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Magnetic media are essential to modern life. They are used in
the form of tape to record sounds and images and to record digital
data. In the form of hard and floppy discs they are used to store
computer data. When applied in the form of a magnetic strip to
a card, magnetic media control our access to money from cash dispensers,
entry to doors and to many other things.
The basic principles for recording signals on a magnetic medium
were set out in a paper by Oberlin Smith in the 1880's. The idea
was not taken any further until Valdemar Poulsen developed his
wire recording system in 1898. Magnetic tape was developed in
Germany in the mid 1930's to record and store sounds. The use
of tape for sound recording did not become widespread, however,
until the 1950's. The BBC, for example, was still using disc recorders
until around 1965.
The recording of images on magnetic tape came later. As with sound
recording, there were several systems before tape recording came
into common use. The first known recordings of images by a non-photographic
method were made by John Logie-Baird in 1924. The images were
recorded on to 78 rpm discs which are now in the National Sound
Archive in London. The first practical recordings of television
programmes were made using special film cameras filming video
screens. The first video recording machine using tape was made
by the BBC in 1955. It used a half inch tape running at 120 inches
per second - just over 3 metres per second. This was swiftly superseded
by the introduction of the Ampex Corporation's 2 inch video tape
system. The arrival of new formats for recording video pictures
has steadily increased since then. It has been calculated that,
taking the different broadcast standards and electricity supplies
into account, images have been recorded on over 100 different
formats in the 40 years since video-tape recording started.
Although some dictating machines using a disc coated with a magnetic
pigment were in use from the 1950's, disc based media did not
develop until computers became widespread. The steady increase
in the storage capacity and decrease in physical size of both
hard and floppy disks has paralleled the developments in the tapes
used for sound and image recording.
Magnetic Tapes
Magnetic media in the form of tapes on open reel or housed in
cassettes and cartridges are the most widespread carriers for
audio and video data and are widely used for the storage of large
quantities of computer data. They are a reliable, lowrisk
and economical storage medium. Archivists can rely on a long period
of experience in the care and handling of magnetic tapes in archives.
If free from production faults, they can be preserved for many
years. The oldest audio tapes are now over 50 years old and still
perfectly readable.
Types of Magnetic Tape Construction
Early audio tapes used cellulose acetate as the support film material, which is also used for safety film. Cellulose acetate has a tendency to become brittle through hydrolysis caused by the moisture contained in the atmosphere. This brittleness generally causes serious problems when playing old audio tapes. Tapes with severe cases of hydrolysis can suffer from the socalled "Vinegar Syndrome", an auto-catalytic process whereby acetic acid is set free in ever increasing quantities and thus creates an accelerating effect on the decay process. This has been particularly experienced in film archives, especially in hot and humid climatic areas. Affected films become soft and limp, ending up as powder or slime. While, in theory, this may also happen to acetate audio tapes, no disastrous losses similar to those in the film world have been reported. Still, acetate tapes, which were produced until the mid 1960s, are at risk and transfer onto other carriers must be envisaged.
Another group of historical audio tapes used polyvinyl chloride
(PVC) as the base film material. As with vinyl discs, these tapes
have not exhibited any systematic instability; the long term prospects
are, however, unknown.
Polyester is the base film material which is used for all modern
audio and all video and computer tapes. It has the greatest resistance
of all base materials to mechanical stress and the influence of
humidity. No systematic stability problems have occurred so far
but, again, its stability over very long periods (centuries) is
unknown.
Many varieties of magnetic materials have been used for the pigment
layer, for example the various oxides of iron used from the very
first tapes until today and chromium dioxide. Only metal powder,
as used in more recent high density tape formats, has given cause
for serious concern. Early tapes metal powder tapes suffered from
corrosion but this problem now seems to be under control. There
is, again, no precise answer to the question of how long metal
particle tapes will keep their information undistorted and readable.
It must be emphasized, however, that, contrary to layman's expectations,
the magnetic information on properly stored and handled tape does
not fade away.
The greatest problem related to magnetic tape is the stability
of the pigment binder - the glue that holds the magnetic particles
together and to the base film. A considerable number of audio
and video tapes, especially amongst those produced during the
seventies and eighties, are suffering from pigment binder hydrolysis.
The atmospheric moisture is absorbed by the pigment binder causing
the polymer to hydrolyse and lose its binding property. Tapes
of this kind deposit a smear of magnetic particles onto the replay
heads. This clogs the heads and swiftly makes the tape unreadable.
In extreme cases, the oxide layer the completely delaminates from
base in large segments when the tape is played. Processes to render
such tapes playable again are available, but the restoration process
is cumbersome, time consuming and cannot restore the most severely
affected tapes. This problem has been found especially in hot
and humid areas where many tapes do not last longer than a few
years.
Types of Magnetic Tape Housing
There are three basic methods for the immediate storage of tape:
open spool, cassette and cartridge. The tape on an open spool
has to be threaded on to the machine and the free end secured
on a second spool by hand - a time consuming task and easily performed
incorrectly. Tape in a cassette is enclosed in a shell and the
two ends of the tape are securely fixed to captive spools. A cartridge
is also fully enclosed but the tape is in the form of a continuous
loop. Cassettes and cartridges are easier to load on to a machine
than open spool tapes and are also suitable for use in robotic
storage systems. Cassettes are common for modern video and computers
but relatively few are used for professional audio. Cartridges
are most commonly encountered with data but some are used for
audio - particularly for short items such as station idents and
commercials.
Open Spool Tapes
Open spools were until recently the main form of tape used for
professional audio recordings. Continental European tradition
generally uses professional tape on flangeless hubs only, a practice
that requires additional care when handling the tapes. Some expensive
professional digital audio formats like DASH and PD (ProDigi)
use reel-to-reel tape and stationary head technology. Early video
and many data tape formats also used tape in open spool form.
Cassette Tapes
Cassettes are used for many purposes. They range from the Compact
Audio Cassette or Musicassette through the many types of video
cassette to the latest Digital Audio Tapes with rotary heads (R-DAT).
They are probably the most widely used form of tape used in modern
systems.
The Compact Audio Cassette was originally designed for use with
dictating machines. Its convenient size led to it becoming used
for the issue of commercial music recordings and for home recording.
Except as an access tape, it was not normally used for professional
work. In addition to the Compact Cassette, there have been several
other cassette tape formats used for dictating machines.
Many types of cassette have been used for analogue video recordings both professionally and in the home. The commonest is the ubiquitous VHS cassette. Other formats include the ¾ inch
U-Matic - a semi-professional format - and the ½ inch BetaCam
used by many broadcasters around the world. All video tape formats,
analogue and digital, use rotary head technology. Some of these
digital video formats were also adapted for the storage of general
computer data.
Rotary head technology is used for the digital audio format R-DAT,
while stationary head technology is employed for the DCC (Digital
Compact Cassette), a data-reduced digital consumer audio format
designed to replace the Compact Audio Cassette.
A variety of cassette formats are used in the computer world as
back-up tapes for the information held on hard discs - the so-called
streamer tape formats. These include the QIC- 80, Exabyte and
a derivate of the R-DAT audio format.
The R-DAT format potentially makes an ideal data backup media.
However, there is little experience of their longterm storage
qualities. Opinions are divided. Some experts say that a five
year re-copying term is appropriate, others claim that DAT are
not suitable for longterm storage. For safety reasons, a
two year recopying term is advisable until more is known of the
long-term performance of these formats.
Tape Cartridges
The primary use for cartridges is for storing computer data but
a variant was extensively used to record short sound sequences
for commercials, station identifications and the like. These audio
cartridges were either monophonic or stereophonic (two tracks).
The cartridges used for computer data, however, use 24 tracks
which permits a storage capacity of 12700 bpi. Due to the sequential
recording, the average accesstime is relatively long.
Magnetic Disks
There are two types of magnetic disk - the hard disks and the
floppy disks. While reading and writing, the disk is rotating
around its centre. The data are recorded in circular tracks, sector
by sector. Because of the sectorial access to the data, the average
access time is relatively short .
Floppy disks are thin, flexible plastic plates covered with a
magnetic oxide layer and protected by a firmly fastened square
plastic jacket. At present, the common format is the 3×5
inchdisk. The older 3×0,
5×25 and 8×0
inch disks are no longer in use and it is difficult to find drives
for them. The storage capacity of a 3×5
inch disk is 1×4 MB. 3×5
inch disks with a capacity of 2×88
MB have been developed but are not very common yet.
Data interchange on floppy disks usually causes no media problems
provided that a drive for the physical format of the disk is available.
Disks are not suitable for longterm storage. They can deform
because of the instability of the plastic material and damage
the drive. They should, therefore, only be used for a limited
period of time.
Hard disks are usually found installed permanently in computer
systems and used for very fast access, short term storage. Removable
hard disks exist but they are not common. Although hard disks
are reliable, it is advisable to make back-up copies of data stored
on them. Storage capacities in excess of 2 GB are now common and,
when hard disks are used in an array (RAIDs), very large storage
capacities can be achieved - albeit at great cost compared with
other storage formats. Hard disks in RAIDs will be in continuous
use and have a life expectancy of several years
Data Density versus Data Security
The history of magnetic storage media is the history of ever increasing
data density. This has been achieved by the steady decrease in
size of the elementary magnetic structure - from iron oxide via
chromium dioxide to "pure metal" as used in metal particle
tape and hard disks. In parallel with this, has come the development
of ever smaller gaps in the reading heads, very thin base films
(some R-DAT tapes are only 9mm "thick")
and very narrow tracks widths (13mm
on R-DAT). By use of these developments, ever increasing quantities
of information can be recorded on ever decreasing sizes of carriers.
The danger is, however, that the recorded information becomes
increasingly vulnerable. It is generally true to state that, because
of their increased data density, modern formats are less reliable
than older formats with their lesser storage capacities. Correct
recording and reading of information onto or from modern magnetic
formats is highly dependent on the physical and chemical condition
of the recording medium being in pristine condition, the replay
equipment functioning perfectly and an environment free from disturbing
factors such as smoke, dust, and other pollutants.
The Stability of Magnetic Carriers
The main factors that affect the stability of carriers and the
retrieval of information can be summarised as:
Humidity is the most dangerous environmental factor. Water
is the agent of the main chemical deterioration process of polymers:
hydrolysis. Additionally, high humidity values (above 65% RH)
encourage fungus growth, which literally eats up the pigment layer
of magnetic tapes and floppy disks and also disturbs, if not prevents,
proper reading of information.
Temperature is responsible for dimensional changes of
carriers, which is a particular problem for high density tape
formats. Temperature also determines the speed of chemical processes:
the higher the temperature, the faster a chemical reaction (eg
hydrolysis) takes place; the lower the temperature, the slower
the chemical reaction.
Mechanical integrity is a much underrated factor in the
retrievability of data recorded on magnetic media: even slight
deformations may cause severe deficiencies in the play back process.
Most careful handling has, therefore, to be exercised, along with
regular professional maintenance of replay equipment, which, if
it malfunctions, can destroy delicate carriers such as R-DAT very
quickly. With all tape formats, it is most important to obtain
an absolutely flat surface of the tape wind to prevent damage
to the tape edges which serve as mechanical references in the
replay of many high density formats. All forms of tape - open
reel, cassettes and cartridges - and floppy disks should be stored
upright.
Dust and dirt prevents the intimate contact of replay heads
to media which is essential for the retrieval of information,
especially with high density carriers. The higher the data density,
the more cleanliness has to be observed. Even particles of cigarette
smoke are big enough to hide information on modern magnetic formats.
Dust may also be responsible for "head crashes" of computer
hard disks and of rotary head formats, which inevitably leads
to irretrievable loss of data. It goes without saying that in
addition to the mechanical problems caused by dust, fingerprints
and smoke, chemical pollution caused by industrial smog can accelerate
chemical deterioration. The effective prevention of dust and other
kinds of dirt and pollution is, therefore, an indispensable measure
for the proper preservation of magnetic media.
Stray magnetic fields, finally, are the natural enemy of
magnetically recorded information. Sources of dangerous fields
are dynamic microphones, loudspeakers and head sets. Also magnets
used for magnetic notice boards etc, possess magnetic fields of
dangerous magnitudes. By their nature, analogue audio recordings,
including audio tracks on video tapes, are the most sensitive
to magnetic stray fields. Analogue video and all digital recordings
are less sensitive. For the safeguarding of analogue audio recordings
is necessary to keep to the following maximum magnetic stray fields:
It should be noted that normally a distance of 10-15cm is enough to diminish the field strength of even strong magnets to acceptably low values.
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Fluctuations of target temperature and humidity values should
be kept to a minimum. Operational areas (studios) should, therefore,
have the same climatic condition as the access storage areas.Tapes
must be allowed to slowly acclimatise to the change in conditions
when brought out of or returned to the preservation storage.
It is of utmost importance that both temperature and humidity
are controlled simultaneously. Damage to tapes can occur if attempts
are made to cool the storage environment without dehumidification.
Such action will normally lead to an excessive rise in relative
humidity.
Summary
Although magnetic media are generally quite stable and analogue
magnetic tape has been in existence for over 60 years, none of
the magnetic media are designed with longevity in mind. Even if
all recommended standards and practices for the preservation of
original magnetic carriers are carefully observed, it not possible
to preserve them for long periods. Sooner or later copies will
have to be made. Because of the increasing deterioration of audiovisual
information when copying in the analogue domain, a preservation
policy based on copying can only be successful in the digital
domain. For digital documents, audiovisual as well as computer
data, the problems related to the preservation of the carriers
are increasingly overshadowed by the problems of the obsolescence
of hardware and associated software. The management of future
migration is, therefore, becoming the central issue of audiovisual
as well as of general data preservation. Self-checking and self-regenerating
digital mass storage systems are likely to become a powerful tool
in future preservation policy.
The digitization of analogue carriers and the migration of information
already in the digital domain are gigantic tasks. Because these
will take time to complete, digitization/migration projects must
be arranged in a hierarchical order. Priority must be given to
those documents which are in frequent demand and to those which
are at immediate risk. Meanwhile, those documents which are in
good condition can wait. They must, however, be stored with all
possible care in order to keep them in the best possible physical
condition until their turn for digitization or migration comes.
Standards
There are few relevant international standards at present. The
American National Standards Institute (ANSI) and the British Standards
Institute (BS) have issued some useful national standards. The
Audio Engineering Society (AES) and ANSI are jointly working on
a series of new standards dealing specifically with the storage
of magnetic materials.
International
AES22xxxx | Draft AES Recommended Practice for Audio Preservation and Restoration Storage of Polyesterbase Magnetic Tape |
AES/EBU | Recommended practice for digital audio engineering - serial transmission format for linearly represented digital audio data |
CCIR 601 | Digital video - composite format |
ISO/IEC 11172 | MPEG-1 video compression system |
ISO/IEC 13818 | MPEG-2 video compression system |
IASA TC03: | The Safeguarding of the Audio Heritage: Ethics, Principles and Preservation Strategy. 1997 |
SMPTE RP 1901996 |
Recommended Practice: Care and Preservation of Audio Magnetic Tape |
National
ANSI X3.14-1983 | American national standard for information systems: recorded magnetic tape for information exchange |
BS 4783:1972 | Recommendations for the care and transportation of magnetic tape |
BS 4783:1988 | Recommendations for storage, transportation and maintenance of magnetic media for use in data processing and information storage. Part 2: Recommendations for magnetic tape on open spools |
BS 4783:1988 | Recommendations for storage, transportation and maintenance of magnetic media for use in data processing and information storage. Part 3: Recommendations for flexible disk cartridges |
BS 4783:1988 | Recommendations for storage, transportation and maintenance of magnetic media for use in data processing and information storage. Part 4: Recommendations for magnetic tape cartridges and cassettes |
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