Optical media preservation
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The
Types of media
The two broad types of optical discs are
The reflective and data layers of CDs are just below the label and a thin sheet of polycarbonate substrate. A much thicker layer of the substrate supports and protects the bottom of the disc. The reflective and data layers of double-sided DVDs are in the center of the disc structure, housed between two equal layers of polycarbonate substrate. Because the data layer of CDs is more exposed than double-sided DVDs, a thin lacquer layer is used to protect the surface of the CD. The top of a CD is delicate and fragile; the bottom is merely a transparent protective covering.[2]
For preservation purposes: Gold CD-R (Compatible Disc-Recordable) and DVD-R (Digital Video Disc-Recordable or Digital Versatile Disc-Recordable) discs are preferred by experts over aluminium and silver for reliable long-term backup storage—the reflective layer of the optical disc is gold.
Permanent and long-term storage are distinct. “[D]igital archiving experts commonly acknowledge that no carrier is permanent. Instead, one must maintain data transferred to storage and provide access and ensure integrity of the information for the long-term.” As technology changes data can be migrated from an older to a newer type of media to avoid media failure or “format obsolescence”—a real threat for technology when it is no longer supported. If the machine required to play and read the discs is not kept in working order and maintained, data loss may result.[3] Risks involved with optical media are covered below.
Issues which affect data longevity of nominally archival-grade discs include the following: dye failure (discs with premium organic and pthalocyanine-based long-life dyes are more suitable); bonding failure (premium bonding agents and edge-to-edge coverage improve longevity); scratches, minimised by careful handling and a scratch-resistant coating; production quality (some factories have better quality control standards, and discs from a batch known to be good may be more reliable than another batch).[4] Testing is required since conditions vary from machine to machine and from disc to disc; environmental control is required to prevent damaging conditions. When these conditions are met it is believed that the life of an archival-quality CD-R or DVD-R can be as long as 100 years, compared to the typical five to ten years for non-archival quality optical discs. The ISO 9660 standard specifies a stable room temperature of 18 - 23 °C (64-73 °F) with relative humidity of 30% to 50%.[5] Keeping multiple copies of discs is necessary for added protection. “One Master, stored under optimal conditions, one Working copy to be used for access purposes or copying, and one Safety copy to be stored at a different location.”[6] According to the Institute of Conservation the container most suitable for storage is a rigid high-quality case made from inert polyester placed vertically.[7]
There is still speculation on how reliable optical media is. There are no accepted standards for blank discs and recording devices, so acceptable performance cannot be relied upon.[8] The longevity of rewritable formats—CD-RW, DVD-RAM, DVD-RW and DVD+RW—is less well known; they are also susceptible to accidental overwriting.
Optical disc types
Dyes used in DVD-Rs and CD-Rs include Phthalocyanine (greenish) Cyanine (blue) and Azo (dark blue). Silver, silver alloys and gold are used as reflective layers on recordable CDs and DVDs. Both gold and silver will outlast the organic dyes, which will decay over time. Aluminium is not used because it may cause reactions with the dyes.
Damage to optical discs
Optical discs are not subject to wear as they are read without mechanical contact with the surface, but are susceptible to scratches from handling. The risk of scratching is minimised by handling discs only by their edges and the center hole or hub.[11] Machines used to read discs can damage them if mechanically faulty.
Scratches
Scratches on the laser-reading side of an optical disc
Data recorded on discs has redundancy, so that error detection and correction can compensate for some degree of damage. The depth and width of scratches as well as the direction in which scratches on the bottom run all determine whether or not the data on the optical disc will be readable. Small scratches on the substrate generally have no effect on the readability of a disc as the laser is reading through the substrate to the data layer. If a scratch is deep or wide enough to affect laser focus, error correction is usually possible, but scratches can be too deep and wide or too close together for error correction to be successful. If a scratch runs along a track or is deep enough to scratch the data layer, data may be lost.
Scratches on the top of an optical disc
Because a CD's reflective metal layer and data layer are both directly beneath the thin lacquer surface of the label, data can be destroyed by even a small scratch on the top of a CD. The instrument used to label CDs should be seriously considered. Pens or markers with hard tips, or with solvents that can affect the protective layers, can scratch or damage the data layer; water-based felt-tip pens are safest.[2] The data layer of a double-sided DVD is in the middle of the disc, surrounded by the substrate on both sides so scratches to either side are equivalent.
Environmental considerations
Optical discs can be damaged by exposure to extreme temperatures; direct sunlight may overheat the disc or damage the data layer with
The polycarbonate substrate of discs exposed to high humidity or immersion may absorb some water, and should be allowed to dry out in a less humid environment before being used. After drying discs should be usable unless minerals left behind from the water react with the components of the disc, damaging the layers and making it unreadable, as sometimes happens.[2]
Disc quality scanning
By measuring the rate of correctable data errors, potential future data losses caused by media degradation can be predicted.
Types of correctable data errors include so-called C1 (also known as E31) and C2 errors (also known as E32) on CDs, and Pairity Inner Errors (PIE), Pairity Inner Failures (PIF) and Pairity Outer Errors (POE) on DVDs, while uncorrectable errors are CU (or E32) on CDs and Pairity Outer Failures (POF) on DVDs.[12][13]
A high rate of errors may also be caused by scratches and dirt on the surface, low quality media, recording speeds incompatible with the specific media, and data written using a malfunctioning optical drive.
Too many consecutive smaller data errors could lead to data loss. Due to the weaker error correction used on
Support for error scanning does vary among vendors and models of
Cleaning of optical discs
Optical media discs often require professional preventative or routine cleaning to ensure data accuracy and accessibility; those with no professional experience may scratch the disc surface in their attempt to clean the disc. Build up of dust and oily contaminants on the disc surface, and fingerprints can typically impede the laser beam's ability to penetrate the substrate to read the data layer, and more often impede writing. Minor interferences with reading is handled by error correction technology. If an audio CD (with a much lower accuracy threshold than a data disc) becomes dirty, it can be cleaned safely with a dry, soft lint-free cloth, holding the disc by the edges or by the center hole. Light dirt that is not removed by this method can be removed with a cloth dampened with water or a suitable optical disc-cleaning fluid. It has been advised that excess dust be blown off an optical disc before reading, to avoid buildup of dust in the reader, particularly on the laser.[15]
Storage of optical discs
Individual storage containers protect optical discs from scratches and dust. It has been recommended that discs be stored vertically, if possible in a cabinet or drawer less susceptible to changes in temperature or humidity. For long-term storage it has been advised that any paper inside the case—liner, booklet—be removed to minimise the collection and retention of moisture inside the case.[15]
Repair and reclamation of data from optical discs
If scratches on the laser-reading side of an optical disc prevent it from being read, it may be possible to recover all or most of the content once, and transfer it to another storage medium. There are software packages that analyse data on a damaged storage medium and can recover some or all otherwise inaccessible information.[2] Commercial companies offer data recovery services.
Also, the disc itself can be repaired. There are various machines that will repair discs by polishing, buffing or grinding the playable surface. Usually the disc will appear like new and most importantly the data can once again be read by the laser. Some game shops, pawnbrokers and supermarkets provide a disc repair service. However, since this is an ablative process in which a layer of the substrate is removed, it is only safe to do a few times before risking the loss of the whole disc.
See also
- M-DISC (DVD and Blu-Ray for long term digital preservation)
References
- ^ Pohlmann, K. C. (1992). The compact disc handbook. Madison, Wisconsin: A-R Editions INC.
- ^ a b c d e Bennett, H. (2003). Understanding CD-R and CD-RW. Cupertino California: Optical Storage Technology Association.
- ^ Bradley, Kevin. (2006) Memory of the World Programme: Sub-Committee on Technology: Risks Associated with the Use of Recordable CDs and DVDs as Reliable Storage Media in Archival Collections–Strategies and Alternatives. pp. 3–4. Paris: UNESCO. Accessed on October 8, 2007.
- ^ Frazier, Ronald W. (December 6, 2006) [Document on Using CD’s and DVD’s for Long Term Storage.] Online posting. The Archives & Archivists (A&A) List. Accessed on October 14, 2007.
- ^ The University of the State of New York. Guidelines for Ensuring the Long-Term Accessibility and Usability of Records Stored as Digital Images: Government Records Technical Information Series. p. 5. Albany: University of the State of New York, 1998.
- ^ Bradley, Kevin. (2006) Memory of the World Programme: Sub-Committee on Technology: Risks Associated with the Use of Recordable CDs and DVDs as Reliable Storage Media in Archival Collections–Strategies and Alternatives. p. 11. Paris: UNESCO. Accessed on October 8, 2007.
- ^ ICON Conservation Register. (2006) Caring for Digital and Electronic Media Accessed on October 12, 2007.
- ^ Bradley, Kevin. (2006) Memory of the World Programme: Sub-Committee on Technology: Risks Associated with the Use of Recordable CDs and DVDs as Reliable Storage Media in Archival Collections–Strategies and Alternatives. pp. 6–7. Paris: UNESCO. Accessed on October 8, 2007.
- ^ Byers, F. (2003). Care and handling of CDs and DVDs: A guide for librarians and archivists. Council on Library and Information Resources and National Institute of Standards and Technology.
- ^ Toshiba Corporation (2008). Toshiba Announces Discontinuation of HD DVD Businesses Accessed on July 25, 2008
- ^ Library of Congress. (2002). Cylinder, disc and tape care in a nutshell.
- ^ QPxTool (error scanning software) - Frequently asked questions
- ^ One DVD “DATA” Sector - LightByte
- ^ "QPxTool glossary". qpxtool.sourceforge.io. QPxTool. 2008-08-01. Retrieved 22 July 2020.
- ^ a b Henderson, K. L., & Henderson, W. T. (1991). Conserving and preserving materials in nonbook formats. Urbana-Champaign, Illinois: University of Illinois