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Background: The Library of Congress has a large collection of audio content on compact discs, dating from the earliest days of CD manufacture. The CD-DA (Digital Audio) and CD-ROM (Read-only Memory) formats are structurally identical. The data is molded into the disc at the time of manufacture and cannot be changed. Unfortunately, these media are machine-dependent, and continued access to the digital content is contingent on the availability of compatible hardware and software. Additionally, these media are subject to deterioration just like any other material.
CD-ROMs are composed of layers: a polycarbonate substrate; a metal reflective layer; and a protective top coating, which is often printed with the contents of the disc. The chemical composition of these layers varies depending on when and where the disc was manufactured. Aging, storage environment, and handling can cause changes in the materials leading to deterioration and loss of data, sometimes referred to as "CD-Rot" or "Laser-Rot". Oxidation, hydrolysis, and damage caused by mechanical stress can result in errors in signal playback, which can be assessed by measuring a disc’s “block error rate” or BLER. When BLER exceeds 220/s, the maximum limit specified in the ISO 10149 standard for CD-ROM, the disc can begin to develop uncorrectable errors, indicating that information may have been lost, or the disc may become unplayable altogether.
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 the loss of data 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. Identifying compositional characteristics of CD media that can be linked to failure mechanisms would enable curators of digital media to flag more vulnerable CDs for replication or backup 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.[PDF: 14 p. 357 KB]
Shahani, C., Youket, M., Weberg, N., Compact Disc Service Life: An Investigation of the Estimated Service Life of Prerecorded Compact Discs. [PDF: 32 p. 430 KB]
Shahani, C., Youket, M., Weberg, N., The Effects of Laser Engraving on the Estimated Service Life of Prerecorded Compacts Discs (CD-Rom). [PDF: 19 p. 391 KB]
Project Description: Several different studies have been undertaken or are underway.
In 1996 a selection of 125 CD-Audio discs from the LC collection were tested for errors at baseline and tracked over the course of 15 years by periodic testing to observe the effects of handling and storage on disc error rates. In 2005 an additional 1200 discs were selected from the collection to more fully represent the distribution of dates of manufacture of CDs held at LC. The discs were tested for BLER and returned to the controlled conditions of the storage vaults at the Library's Packard Campus in the National Audio-Visual Conservation Center in Culpepper, VA. Periodic testing of these discs is expected to show how controlling the environmental conditions and minimizing fluctuations in temperature and relative humidity affect error rates.
As a complement to these natural aging studies, the Library conducted experiments utilizing accelerated aging techniques to compare relative stability and estimate the longevity of CDs. One study used a single elevated stress condition to examine the effect of adhesive security labels on disc degradation. Another study employed thermal cycling to examine the effects of rapid shifts in temperature on disc stability. A study using five different stress conditions was conducted in an effort to determine a typical life expectancy for CD-ROM media. This study followed the test method described in ISO 18921. By incorporating statistical analysis into the accelerated aging protocol, this method uses extrapolation of the end-of-life (EOL) data obtained at accelerated conditions to estimate a real-time service life for the disc. As an adjunct to this study, a set of duplicate discs was included to evaluate the effect of a proposed strategy to enhance the security of collection CDs by the application of a laser-engraved property mark in the inner hub.
Outcomes/Findings: The results of these efforts are summarized below:
- Data from the first fifteen years of the natural aging study confirmed that read-errors, measured as BLER, increase over time when discs are stored at standard room conditions. The majority of the discs appear to be quite stable, showing relatively little change in overall error rates. A small number have error rates that exceed the limit specified in the ISO standard, representing a 4 % overall failure rate for this specific population of discs. Analysis of the data for individual discs showed that BLER increases at different rates. Some discs showed sharp increases in BLER due to physical damage from handling. Poor quality discs, those with high BLER at the start of the study, continued to accumulate errors faster than the rest.
- The results of the accelerated aging study using the ISO 18921 test method showed a wide range of life expectancies for CD-ROM media. Statistical analysis of the EOL data obtained at accelerated conditions showed that ~ 70% of discs in the test population had an estimated longevity exceeding 100 years. The data indicated that ~ 4% of the discs would reach EOL within 10 years. This failure rate is in agreement with observations from the natural aging study. Data from this study was used to plot the effects of changes in temperature and relative humidity (RH) on disc life. The plotted data indicates that as temperature and RH are reduced, mean life expectancy increases. Based on the data from this study, storing discs at 10°C and 35 % relative humidity, conditions that are equivalent to the 50°F / 35% RH standard storage vaults at the Library's Packard Campus, could produce an 11-fold increase in media life than would be expected at 25°C and 50% RH.
- A variety of physical manifestations of degradation were observed in the discs in both the natural and accelerated aging studies. Preliminary chemical and physical analysis indicates several different modes of degradation at work. Subsequent analyses will lead to a better understanding of failure mechanisms in these systems.
- Accelerated aging studies showed that the application of adhesive security devices and rapid shifts in temperature and humidity reduce disc life, but laser engraving identity codes inside the hub have no detrimental effect on the Library's compact disc holdings.
Updates and Images:
Image of microscopic analysis of defects in a naturally aged CD, using custom
CD-scanning stage with raking polarized light, Zeiss Stemi SV-11 microscope,
and Image Pro digital imaging and analysis software
May 2009: Presentation, American Institute for Conservation, Electronic Media Group: Characterizing Optical Disc Longevity at the Library of Congress, published in AIC post-prints, 2010.
April 2010: Presentation, Library of Congress, Topics in Preservation Science 50th Symposium: Update on LC's Electronic Media Research.
April 2010: Presentation, Andrew W. Mellon Symposium, Technical Conservation Issues of Time-Based Media: CD-R, CD-ROM and Other Time Based Media Research at the Library of Congress.
November 2010: Presentation, Eastern Analytical Symposium, Digital Media in Cultural Heritage: New Preservation Technologies Session: Research Studies on the Longevity of Digital Optical Media at The Library of Congress.
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