Stability comparison of recordable optical discs—a study of error rates in harsh conditions

Journal of Research of the National Institute of Standards and Technology, Sept-Oct, 2004 by Oliver Slattery, Richang Lu, Jian Zheng, Fred Byers, Xiao Tang

The reliability and longevity of any storage medium is a key issue for archivists and preservationists as well as for the creators of important information. This is particularly true in the case of digital media such as DVD and CD where a sufficient number of errors may render the disc unreadable. This paper describes an initial stability study of commercially available recordable DVD and CD media using accelerated aging tests under conditions of increased temperature and humidity. The effect of prolonged exposure to direct light is also investigated and shown to have an effect on the error rates of the media. Initial results show that high quality optical media have very stable characteristics and may be suitable for long-term storage applications. However, results also indicate that significant differences exist in the stability of recordable optical media from different manufacturers.

Key words: archiving; CD-R; digital preservation; DVD-R; error rates; life expectancy.

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1. Introduction

Recordable optical disc media contains an organic dye layer whose transparency can be altered either to absorb a laser beam or to allow the beam to pass through to a reflective layer behind the dye [1,2]. The nature of this organic dye is such that when the internal energies of its molecules reach a particular threshold, an irreversible chemical reaction occurs, and the dye layer loses its transparency. This property allows a high-energy beam to "write" data by burning "pits," in the form of dark marks, to the disc during recording. A low powered laser reads the data by either passing through the transparent dye layer (without causing any molecular change) to the reflective layer or by being absorbed by the nontransparent marks in the dye.

Due to the organic nature of the dye, degradation and breakdown of the transparent portion of dye layer will occur over a long period of time as a natural process. This process, which has its roots in chemical kinetics, can take several years in normal environment conditions [3]. Higher temperatures and humidity will accelerate this process by increasing the thermal and kinetic energies of the dye molecules.

It is well known that temperature and humidity are among the most important factors affecting the life expectancy of optical discs. Yet, there is another important factor that has not been so well investigated. Light exposure can increase the rate of dye degradation precisely because the organic dye used in recordable media is light sensitive. This study also addresses this issue.

The effect of these processes can be modeled using various techniques including the Eyring model [4], which is derived from the study of chemical kinetics. The Eyring equation can model the effect of two stresses, such as temperature and relative humidity, on the rate of a reaction or degradation, which can be related to the time-to-failure of the optical disc.

The end of life of a disc can be defined as the time when an uncorrectable error occurs. Although the disc may still be readable after this point, some information has been lost. Consequently the life expectancy of a disc is the period of time in which the information recorded on the disc can be retrieved without loss. In an ideal case, the real time taken for actual failure to occur would be measured and used as the time to failure. However, this measurement is impractical to explore the degradation process, since a single end point cannot describe the complex process that led to failure. Instead we use the maximum value of some error rate monitor, whose gradual change can serve as an indicator of the media stability. In this study, the block error rate (BLER) [5] is used to monitor CDs and the parity inner (PI) [6] error rate, as summed over eight consecutive error correction blocks (PIE Sum8) [6], is used to monitor DVDs. A high BLER rate indicates a potential onset of uncorrectable errors (E32) [5] in CDs, and likewise a high PI error rate indicates a potential onset of uncorrectable errors (PO) [6] in DVDs. In both cases, these error rate monitors are used to characterize the extent of media deterioration.

2. Experimental Equipment and Procedural Overview

All testing occurred at the National Institute of Standards and Technology (NIST) as part of the digital data preservation program ongoing in the Information Access Division (IAD). Two types of environmental chambers were used for artificially aging the media. Both chambers were designed to allow aging of the media under a controlled environmental condition.

Temperature and humidity: A Blue M (model: FRM-256B) (1) environmental chamber was used to control the temperature and relative humidity through various settings of temperature (-18 [degrees]C to -93 [degrees]C) and relative humidity (5% to 98%). The specified control accuracy is [ or -]0.5 [degrees]C for temperature and [ or -]1% for relative humidity (RH) respectively. The test stresses of aging used are given in Table 1.

A complete incubation cycle for temperature and RH accelerated testing is shown in Fig. 1. Once at the stress condition, the temperature and RH were held constant for a period of approximately 45 h followed by a gradual return to ambient conditions. Discs were analyzed after each incubation cycle. This cycle was repeated under the same stress condition until the error rate of most discs in the group increased to exceed an upper limit of the error rates (as indicated in the DVD and CD specifications) or until the disc became unreadable.