CD-ROM Longevity Research
Background: The Library of Congress has large CD-Audio collections dating from the earliest days of CD manufacture. CD-ROMs are convenient media for access and temporary storage. Unfortunately, such media are machine-dependent, so access is contingent on surviving and working hardware and software. Additionally, such media is subject to deterioration, just like any other material. CD-ROMs are made of many layers, and the materials used in these layers change over time, depending on when the CD-ROMs were made and how they age. Depending on susceptibility, wear, and environment, various layers of a CD may undergo oxidation, hydrolysis or mechanical stress, leading to damage (variously described as “rot” or “pin holes” or “mirroring”, etc.). These and other phenomenon may lead to errors in signal playback, which can be assessed by measuring a disc’s “block error rate” or BLER. The greater the BLER, the greater the loss of information.
The Library of Congress has long been committed to gaining an understanding of the longevity of optical storage media. A primary objective of the CD-ROM longevity research is to assess the preservation needs of the CD-Audio collections at the Library of Congress, and to devise strategies to minimize any data losses that may result as the collections continue to age. To this end, an essential first step is to gain an understanding of failure modes and mechanisms by monitoring errors that may accumulate upon aging of CDs. New insights might also be gained through a scientific examination of any visually perceptible defects as well. It is anticipated that compositional characteristics of individual CDs can be linked to failure mechanisms in a way that would enable those charged with curation of digital media to flag more vulnerable CDs for replication or backup so as to preserve the data before any of it is lost.
Contributing Study: Shahani, C., Manns, B., Youket, M., Longevity of CD Media: Research at the Library of Congress, http://www.loc.gov/preserv/studyofCDlongevity.pdf
Project Description: Several different studies have been undertaken or are underway. One tracked the disc quality of a specific population of CD-Audio discs from the LC collection over a course of 10 years with periodic testing to check for the accumulation of errors with age. A later study used accelerated aging techniques and statistical analysis to estimate the life expectancy of a set of CD-Audio discs. Additional efforts addressed the questions of whether the presence of security labels or laser engraving caused a more rapid degradation of the disc than would be expected in their absence.
Outcomes/Findings: The results of this effort are summarized below:
- The tracking study generally showed relatively little change in disc quality (as manifested by BLER) over the course of ten years for this specific population of discs, though some discs did fail with regard to successful playback (4 %).
- Accelerated aging studies (conducted using various elevated temperatures and relative humidities) produced a wide range of life expectancies of media currently held at the Library and the results of the studies fit the mathematical expectations of the ANSI IT9.21-1996 test standard.
- A number of failures were seen in the accelerated aging study that require subsequent chemical and physical analysis, which will allow the development of a better understanding of failure mechanisms in these systems.
- There were no detrimental effects observed from laser engraving identity codes on the Library compact disc holdings.
- The type of security label tested in this study did seem to cause increased disc failures compared to those discs without the label.