Longevity

One of the key questions about CD recording is: how long will it last - where 'it' is a write-once or erasable blank either before or after recording. There is a little information about write-once blanks which I am trying to summarize below and a lot less about erasables.

A write-once blank uses a dye which is not entirely stable over time either before or after it is written. Using accelerated life tests, manufacturers estimate that a type of disc will remain writable for between five and ten years after manufacture. After that, the probability of write errors will increase - I suspect because the power requirement will become non-uniform and regions will not record properly. After being written, the areas which have been written should hold well, but the unwritten areas will effectively be hardened and the disc will develop excessive errors if you write another session years later.

Some comments are critical. One is that there are assumptions about handling of the discs which can be significant. If you expose any blank to intense sunlight for a period of time - whether written or not - you will find that it changes visibly and functionally. That does not mean that you need to keep the discs in the dark, but that exposure to heat or light will accelerate the breakdown. With normal handling, a written disc will probably outlast you and certainly your interest. A related point is that poor labelling - a bad adhesive, leeching ink or solvent - can destroy a disc over time. Finally, accelerated life tests are performed by overloading the medium with the forces which are believed to cause the decay. Needless to say, none of the media we use today has had a century or even a decade of storage to validate the tests that are reported.

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The information above is from Kodak Ultima discs and is of interest because it indicates the way in which manufacturers support their claims. Accelerated life test is necessary because decades are not available for testing. The problem is, no one knows how closely the results match real conditions. The fact that Kodak's discs would last longer in a jar with water on the surface of Venus doesn't say much about how they'll last where humans can survive - which does include summer in Texas, despite the impression of Houstonians.

Erasables work quite differently and are subject to different forms of decay. I have been able to find neither claims for life expectancy nor accelerated life tests on them. It is clear that the story is more complex than for write-once if only because we know that erasables lose reliability when put through erase cycles. My guess is that the distinction in bit-sized regions between crystalline and amorphous becomes lost, that the regions become more or less locked in one state or the other when written repeatedly. If that's true, then the same effect will presumably limit the life when the disc is exposed to heat, light or cosmic rays. Does that mean that erasables will last longer than write-once or not? Yes, it does. It means that their expected life is longer or shorter, but I cannot determine which. So far, experience suggests that high-speed erasables have much shorter lives than write-once.

A final comment seems appropriate. The failure modes known and hypothesized here do not lead to the disc suddenly 'breaking' in any sense. Reading a CD entails correcting errors. As the disc degrades, the rate of errors increases. An unwritten blank may become gradually less consistent in reacting to the write process so that regions are not written uniformly. Regardless, you are not likely to find that a spindle becomes suddenly useless the day after its tenth birthday or that an erasable which has worked beautifully for 999 erase cycles turns transparent on the 1000th. Instead, there will be a gradual increase in the risk of a faulty data read or a gradual degradation in audio quality.

Let's put it all together to see what we can do for long life of write-once media:
1. Use a line of discs with a good reputation to minimize the risk of systematic failure
2. Treat the discs well, especially with regard to labelling.
3. Take manufacturers' data for what they are: well-intentioned, self-serving guidance.

E-mail me at cdrecording@mrichter.com
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