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Presented by Ross Harvey, Division of Information Studies, Nanyang Technological University (Singapore) at the 2nd National Preservation Office Conference: Multimedia Preservation - Capturing the Rainbow, in Brisbane, 28-30 November 1995.
  • Abstract
  • Introduction
  • Rothenberg et al.
  • Magnetic Tapes
  • Optical Disks
  • Other Media
  • Training Needs
  • Conclusion
  • References

Abstract

A critical issue for the preservation of interactive multimedia is how to ensure that the digital data of which it consists maintains its integrity and remains usable. There are two possible approaches to take when considering the preservation of digital data: to preserve the artefact on which it is stored, or to direct efforts towards migrating the data (or digital 'object') to new systems as they are introduced. This paper describes the first approach and examines its implications for organisations which are committed to maintaining digital data for any length of time.

The digital storage media examined are magnetic tapes and optical disks (including magneto-optical disks). For each medium the claims of manufacturers about their longevity, results of accelerated aging tests, and observations from field sites are presented. Recent research, including that presented by Jeff Rothenberg and by the National Media Laboratory, St. Paul, Minnesota, is noted.

The paper concludes that there are at present too many unknowns to commit digital data to currently-available artefacts for anything other than short-term storage. The preferred option is to direct preservation efforts towards solutions which preserve the information content - the digital 'object' - rather than the digital 'artefact'.


Introduction

A critical issue for the preservation of interactive multimedia is how to ensure that the digital data of which it consists maintains its integrity and remains usable. There are two possible approaches to take when considering the preservation of digital data:
  • to preserve the artefact on which it is stored; or
  • to direct efforts towards migrating the data (or digital 'object') to new systems as they are introduced.
My task in this paper is to describe the first approach and examine its implications for organisations which are committed to maintaining digital data for any length of time. This task has been made significantly easier by the publication in June 1995 of John Van Bogart's report Magnetic Tape Storage and Handling: A Guide for Libraries and Archives [1]. It would be churlish of me to lament the fact that an equivalent report for optical disks does not exist, but one can at least hope that this will soon appear.

The Issues

First, some working definitions:

  • multimedia are physical formats in which information in more than one medium is stored: because multimedia in use today are recorded digitally, I am concerned here with digital data.
  • archival appears to have many meanings. Archivists and librarians mean life spans of several hundred years. Manufacturers of compact discs talk in terms of decades. Computing people may talk of up to two years. Note a recent statement: 'Archivability' is defined as 'How long the media will last on the shelf and still be playable . . . [it] is normally thought of as being a medium problem, but it also becomes a machine problem after the machines cease to be manufactured [2]. This last point - machine obsolescence (and software obsolescence too) - are other significant parts of the equation.
Others more eloquent than I have captured the essential issues in words. Don Waters, Yale University Library, in his position paper Some Considerations on the Archiving of Digital Information notes:
Preserving the media on which information is electronically recorded is now well understood to be a relatively short-term and partial solution to the general problem of preserving digital information. Even if the media could be physically well-preserved, rapid changes in the means of recording, in the formats for storage, and in the software for use threaten to render the life of information in the digital age as, to borrow a phrase from another arena of discourse on civil society, 'nasty, brutish and short.[3]
Archivists have been forced to develop strategies to preserve electronic records because they have had in their care for many years significant quantities of such records. In 1988 this thinking was put into print by the American David Bearman:
We must begin by accepting the information life of specific recording formats as a fact of physics. While we can influence the production of new media and formats and encourage current information recorders to use formats with longer lives, the 'format life' of any given format is the outside boundary beyond which we cannot rationally plan to retain the information without transforming the medium. [4]
The Commission on Preservation and Access's Annual Report July 1, 1991-June 30, 1992 summarises the issues for libraries. Its President, Patricia Battin, argues that 'we must remove the burden of archival copy from the . . . artefact' and refocus on 'the concept of managing continuing access to information stored on a variety of media and requiring a variety of ever-changing access hardware and software.' To this end she believes that 'preservation policies must now focus, not on the permanency of the medium, but on the management of permanency in the digital environment.' [5]

So, in relation to electronic records, we are rapidly moving away from the conventional preservation approach - attempting to preserve the artefact. With the rapid rise of electronic records, it is now obvious that the notion of 'saving object X for Y years' will become obsolete, or perhaps applied only to specific categories of information-carrying artefacts such as the book. The primary questions become those of what is worth keeping, and for how long; only when we have answered these can we look at the question of what medium to convert to. The issues here are fragility of each medium, rapid rate of obsolescence of the operating apparatus (software, operating system, etc.), the ease of altering the data, ownership of information (copyright, etc.), and who takes responsibility for its preservation.

As earlier indicated, my role in this paper is to concentrate on the matter of what medium to convert to. To address this question requires knowing about the physical and chemical makeup of the media and about optimum conditions for their storage and handling. Information about these is available: but it is not readily accessible to the consumers (by which I mean librarians, archivists and indeed anyone without the scientific and technical education to understand this information). This point is made by the writers of another report initiated by the Commission on Preservation and Access, this one entitled Research on Magnetic Media-Phase 1:

While there is ongoing research and data available on durable or 'robust' magnetic media, archivists and librarians do not have ready access to this information and are generally unable to interpret the technical data resultant from the research. We know or understand little about the nature of the media, how they react to ambient conditions in storage and during use, and how these properties relate to the long-term preservation and use of information recorded on magnetic media. . . . In order to make decisions regarding migration of recorded information . . . archivists and librarians need to be able to predict the life expectancy of magnetic media when stored under a diverse and variable set of environmental conditions'. [6]
In short, the key points are:
  • we need to decide how long we want to keep digital information
  • there is a choice between keeping the physical objects themselves (the 'digital artefacts') in usable condition, or keeping the data contained in the physical object (the 'digital object') in a state in which it can be used.[7]
To these must be added a third point:
  • if we choose to preserve the digital artefact, then we must be aware that it is a short-term expedient.

Rothenberg et al.

Jeff Rothenberg's article in the January 1995 Scientific American has succeeded in focusing popular attention on this question of the short times we can expect our digital artefacts to last. His conservative estimates ('expected lifetimes are estimated conservatively to guarantee that none of the data are lost', he notes) are [8]:
Magnetic tape       1 year           equipment obsolescence 5 years                           
Videotape           1-2 years        equipment obsolescence 5 years                           
Magnetic disk       5-10 years       equipment obsolescence 5 years                           
Optical disk        30 years         equipment obsolescence 10 years.                      
Rothenberg's estimates caused a flurry of refutation, including a letter from staff of the National Media Laboratory to Scientific American which indicated that 'the physical lifetimes for digital magnetic tape are at least 10 to 20 years.' [9]

Current advertising in the press presents another story. We cannot blame the readers of popular computing magazines or the computer supplements of daily newspapers if they appear to be confused. A completely unscientific sample of current advertising provides the following:

  • Reports in The Straits Times (Singapore) variously state that the life-span of CD-ROM is approximately 100 years (1 March 1995), almost indefinite (7 March 1995), and for CD-R (Compact Disc-Recordable) a 'long shelf life . . . 10 years' (8 February 1995)
  • Digitised images stored on CD-ROM will be 'yours to keep without deterioration in quality. Hey! It will still be in mint condition for posterity's sake when your descendants see it!' (HomePC (Singapore, October 1995): p.71)
  • By comparison, an advertisement by Pinnacle in PC Magazine (21 November 1995, p.43) claims a shelf life for CD-R of one hundred years:
[Figure 1]
(Source: PC Magazine 21 November 1995, p.43)
Let me again note (although it is not within the province of this paper to dwell on this aspect) that even where the life expectancy of the digital artefact can be estimated to be in decades, the equipment's life-cycle is only at the most ten years. Even given the possibility of developing software emulators address this issue, there is still a major problem.

This paper examines primarily the two main digital storage media, magnetic tape and optical disks.

Magnetic Tapes

The only magnetic medium I will note here is magnetic tape, as it is the primary magnetic medium in serious use for storing non-current digital data. This section is a summary of Van Bogart's already mentioned report Magnetic Tape Storage and Handling: A Guide for Libraries and Archives. While his report refers to video- and audiotapes, the author has advised me that it can be extrapolated to data tape, whose structure and composition is almost identical. There are, however, a few differences. [10]

Physical Structure

Some basic knowledge about the structure of magnetic tape allows a better understanding of its preservation problems. The information is stored in the alignment of magnetic particles which are suspended within a polymer binder. This binder adheres the magnetic information carrying layer to a base or substrate; it also provides a smooth surface to ensure that the tape passes smoothly through the tape heads. Other substances are added, for example a lubricant to reduce friction, and a head cleaning agent.

[Figure 2]
Cross Section Of Magnetic Tape
(Source: Van Bogart (1995) Figure 2)
Effect of Physical Structure on Longevity

The binder is the most significant factor which determines the longevity of magnetic tapes. It may soften or become embrittled through hydrolysis, a chemical reaction which requires water to be present for it to occur. The polyester linkages in the binder break - the more moisture in the air, the more likely hydrolysis is to occur. Hydrolysis leads to the 'sticky tape' phenomenon where the binder can stick to the recorder heads and lead to clogging of the heads, dropout and other problems. Another problem is lubricant loss, which occurs with the passage of time, even if the tape is unplayed. Less lubricant means increased friction and the 'sticky tape' phenomenon can occur.

The magnetic particles store data in the form of changes in the direction of the magnetism in the particles. These particles (or pigments) vary in their ability to remain magnetically stable (which directly affects the quality of data recorded) according to what they consist of. Although the most stable are iron oxide and cobalt-modified iron oxide, these are not used for high quality tapes where metal particulate (MP) and chromium dioxide (CrO2) pigments are used because of their superior characteristics for recording higher frequencies and allowing higher signal outputs. Van Bogart notes 'there is not much that can be done to prevent the magnetic deterioration that is inherent in the metal particulate and chromium dioxide pigment types' [11] but indicates that storing the tapes in lower temperatures slows the rate of deterioration.

The substrate is most commonly made of polyester film (Mylar, polyethylene terephthalate, or PET). [12] Polyester film is well known to be chemically stable and will easily outlast the binder; rather, its problems arise from mechanical problems such as stresses on the tape caused by fluctuations in temperature and humidity levels in storage areas. These can result in mistracking during playback. Deformation of the substrate can also arise if the tape is not appropriately stressed when it is wound or rewound.

Other factors which affect data loss include whether the recording is helical scan (for example videotapes) or longitudinal scan (for example analog audio tapes) and, of course, the quality and maintenance of the tape recording device itself. [13]

Improving Longevity

It follows from the above description of physical structure of magnetic tape that ways to improve their longevity are based around:

  • care and handling: quality of storage conditions, care in handling, number of times the tape is accessed
  • quality of the tape
  • future availability of the technology to play back the tape.
Only over the first of these can we exercise any real control as custodians of digital data.

I refer the readers to Van Bogart's report [14] for detailed descriptions of the care and handling required, and continue here with storage conditions. Because binder hydrolysis is the key factor in tape deterioration, and as this depends on the moisture content of the tape, lowering humidity levels means reduced rates of hydrolysis and lower temperatures slow down the rate of hydrolysis. Similarly, the magnetic pigments degrade more slowly at lower temperatures. Reducing the variation of fluctuation of temperature and humidity levels also assist. Storage at high temperatures (indicated by Van Bogart to be higher than 23oC) increases the tightness with which the tape is packed, thereby increasing the distortion of the tape backing and resulting in an increase in permanent dropouts. Storage at relative humidities higher than 70% can also result in increased tape pack stresses as the tape absorbs moisture and expands. Fungal growth may also occur at high temperatures and humidities.

[Figure 3]
Temperature and Humidity Conditions and Risk of Hydrolysis
(Source: Van Bogart (1995) Figure 6)
Attention also needs to be paid to minimising variations in temperature and relative humidity in the facility, to maintaining air quality at a high level, and to reducing dust and debris, and again I refer the reader to Van Bogart's report. Conditioning (acclimatisation) is also required if the tape is stored in a different environment from that in which it is used. Van Bogart summarises current conditions being proposed in drafts of storage recommendations by various standards organisations. [15] Here is my summary of his summary:
KEY FEATURE        ACCESS STORAGE (storage  ARCHIVAL STORAGE      
                   for media that allows    (storage that         
                   immediate access and     preserves the media   
                   playback)                for as long as        
                                            possible)             

Acclimatisation    No                       Yes                   
required before                                                   
playback?                                                         

Media life         At least 10 years        The maximum possible  
expectancy                                  for the media type    

Temperature        Room ambient (15-23oC)   As low as 5oC         
                   Maximum variation 4oC    Maximum variation     
                                            4oC                   

Humidity           Room ambient (25-75%     As low as 20% RH      
                   RH)                      Maximum variation     
                   Maximum variation 20%    10% RH                
                   RH                                             
Life Expectancies

I still have not addressed the question of 'how long'? Taking the key factor of binder hydrolysis (and he is at pains to point out that there are other reasons why tapes can fail, and that his estimate is capable of qualification in many areas) Van Bogart provides this illustration:

[Figure 4]
Life Expectancies for a Hi Grade VHS Tape
Estimated by the degree of binder hydrolysis using an end-of-life criteria of 12%
(Source: Van Bogart (1995) Figure 10)

For the kinds of ambient room temperatures in Melbourne, say - 25oC and 50% RH, and assuming an unlikely low level of fluctuation - this table suggests an estimated life expectancy of about 10 years. Living in Singapore - 30oC and 80% RH - I must reconcile myself to something more like 1-2 years.

Optical Disks

Unfortunately I know of no source for optical disks as current and concise as Van Bogart's is for magnetic tapes. [16] An almost bewildering variety of types can be included under the name 'optical disk', but I am concerned here with 'optical storage products which use light - specifically, the light from lasers - to record and retrieve [information from] … light-scattering holes, bumps, or bubbles.' [17] These fall into two general categories: read/write and read-only. Read/write is further divided into two categories: write-once and rewritable. The first optical disks to appear were WORM (Write Once Read Many) disks at the end of the nineteen-seventies, so we have almost twenty years of practical and anecdotal experience to draw on. One clear implication to be drawn from this evidence is the importance of standards, with numerous stories about institutions committing themselves to one format only to find that it is no longer manufactured after a year or two - but this is not the main concern of this paper.

Physical Structure

All optical disks use basically the same structure, the main difference being the way in which the data is recorded. [18] My examples will concentrate on CD-ROM, but the same physical structure and consequent problems apply more generally.

Videodisks are made by a laser which burns minute holes into a glass master, which is then used to make a metal master from which plastic discs are stamped. An acrylic protective coating is applied. Compact discs are similarly produced by a laser which burns pits into a coating on a glass master from which a metal master is produced. This stamps a plastic base or substrate which is next coated with a thin layer of metal, usually aluminium, and is then covered with a protective polymer (or sometimes lacquer) layer.

[Figure 5]
(Source: Straits Times)
Effect of Physical Structure on Longevity

The metal reflecting layer is considered to be the most susceptible of the factors, largely because the aluminium usually used is more vulnerable to oxidisation than other metals or alloys. Oxidisation leads over time to corrosion which obscures the distinction between pit and surface (that is, between 0 and 1, the way in which digital data is stored), and the data becomes unreadable. Some manufacturers have used other metals or alloys (platinum or gold, for instance) but the manufacturing costs are consequently significantly higher. The polymer base, whose primary function is to support the metal substrate, can itself be permeable to oxygen, thus affording incomplete protection against oxidisation. It can also contain rough spots or other defects which promote localised corrosion. Although optical disk manufacturers use alloys which are more resistant to oxidisation, this can only retard, not halt, deterioration. The protective polymer coating can also fail and again allow oxidation to occur. [19] The bonding materials need further investigation: how are the layers bonded? What is known about the stability of the process? And the ink used for printing onto the disc has been noted as causing oxidation of the metal layer because it caused breakdown of the polymer coating.

Improving Longevity

As for magnetic tapes, ways to improve the longevity of optical disks are based around:

  • care and handling: quality of storage conditions, care in handling
  • quality of the disk
  • future availability of the technology to play back the disk.
And again as with magnetic tapes, we can only exercise any real control over the first of these.

It is generally assumed that optical disks are less vulnerable to damage caused by poor handling than are magnetic tapes, although we are probably all familiar with the temporary unreadability resulting from fingerprints on a CD-ROM. All information storage media need careful and respectful handling, and optical disks are no exception.

A 3M employee makes the comment that the protective coating on a CD is very thin indeed and 'where there's a fault, normally it is that the seal coat doesn't cover everything and something gets in'. [20] It follows then that anything which can minimise the possibility of a fault occurring is worth pursuing. As one example, it is inadvisable to apply adhesive labels to CD-ROMs. Storage at extremes of temperature and humidity can also, clearly, affect the physical structures: for example, as plastic substrates can absorb moisture and oxidisation of the metal layer can occur as a result, then high humidity conditions should be avoided. For the same reasons storage areas in which temperature and humidity fluctuates, resulting in condensation, should be avoided.

Life Expectancies

Peter Adelstein noted in 1993 that while some excellent studies had been carried out on optical disk longevity, there were at that date no national or international specifications. [21] This still appears to be the case today. At one end of the spectrum is an estimate of three to five years for CD-ROMs reported as the view of NARA (National Archives and Records Administration) in 1992. The main problem 'according to Ken Thibodeau of NARA, is that the aluminum substrate on which the data is recorded is vulnerable to oxidation . . . The plastic that protects the substrate is oxygen-permeable, so it provides no protections against the oxidation process.' [22] In late 1994 NARA was still not considering CD-ROMs as an acceptable archival medium:

NARA views CD-ROM as an acceptable transfer medium for permanent records, but has not yet sanctioned it as an archival medium. This means that federal government records that have long-term or permanent value . . . may be transferred to NARA on CD-ROM media but will not be stored permanently on CD-ROM. Once NARA receives such records on CD-ROM, NARA will copy them onto 3480 class magnetic tape cartridges - the only currently acceptable electronic archival medium for permanent storage. [23]
Of more general applicability is a report of accelerated aging studies of CDs carried out at 3M, with input from the National Media Laboratory, in 1992. These resulted in 'a 25-year warranty that assures 100 year life-time at room temperature': that is, the lower estimate takes account of 'general storage fluctuation, as long as it's non-condensing', [24] and 100 years is more like the lifetime to be expected from high quality storage conditions.

Studies are still continuing on the life expectancies of optical disks and will certainly need to continue, especially as new kinds become available and enter into common use. An announcement was made recently that the National Media Laboratory is setting up stability studies for CD-R. [25] As yet no useful results are being reported in the non-scientific, library or archival studies literature.

Saffady [26] summarises manufacturers' lifetime estimates for read/write optical disks:

RECORDING TECHNOLOGY         LIFETIME  ESTIMATE     
Ablative technology          10-40 years            
Thermal bubble               10-50 years            
Dual alloy                   100 years              
Dye-based                    15 years               
Magneto-optical              10-30 years            
WORM phase change            15 years               
Rewritable phase change      10 years               
Although these estimates are subject to change as more accurate tests are devised and applied, their implication is clear: the lifetime of optical disks of all kinds, and especially CD-Rs, is greater than the technological obsolescence factor of their recording and playback technology.

Other Media

Many other media exist for the storage of digital data and new media are being developed and promoted on a regular basis. (My favourite is the promising write-once medium called Digital Paper whose 'effectiveness continues to be hampered by its perplexing nonexistence.' [27]) I have conveniently ignored these newly-developing removable storage media - magneto-optical, Zip drives, Sysquest, for example. Those which establish themselves commercially will clearly need to be tested to determine their life expectancies.

Training Needs

What are the implications of the above for training needs in multimedia preservation?

The primary requirement is that of ensuring that the trainers (using this term to include educators at all levels, from on-the-ground library staff to teachers at tertiary institutions) have ready access to the kind of interpretation of the scientific and technical data which Van Bogart has supplied in his report on magnetic tape. Although trainers have an obligation to ensure that their knowledge and skills base are kept up to date, their task is not always easy, especially if the required data is effectively lost to them because they lack the scientific background to interpret it. While all trainers in the preservation area should have a modicum of scientific knowledge, they often do not: humanities and social science backgrounds prevail amongst librarians and archivists. The Commission on Preservation and Access provides the kind of interpretation which is required, but perhaps there is also a more local role to be played? One envisages the National Preservation Office taking responsibility for a series of papers aimed at trainers, regularly updated and widely disseminated, which summarise current findings and recommendations.

On a more popular level, the misconceptions that we saw in the 'Tape is out. Optical is in' advertisement need to be countered more effectively than is currently the case. If the person in the street is convinced that their Kodak photo CD-ROM will last for ever (whereas their colour film, they know from direct experience, will not) because advertising has consistently and loudly told them that CDs are forever, so, then the more informed but considerably quieter voice of the librarian or archivist will not readily change their mind. This suggests that a different kind of publicity campaign is needed. Perhaps the computer equipment manufacturers, with a significant interest in more sales of their products, can assist with funding to promote the idea that migration of data is the key to this issue?

Conclusion

Horses for courses?

All of the above ineluctably points to the need to pose (and answer satisfactorily) three questions:

  • how do we decide what information we want to retain?
  • why do we want to retain that information?
  • how long do we want to retain it?
These questions are not easy to answer. The best thinking on the matter has been carried out by archivists, for their profession has developed criteria to apply in selecting categories of data to retain. If we assume here that we have answered the 'what' and 'why', these answers allow us to then decide how long we need to retain each category, and leading on from that the appropriate preservation strategy to apply. Some hypothetical examples:
  1. 'in-house' staff training multimedia in CD format: short-term retention (say five years) - conserving the artefact itself and storage at room ambient conditions (assuming that they are within acceptable workroom limits, say 20-24oC and <55% RH [28]);
  2. business records, say a combination of images and machine-readable text on optical disk, which need to be retained for legal reasons for a minimum of ten years - conserving the artefact itself may be sufficient, as long as care is taken to ensure that the data remains readable and the equipment and software to access it remains in working order; special storage facilities would be required;
  3. medical records on optical disk which need to be retained for the shorter of the patient's life-time or for twenty years - conserving the artefact itself may be sufficient, as long as care is taken to ensure that the data remains readable and the equipment and software to access it remain in working order; special storage facilities would be required; but refreshing of data and migrating it to new systems as they are introduced will probably also be required;
  4. national heritage material such as multimedia published in Australia in a variety of formats: 'permanent' retention - the solution is NOT to preserve the digital artefact, but rather to concentrate attention on the digital object (for instance by refreshing of data and migrating it to new systems as they are introduced).
In fact this last point is surely the inescapable conclusion to be drawn from this paper: that the equipment (and software) obsolescence factor is where our efforts must be concentrated. In one sense the precise life expectancies of the digital artefacts do not matter. What matters is that they are for periods less than the effective life span (obsolescence period) of the equipment. Although it is platitudinous to say this, it is worth repeating here: equipment costs are decreasing rapidly. [29] What seem unlikely now may be easily affordable in only a brief period of time.

References

Sections of this paper first appeared in the proceedings of the LAS/PPM Multimedia Workshop, 29-30 March 1995, Singapore.
  1. Van Bogart, John. Magnetic Tape Storage and Handling: A Guide for Libraries and Archives (Washington, DC: The Commission on Preservation and Access and National Media Laboratory, 1995); also available on the World Wide Web at URL http://www.nml.org/resources/misc/commission_report/contents.html.
  2. Email from Jim Wheeler to Data Recording list (DATA_RECORDING@NML.ORG), 23 August 1995.
  3. Don Waters, Yale University Library, January 1995; available on the IFLA World Wide Web site; italics added for emphasis.
  4. Bearman, David. "Archival Methods", Archives and Museums Informatics Technical Report, 9 (Pittsburgh: Archives and Records Informatics, 1988, reprinted 1991), p.24; italics added for emphasis.
  5. The Commission on Preservation and Access Annual Report July 1, 1993-June 30, 1994, p.1.
  6. "Research on Magnetic Media-Phase 1" made by Chris Ward et al in conjunction with Commission on Preservation and Access , January 1994; available on the CoOL (Conservation Online) World Wide Web site; italics added for emphasis.
  7. I am aware of, but am conveniently ignoring, other aspects here. One is of the necessity to preserve metadata, or as Peter S. Graham calls it 'intellectual preservation', that is, information about the integrity and authenticity of the information as originally recorded. See URL http://aultnis.rutgers.edu/texts/dps.html for Graham's text 'Long-term Intellectual Preservation' (version dated 18 July 1995 seen).
  8. Rothenberg, Jeff. 'Ensuring the Longevity of Digital Documents', Scientific American (Jan 1995): 24-29.
  9. Available on the CoOL (Conservation Online) World Wide Web site.
  10. I must again note here my indebtedness in this paper to John Van Bogart. The reader should understand that I use his words here at times without direct acknowledgment. The provisos are noted in an email message to me from John Van Bogart, 21 November 1995. Summarised, the differences are: 1) Cleanliness: higher levels of cleanliness of storage environments and of tape drives are required, as missing information on a data tape caused by dropouts (missing signal caused by dust or debris) are not compensated for by the brain, as they are on audio- or video-tape; 2) Tape acclimatisation: if a tape is not fully acclimatised before play, mistracking may occur; the result of this in a data tape may be that data files cannot be read, whereas in a videotape it may only be an annoying band on the screen ; 3) Loading/Unloading from drive: because tape drive mechanisms wear out, the tape may become caught. Care is required when ejecting tapes: 'NEVER attempt to eject a tape while it is in a read/write operation'; 4) Periodic retensioning: data tapes require this more frequently than audio- or videotapes.
  11. Van Bogart (1995), p.5.
  12. Older tapes were made from other materials such as acetate, which is less stable as a substrate. See Van Bogart (1995), p.6, for further details.
  13. See Van Bogart (1995) pp.7-8 for more information; a recent email gives in detail some of the mechanical problems which can arise with 8mm drives (Patricia Adams to Data Recording List (DATA_RECORDING @NML.ORG), 20 November 1995).
  14. Van Bogart (1995), pp. 13-14, 23-27.
  15. Van Bogart (1995), p.18.
  16. I am very conscious that most of the sources I have used are not as current as I would like. Although this may be due to my literature-searching abilities, it could also suggest that current interests are focused elsewhere than on the life expectancy of optical disks.
  17. Saffady, William. Electronic Document Imaging Systems: Design, Evaluation, and Implementation (Westport, CT: Meckler, 1993), p.63.
  18. See Lieberman, Paula. 'Taking Measure of Magnetic, Optical, and Magneto-Optical Media and Drives', CD-ROM Professional 8, 7 (July 1995) and Saffady (1993) for more information about the various types of recording methods used by CD-Rs such as phase change, magneto-optical, dye.
  19. Saffady (1993), p.116.
  20. Arps, Mark. 'CD-ROM: Some Archival Considerations' in Preservation of Electronic Formats & Electronic Formats for Preservation, ed. Janice Mohlhenrich (Fort Atkinson, Wisc.: Highsmith, 1993), p.96.
  21. Adelstein, Peter Z. 'The Stability of Optical Disks: A Science-Standards Review', The Commission on Preservation and Access Newsletter 58 (July 1993): 3-4.
  22. Publishing E-Journals: Publishing, Archiving and Access List (VPIEJ-L@VTVM1.BITNET), ca March 1992.
  23. Email from Gene Hickock to Data Recording List (DATA_RECORDING@NML.ORG), 27 April 1995.
  24. Arps (1993), p.102.
  25. Email from Robert D. Lorentz, 'Dye Stability of CD-R', 24 Aug 1995, to Data Recording List (DATA_RECORDING@NML.ORG): 'Studies of the stability of CD-R under different conditions (light, heat, humidity) are being established in the National Media Lab. Your experiences with any particular problems enountered would be valuable input to this task. Once results are obtained, they will be available through NML. To find more about NML, try our home page at http://www.nml.org/.'
  26. Saffady (1993), p.118, Table 5-1.
  27. Lieberman (1995), p.72. Another example is the HD-ROM: 'The High-Density Read-Only Memory, or HD-ROM, uses a unique ion beam to inscribe information on pins of stainless steel, iridium or other materials that are built to last. An HD-ROM holds about 180 times more information than a comparably sized Compact Disc Read-Only Memory, or CD-ROM, today's cheapest data storage medium. Storage costs of HD-ROM are roughly one-half percent of CD-ROM costs. . . . "The HD-ROM marks a complete departure from existing data storage technologies . . . For the first time, a non-magnetic, non-optical data storage system can be made from truly robust materials." HD-ROM materials are hard, non-malleable, non-flammable and don't react easily with chemicals. Since the medium isn't magnetic, electromagnetic fields can't destroy the data on HD-ROMs, unlike computer hard drives. . . . 'HD-ROM is virtually impervious to the ravages of time whether from material degradation due to thermal or mechanical shock or from the electromagnetic fields that are so destructive to other storage media."' News release distributed in HPCWire, 23 June 1995..
  28. Van Bogart (1995), p.17.
  29. One example: Lesk gives the equipment cost in 1990 for storing one gigabyte on magnetic disk as $4,000 (Lesk, Michael. Image Formats for Preservation and Access (CPA, 1990), reprinted in Information Technology and Libraries 9, 4 (1990): 300-308). Today the cost is less than $400.


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