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 1880. 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 This was swiftly superseded by the introduction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 discs 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, low-risk and economical storage medium. Archivists
and librarians 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
60 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 so-called "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 3/4 inch U-Matic - a semi-professional format
- and the 1/2 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, but without success.
A variety of cassette formats are used in the computer world as back-up
tapes for the information held on hard discs as well as carriers in mass
storage systems - the so-called streamer tape formats. These include
linear formats like QIC- 80, Exabyte or DLT as well as various rotary
head formats, derivates of R-DAT (DDS) and of various digital video formats.
The R-DAT format potentially makes an ideal data backup media. However, there
is little experience of their long-term 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 long-term 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 12 700 bpi. Due to
the sequential recording, the average access-time is relatively long.
Magnetic Discs
There are two types of magnetic disc - the hard discs and the floppy
discs. While reading and writing, the disc 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 discs 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 inch disc. The older 3.0, 5.25
and 8.0 inch discs are no longer in use and it is difficult to find drives
for them. The storage capacity of a 3.5 inch disc is 1.4 MB. 3.5 inch
discs with a capacity of 2.88 MB have been developed but are not very
common.
Data interchange on floppy discs usually causes no media problems provided
that a drive for the physical format of the disc is available. Discs are
not suitable for long-term 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 discs are usually found installed permanently in computer systems
and used for very fast access, short term storage. Removable hard discs
exist but they are not common. Although hard discs 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 discs are used in an array
(RAIDs), very large storage capacities can be achieved - albeit
at greater cost compared with other storage formats. Hard discs 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 discs.
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 9 m
"thick") and very narrow tracks widths (13m
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 less dense 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.
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