 |
Questions? Call 888-624-8373 |
|
|
|
|
Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 61
Magnetic Recording Media
Magnetic recording media, in the form of tapes and disks, are by far the most
common machine-readable storage media in use today. It is estimated that the
U.S. government alone at present uses over 15 million reels of half-inch computer
tape. In 1985, more than 40 million video cassette recorders were manufactured
worldwide, as well as some 200 million half-inch video cassettes. Today, over
1 billion computer flexible disks are produced annually.
ARCHIVAL CRITERL~
The only other mass memory medium used on such a massive scale is, of
course, photographic film, and it is therefore natural to make comparisons
between the two. Photographic film is the result of more than 150 years of techni-
cal development, and today it is a certifiable archival storage medium even though
many early films {e.g., cellulose nitrate-base film) were patently unstable. Mag-
netic tape has evolved over the past 50 years into a reliable, stable storage medium
despite the problems of its early forebears {e.g., vinyl acetate-base film); however,
it has not yet been awarded archival status.
This discussion reviews the principal reports on magnetic tape stability pub-
lished within the past 10 years. Generally, the conclusion derived is that a good-
quality tape, stored in the proper environment ji.e., 65°F or 18°C, 40 percent RH)
and accorded careful mechanical handling, is likely to remain usable for more than
20 years. The period of 10 to 20 years is of particular significance for all machine-
readable records because it is also the useful life expectancy of the hardware itself.
Today's electronic equipment {e.g., earth satellites, computers, television receiv-
ers, and tape recorders) are not expected to remain in service for more than 10 to 20
years. Two important conclusions stem from this fact: first, the recording media
may well outlast the hardware; and second, it will become necessary to recopy the
tape record every 10 to 20 years on an ever-changing, probably incompatible, new
machine with a new format. This operation, termed file conversion, carries with
61
OCR for page 62
62
PRESER VATION OF HIS TOPICAL RECORDS
it, of course, the potential of ever-increasing data compaction. Concomitantly, the
financial burden of file-converting the entire archival collection perhaps five or six
times per century is likely to be out of the question except for relatively small
collections that have great historical importance, sustain heavy use, or require
rapid access.
In machine-readable records, any realistic discussion of the archival proper-
ties cannot be separated from questions on the longevity of their associated hard-
ware or machines. The long-term stability of the recording medium is necessary,
but it is not a sufficient criterion for its use. With human-readable records, on the
other hand, the long-term stability of the medium is necessary and sufficient.
For these reasons, it must be concluded that magnetic recording media and
other machine-readable recording media {e.g., magneto-optic and optical disks)
cannot be recommended for long-term Say, over 25 years) archival applications.
Similar conclusions have been put forth by the NARS committee {National
Archives and Records Service, 1984) and Mallinson {1985a).
DEFINITIONS
In machine-readable records i.e., magnetic computer, audio, and video
tape, magnetic and optical disks, and phonograph records it is understood that
the recorded information can be usefully recovered only by converting it to a
human-readable form such as paper text, a photograph, or a video terminal display.
In analog recordings, this conversion requires appropriate hardware, and in digital
recordings it requires hardware, software, and documentation. On the other hand,
the information in human-readable records is comprehensible simply by visual
inspection of the record or a magnified image of the record. Only simple optical
hardware, such as microscopes and projectors whose design principles need never
change, are required to read the record completely.
THE ELECTRONIC INFORMATION AGE
Modern civilizations are now entering the so-called Information Age,
wherein the vast majority of their information and records are stored, manipu-
lated, and disseminated by electronic means such as computer networks, earth
satellite relays, and television broadcasting. It seems that the archival community
tends to forget that the principal motive for these technologies is their speed of
access and that this speed is only achieved at an extremely high cost. The
machines themselves {e.g., computers, satellites, and television receivers) are
rarely expected to have a useful life in excess of 10 years. The machine-readable
records are operated at ever-increasing information storage densities, not only to
store more information but also to decrease access times. This is a trend that is
surely inimical to long-term archival preservation.
Since 1956 no less than eight differing, incompatible videotape formats of
increasing storage density have emerged. In fact, coincidentally, since 1952 eight
differing computer tape formats have been used. The sixteen tape formats are
listed in Table 6-1. Each format typically mandates a different machine with its
unique set of demodulators, decoders, and reformatters. This proliferation of
incompatible systems is the root cause of the archivist's dilemma in adopting
OCR for page 63
MAGNETIC RECORDING MEDIA
TABLE 6-1 Video and Computer Tape Formats
Product Current Status
Video tape formats since 1956
2-inch quadrupled
2-inch quadrupled, double density
1-inch helical, type A
1-inch helical, type B
1-inch helical, type C
3/~-inch helical, U-Matic
/-inch helical, Beta-max
/-inch helical, VHS
/-inch computer tape formats since
1952
7-track NRZI, 100 BPI
7-track NRZI, 200 BPI
7-track NRZI, 556 BPI
7-track NRZI, 800 BPI
9-track NRZI, 800 BPI
9-track PE, 1,600 BPI
9-track GCR, 6,250 BPI
18-track NRZI, 19,000 BPI
Obsolete
Obsolete
Obsolete
Obsolete
Obsolete
Obsolete
Obsolete
KEY: NRZI = Non-Return to Zero Inhibit; PE = Phase Encoding;
GCR = Group Code Recording.
63
machine-readable records. The speed of access and the electronic data processing
abilities are indeed attractive, but it must be recognized that the records and their
associated hardware will become obsolete within a couple of decades.
Since the information and communication industries are most definitely not
driven by long-term archival considerations, it seems futile to expect technology
to resolve this problem. Advances in technology continue to cause the machine-
readable problem, and obviously these advances will not solve the problem.
ARCHIVAL PROPERTIES OF MAGNETIC RECORDING MEDIA
Magnetic recording media are made up of three components: the substrate,
the magnetic particles or grains, and the binder system.
In rigid computer disks shard disks d, the substrate is an aluminum alloy. The
magnetic particles are gamma-Fe2O3, and the binder system is usually one of the
epoxy family. Because of the relatively high cost of storing data on rigid disks ( 10-3
cents per bit versus 10-6 cents per bit on taped, rigid disks are rarely considered for
archival applications and, therefore, will not be discussed further. An additional
factor against its archival use is the fact that the majority of today's large hard disk
files "Winchester drivesJ cannot be physically separated from the head-disk assem-
bly tHDAJ, a sealed unit.
Magnetic tapes and flexible disks almost universally have a polyethylene
terephthalate {PETJ film substrate; common trade names are Mylar {DuPontJ,
Celanar {Celanese), and Estar {KodakJ. In a recent publication from the National
Bureau of Standards it was concluded that, given storage at 20 to 25°C {68 to 77°FJ
and 50 percent RH, PET films are expected to have a lifetime of 1,000 years (Brown
et al., 1984J. Consequently, PET films will not be discussed further.
OCR for page 64
64
PRESERVATION OF HISTORICAL RECORDS
The magnetic particles used in most half-inch-wide computer tape are
gamma-Fe2O3. In the most recent half-inch computer tape format ~ 18-track NRZI,
19,000 BPI), CrO2 is the magnetic material used. Most of today's video tapes use
cobalt-surface-modified gamma-Fe2O3. Flexible computer disks use, in the main,
gamma-Fe2O3, with increasing adoption of cobalt-surface-modified gamma-
Fe2O3. It is believed that all these magnetic materials are stable chemical entities
under normal storage conditions. All are produced by high-temperature {above
200°C) processes, which implies great stability around room temperature.
The magnetic stability of these particles is well understood. Their coercive
forces are all above 300 Oe {more than 200 times the earth's magnetic field), and
they are, accordingly, unaffected by the stray fields {about 10 Oe) associated with
most electronic equipment. Their Curie temperatures {the temperature above
which they become nonmagnetic) are above 400°C except in the case of CrO2,
which is only 120°C. These Curie temperatures are so far above the normal
archival storage temperatures that no difficulty is anticipated. Other magnetic
effects of concern include the print-through phenomenon, in which the magnetic
fields from one layer of written tape can slightly magnetize the particles in the
adjacent layers on the reel. The effect is known to be extremely small at room
temperature but increases with temperature. However, at 65°C in a 4-hour test
the print-through signal level typically remains a factor of 500 below the normal
signal levels Bertram and Eshel, 1979~.
The binder systems in universal use in tapes today are of the polyester-ure-
thane type. Because all tapes and flexible disks are intended to be operated with
the writing and coding heads in as close physical contact as possible, the binder
system has been chosen because of its extreme resistance to mechanical abrasion
and its chemical stability. Most manufacturers of tape use slightly differing formu-
lations, and no standards have yet been instituted.
The Achilles' tree! of magnetic recording is the extremely close head-to-
medium spacings required. Accordingly, most of the published reports deal
directly or indirectly with the archival stability of the polyester-urethane binder
systems. The particular area of concern is the hydrolysis of the binders. The basic
reaction is (Brown et al., 1984; Cuddihy, 1980; Bertram and Cuddihy, 1982)
Ester + Water ~ Polycarboxylic Acid + Alcohol
The problem with hydrolosis of the binder system is that its mechanical
properties degrade if it is allowed to progress too far. It is thought that the adhesion
of the binder to the substrate is particularly vulnerable to hydyrolysis brown et
al., 1984~. Earlier work showed that hydrolysis is a reversible reaction and sug-
gested that not only can over-hydrolyzed tapes be recovered but that the system
may equilibrate at a point where the hydrolysis reaction rate is zero {Cuddihy,
1980; Bertram and Cuddihy, 1982~. To attain equilibrium at a satisfactorily low
level of hydrolysis, tapes should be stored at 20°C {68°F) and 40 percent RH
{Bertram and Eshel, 1979; Bertram and Cuddihy, 1982~. Other satisfactory envi-
ronments for limiting hydrolysis are shown in Figure 6-1.
A tape wound on a hub relies entirely on the maintenance of the layer-to-layer
pressures and friction to transmit torque to the outer layers. The layer-to-layer
pressure has been found to vary considerably when the tape reel is exposed to
temperatures and humidities that differ from those that existed when the reel was
OCR for page 65
MAGNETIC RECORDING MEDIA
100
90
80
70
60
I 50
>
OCR for page 66
66
PRESERVATION OF HISTORICAL RECORDS
private communication, 1985~. After longer periods {say, 5 years) of storage, it is
advisable to rewind the tape gently whenever it is withdrawn from the archive.
The published literature contains a host of other sound suggestions for the
archival care of magnetic recording media. The review by Geller { 1983) is particu-
larly detailed and is highly recommended as a guide for action. Of the many
excellent practices suggested, it is perhaps well to note those that concern the
response of magnetic tape to changes in humidity. The coefficient of linear expan-
sion of magnetic tape is almost the same for each percent change in relative
humidity as it is for each degree Celsius change in temperature. Whereas the
temperature may equilibrate in several hours, similar equilibria/ion of humidity
in a tightly wound tape reel may take several days. It follows, then, that if an
archival tape is to be subjected to a change in humidity (e.g., when brought to the
computer room environment), it ideally should be allowed to equilibrate for sev-
eral days even before being rewound or retensioned.
Given proper care, a safe conclusion can be drawn that a good-quality tape is
an archivally reliable storage medium for periods of 10 to 20 years as a minimum,
with much longer periods a distinct but as-yet unproved possibility. The essential
point to be remembered is that today's recording media are most likely to outlive
the period of utility of the other components of magnetic storage systems.
TRENDS
In the future the inexorable trend to higher storage densities will lead to
magnetic tapes and disks that use very thin metallic layers iMallinson, 1985a,
1985b). Projections are that not only will such tapes approach within a factor of
two the storage densities of optical and magneto-optical disks, but also the physi-
cal properties of the metallic storage layers will be very similar. Thus, in magnetic
disks, a 200-A-thick layer of Co-Ni may be used, compared with magneto-optical
disks that have a 150-A-thick layer of Co-Fe-Tb and optical disks with a 150-A-
thick layer of a Te alloy. The archival properties of such metallic thin-film media,
be they magnetic or optical, are not at present known; current estimates are for
lifetimes of 10 to 30 years. Again, the salient point is that the recording medium
may well outlast the hardware.
ARCHIVAL PROPERTIES OF SOFTWARE AND DOCUMENTATION
In a certain class of machine-readable records, namely those employing com-
puter or digital technologies, a further archival problem arises. The mere recovery
of the digital data is not possible without some software. The proper operating
system {called software to distinguish it from hardware) must be available at the
time that data are to be recovered. Sadly for the archivist, software today is chang-
ing more rapidly than hardware; for example, Western Electric's UNIX operating
system has been offered in about 30 versions over the past decade. Offsetting this
serious problem are, of course, some potentially attractive reasons for using digital
recording techniques; compatibility with the computer environment and the abil-
ity to perform perfect error detection and correction are prime examples.
Given the proper operating system for reading out the digital data, a further
requirement arises. Appropriate documentation must be at hand that will provide
OCR for page 67
MAGNETIC RECORDING MEDIA
67
Contro~e3-environment tape storage area. Archival care of magnetic
recor~ingme~a requires attention to come-ons of temperature and
humi~tyas well as carefu~peno~c rewinding.
OCR for page 68
68
PRESER VATION OF HIS TOPICAL RECORDS
the necessary information on the digital codes used, the organization or format of
the record, and several other minor but critical details. Operating systems are
usually resident on computer tapes or floppy disks, thus compounding an already
difficult archival problem. The documentation may be in machine-readable or
human-readable form but, given the human species' well-known tendency to
procrastinate, the needed data may well be incomplete or missing {National Acad-
emy of Sciences, 1982~.
ARCHIVAL PROPERTIES OF HARDWARE
The fact that most electronic hardware is expected to function for no more
than 10 to 20 years raises very serious problems for long-term {more than 20 years)
archival preservation. Even if the operating systems and documentation problems
somehow are dealt with, what is the archivist to do when the machine manufac-
turer declares the hardware obsolete or simply goes out of business? Will there be
an IBM or a Sony in the year 2200? If they still exist, will they maintain a 1980-
1990 vintage machine? Moreover, it must be realized that no archival organization
can hope realistically to maintain such hardware itself. Integrated circuits, thin
film heads, and laser diodes cannot be repaired today, nor can they be readily
fabricated, except in multimillion-dollar factories.
The inescapable conclusion is that, if a long-term archive preserves records in
machine-readable form, it will be committed eternally to file conversion {i.e.,
rerecording the old obsolete versions into the new current format) approximately
every 10 to 20 years. Not only would such an operation be enormously expensive,
but also, in an archive where by definition no records can be disposed of it is a
task that grows exponentially with time. Precisely such file conversions take
place all the time, of course, in today's computer facilities, but the critical differ-
ence is that records in such facilities are continually being retired perhaps to be
sent to an archive!
CONCLUSIONS
The committee's conclusions in the area of magnetic media are as follows:
1. Magnetic recording media today are of sufficient stability that only short-
term {10 to 20 years) storage is practical.
2. Operation of short-term magnetic tape archives in accordance with the rec-
ommended storage practice detailed by Geller (1983) is possible.
3. Magnetic recording media and other machine-readable recording media can-
not be recommended for long-term Over 20 years) storage because of the difficul-
ties in maintaining software, hardware, and documentation; provision for
repeated file conversion can overcome this limitation.
REFERENCES
Bertram, H. N., and E. F. Cuddihy. 1982. Kinetics of the humid aging of magnetic recording tape.
IEEE Trans. Magn., 18(5, September):993-999.
Bertram, H. N., and A. Eshel. 1979. Recording Media Archival Attributes (Magnetic). U.S. Air Force
Systems Command, RADC F 30602:78:C-0181.
OCR for page 69
MAGNETIC RECORDING MEDIA
69
Brown, D. W., R. F. Lowry, and L. E. Smith. 1984. Predictions of Long-Term Stability of Polyester-
Based Recording Media. National Bureau of Standards, NBSIR 84-2988, December. See also
Kinetics of hydrolytic aging of polyester urethane elastomers, Macromolecules, 13:248-252
{1980~; Hydrolytic degradation of polyester polyurethanes containing carbodiimides, Macro-
molecules, 15:453-485 jl982J; Equilibrium acid concentrations in hydrolyzed polyesters and
polyester-polyurethane elastomers, J. Appl. Polym. Sci., 28:3779-3792 jl983~; end Hydrolysis
of crosslinked polyester polyurethanes, Div. Polym., Mater. Sci. Eng., 51:155-161 (1984~.
Cuddiby, E. F. 1980. Aging of magnetic recording tape. IEEE Trans. Magn., 16(4, July]:558-568.
Geller, S. B. 1983. Care and Handling of Computer Magnetic Storage Media. National Bureau of
Standards, NBS SP 500-101, June.
Mallinson, J. C. 1985a. The next decade in magnetic recording. IEEE Trans. Magn., 21 (3,
May]: 1217-1220.
Mallinson, J. C. 1985b. Archiving human and machine readable records for the millenia. Society of
Photographic Scientists and Engineers Second International Symposium: The Stability and
Preservation of Photographic Images. Ottawa, Canada. August 1985.
National Academy of Sciences. 1982. Data Management and Computation, Vol. 1: Issues and Rec-
ommendations. Washington, D.C.: National Academy Press.
National Archives and Records Service. 1984. Advisory Committee on Preservation White Paper:
Strategic Technology Considerations Relative to the Preservation and Storage of Human and
Machine Readable Records. July. Unpublished.
OCR for page 70
Optical disk a recent development in data storage technology. The
magniped view shows digitized data encoded on the disk.
Representative terms from entire chapter:
magnetic recording
Items in cart [1]
