By George Fleming
What causes Compact Disks (CDs) to fail, and what is NSSDC doing about it? While failures of CDs are rare, they do have an expected, if not clearly defined, lifetime. With care, most will likely outlive many readers of this article. A greater problem would likely be the longevity of the read mechanism and the software used to interpret the data. This doesn't mean that we can expect zero failures in the next 25 years, however. If there's an expected 25-year lifetime, a few will fail years before then.
As of December 2000, NSSDC's library has multiple copies of 1,489 titles on factory-stamped CD-ROMs, 11,776 unique CD-Recordable (CD-R) disks and no CD-Read/Write disks. The vast majority of CDs shipped from NSSDC in response to user requests are stamped CDs, but a small percentage are CD-Rs.
Stamped CDs are very stable; their structure is simple, and as long as the (typically) aluminized reflective layer is protected from oxidation, they should last many decades. CD-Rs have a more complex structure, and rely on various organic dyes enabling the writing process. While claims for CD-Rs of a 200+ year lifetime have been made (for instance, see http://www.kodak.com/US/en/digital/cdr/features.shtml), they are still susceptible to degradation by excessive heat and moisture. The age of the CD-R before writing is also a factor. As an aside, due to the way data are recorded, any cleaning of the surface should be done radially from the hub outwards, not along the circumference.
Errors are encountered on all CDs. Errors accumulate due to manufacturing defects, recording errors, physical scratches and fingerprints, the effects of exposure to sunlight, high temperature and humidity, and in the read process. Thus, all CDs employ at least one level of error correction coding (ECC). Audio CDs employ one level, with CD-ROMs employing additional levels of ECC, such as Cross Interleave Reed-Solomon Code (CIRC). Data are interleaved for additional protection. As non-zero block error rates are encountered on all CDs, ECC is necessary. Indeed, any degradation of a CD is hidden by the ECC employed. Errors are quite hidden from the casual user, and are usually not even reported from the CD reader.
NSSDC is presently reviewing various commercially available hardware/software modules that might be procured and attached to a current NSSDC PC to monitor CD degradation.