1.1 Scope of This Guide
This document describes methods for the care and handling of optical discs and is intended for use by librarians and archivists in government, academia, and industry. It draws on accumulated industry knowledge and the results of specific studies by the National Institute of Standards and Technology (NIST).
The document provides guidance on how to maximize the lifetime and usefulness of optical discs, specifically CD and DVD media, by minimizing chances of information loss caused by environmental influences or physical handling. Discrete topic areas include prevention of premature degradation, prevention of information loss, CD and DVD structure, disc life expectancy, and conditions that affect optical discs. Other issues relevant to the management or maintenance of optical systems are beyond the scope of this document. Excluded, for example, are such topics as care and maintenance of the disc drive device and associated hardware and software; digital rights and related legal questions; and methods of making, sending, and receiving digital copies, including analog-digital conversion procedures.
This document is intended neither to represent nor imply a standard. It is merely a consensus of several reliable sources on the prudent care of CDs and DVDs.
1.2 Use of Terms: Information, Content, and Data
Information and content as used throughout this document refer to audio, video, photographic images, graphics, animations, interactive games, computer applications, documents, files or data, and any other digital objects. Data refers to the small pieces of information from which “understandable information” is derived. The term is used extensively in this guide to refer to the bits recorded in the disc. Such bits, or data, are interpreted as ones and zeros by the optical disc drive. Eventually, through a series of manipulations by the hardware and software of a computer or playback system, these bits become “information,” in any of its fully interpreted forms, for the user.
1.3 Comparative Stability of Optical Discs and Other Media
Among the digital media, prerecorded and write-once optical discs are more stable than digital magnetic tape. Neither optical discs nor magnetic tape, however, is as stable as microfilm or paper. With proper care, microfilm and non-acidic paper can last for centuries, while magnetic tape lasts only a few decades (Van Bogart 1995). Just as film types can vary in years of usefulness, one disc type can also last longer than another. Temperature and humidity conditions can markedly affect the useful life of a disc; extreme environmental factors can render discs useless in as little as a few days.
Media deterioration is but one aspect of the preservation challenge. A potentially more immediate threat is technological obsolescence. Technological advances will no doubt make current optical disc types obsolete within several years. If the software currently used to interpret the data on optical discs becomes unavailable, a migration or emulation technology will be needed to access the data. Also, if the current disc-drive technology becomes unavailable, and if disc drives produced in the future lack the backward compatibility to play today’s discs, the information on the discs will likewise be inaccessible. Film and paper are much more stable in this regard, as human language does not change as rapidly as computer software, hardware, or the media format. “Ink on paper,” for example, has been used for centuries, and film has not changed significantly over the years.
The importance of ensuring that information can be read by future generations cannot be overstated. It is vital to have in place a preservation strategy that guarantees the sustainability of the collection for as long as possible. The computer-user “industry standard” for data storage on removable digital media has changed considerably over the past few decades (TASI 2002). As shown in Figure 1, digital media used as recently as 20 years ago are already incompatible with most of today’s systems.
Figure 1: User removable storage-media timeline
Timeline illustrates the changes in common “removable” storage media (Technical Advisory Service for Images (TASI) 2002, extended by author)
1.4 CDs and DVDs: Operation and Variety
CD is short for compact disc. DVD initially stood for digital video disc, then digital versatile disc, but today the term DVD is often used without referring to a specific set of words. Both CDs and DVDs are optical media, meaning media that use light technology (more specifically, laser light) for data retrieval. A disc drive focuses a laser light beam into the CD or DVD to “read” the bits (data) in the disc. The drive can also “write” bits by focusing the laser beam into recordable CDs or DVDs. The laser reads and writes data starting from the center of the disc and proceeding in a spiral direction toward the outer edge. A pre-groove is stamped in all blank recordable and rewritable CDs and DVDs to guide the laser as it writes.
Optical discs are differentially identified to designate specific features such as recordability, rewritability, and accessibility. For example, CD-R, DVD-R, and DVD+R discs are dye-based recordable (write-once) discs-i.e., recordable but not erasable. CD-RW, DVD-RW, and DVD+RW discs are phase-change based, recordable, (rewritable) discs, or discs that permit the erasing of earlier information and the recording of new material in the same location on the disc. DVD-RAM discs are phase-change based, recordable (rewritable) discs formatted for random access, much like a computer hard drive. CD-ROM and DVD-ROM discs are pressed and molded, nonrecordable, read-only discs. Brief definitions of the various types of optical disks can be found in the glossary; disc structure is covered in greater detail in section 3.