bottom piece of plastic, it is created by pressing a glass master into the surface. Where the result is flat (parallel to the face of the disc), the reflection from the metallized layer is strong; where there is a slant to the plastic, the illumination is scattered and there is no significant return signal. Thus, the maximum brightness is high and the minimum is quite low.

An erasable disc (CD-RW) is similar in some ways to the pressed disc. The top and bottom layers are the same, but the plastic is smooth. The big difference is that instead of a metallized layer which simply reflects the light uniformly, the erasable uses an alloy which changes state when illuminated strongly. In the crystalline state, the alloy reflects light quite well. But it can also be switched to an amorphous state in which its reflectance is substantially lower. The frequency of illumination for reliable reading of an erasable is different from that for a pressed disc or for a CD-R, so special hardware is needed in a reader to allow it to get the signal back. It is also significant that the change of state is not completely reversible. After a number of cycles, the alloy becomes stubborn at a spot and insists on being either crystalline or amorphous, ignoring all the urging that the writing laser may offer. As a result, the disc will gradually develop errors over repeated erasures; eventually, they will be too numerous for error correction to deal with and the disc will be a coaster.

A write-once disc (CD-R) uses yet another arrangement. The layers are similar: thick plastic, smooth surface, metallized reflecting layer, thin plastic over the top. The essential difference is that the thick plastic layer has a dye in it which changes color when illuminated by the writing laser. Before illumination, it may be relatively transparent to the reading frequency; after illumination, it will darken. The dye is deliberately unstable (otherwise, it would not change transparency) in two different senses. One is that it is able to be written - to respond to the writing laser by changing color - only for about five years; the other is that over time and depending on handling, the dye can fade. When working right, the dye absorbs some of the light when it would ideally be clear and allows some through when we want it to be opaque, so it does not offer the contrast of a pressed disc.



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