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Your
YOUR
valuable
VALUABLE
magnetic
MAGNETIC
tapes
are
TAPES ARE
dying
DYING
If you’re storing valuable information on old
video, audio or computer tapes, you might be
surprised about how well those tapes last! If
you don’t take a lot of care, they can become
useless in just a few years!
By Julian Edgar
12 Silicon Chip
W
HILE MOST PEOPLE think
of magnetic tape as a longterm storage medium, the
actual length of time that the recorded
information remains usable depends
heavily on storage conditions and the
quality of the tape. If you are not careful, tapes can deteriorate very rapidly.
But before we look at how you
should be preserving your precious
tapes, what exactly are the causes of
tape degradation?
When a magnetic tape is recorded,
the individual magnetic particles on
the tape are oriented according to the
influence of the recording head. Tiny
bar magnets are created which have
a length equal to a half wavelength at
the frequency being recorded. These
bar magnets are “hard” or permanent
because of the coercive field strength
of the magnetic material.
Strong external magnetic fields can
alter the magnetic orientation of these
particles. If that happens, the signal
is degraded or lost. But with a few
exceptions, such field strengths are
comparatively rare. Even power lines
with current flows of 1000A have no
effect on magnetic tape recordings at
a distance of more than one metre.
Standard household power wiring
with current flows of up to 15A have
a maximum resulting field strength of
0.16kA/m, which lies between 0.3‑1%
of the coercive field strengths of most
common magnetic tapes. These fields
therefore have no effect on magnetic
tape. However, as can be seen in
Fig.1, the distance from the magnetic
field source to the tape is critical, so
care should be taken that tapes aren’t
brought very close to the sources of
magnetic fields.
Keep away from magnets
Tapes should be kept away from
strong permanent magnets, such as
those in loudspeakers, dynamic headphones and microphones.
Also keep tapes away from power
transformers and from motors in
domestic appliances as these can produce quite strong magnetic fields at
switch‑on. Which appliances would
these be? Vacuum cleaners, food processors and power tools are the main
ones to watch. Of course, if you value
your tapes, you wouldn’t have them
strewn around on the floor while you
do the vacuuming, would you?
One particular trap which many
people fall into is to put a video tape
Fig.1: the strength of a magnetic field drops rapidly with increasing
distance from its source. This means that magnetic tapes are
generally quite safe from erasure if kept a reasonable distance away
from such sources. (Emtec Magnetics – formerly BASF).
on top of their TV set. This is probably
the worst place in your home for a
video or audio tape.
Why? Because your tape is bound to
be affected by the very strong magnetic
field produced when the degaussing
coil around the picture tube is momentarily energised at switch‑on.
Security x‑ray equipment in airports uses quite small field strengths,
with a negligible effect on magnetic
tape.
The presence of an external magnetic field during the actual recording
process has a greater potential to affect
the recording than during subsequent
tape use and storage.
Curie temperature
The residual magnetism of recorded
Fig.2: The degree of print‑through and the frequencies affected depends on
both the tape speed and the tape thickness. (Emtec Magnetics)
JUNE 1999 13
Fig.3: the amount of print- through depends on the way in which
the tape is wound. High storage temperatures make it worse.
(Emtec Magnetics).
tape is stable only below a certain
temperature, termed the Curie Temperature. At temperatures higher than
the Curie figure, the magnetisation is
destroyed. The Curie Temperature of
iron oxides is approximately 350°C,
which is not of great concern. Any
tapes subjected to these sorts of temperatures are likely to be unplayable
simply because the cassette has melted or the tape itself has disintegrated!
The situation is different for chromium dioxide tapes. This material
has a Curie temperature of close to
130°C. Such temperatures are possible
in some circumstances of tape use,
although again, a cassette exposed to
this temperature is likely to be severely distorted and hence unplayable.
On the other hand, even extreme
cold has no long term effect on magnetic tapes.
also affect the nature of print‑through.
When the emulsion is placed on the
inside, the effect will be a heavy
er
Keep your tapes away from pow
on
e
tap
a
sit
’t
transformers and don
be
ld
cou
it
ere
wh
top of your TV set
gaffected by the very strong ma
g
sin
aus
netic field when the deg
is
e
tub
e
tur
coil around the pic
momentarily energised at
switch‑on.
pre‑echo and a light post‑echo. With
the tape wound so that the emulsion
is on the outside, a light pre‑echo and
a heavy post‑echo will occur. Fig.3
shows these effects. Print‑through
increases at higher temperatures.
The coercive strength of tapes
depends on the size distribution of
the magnetic particles used in their
manufacture. Tapes with the most
uniform particle size have the highest
echo suppression ability.
Because print‑through occurs
without the aid of the high frequency
bias recording signal, the simple mechanical stress of fast‑forwarding and
rewinding the tape can be enough to
reduce the effect.
Print‑through also occurs on digital
and encoded recordings but because
the spurious data is far beneath the
signal recognition threshold, it can
be ignored.
Repeated playing of tapes reduces
the residual magnetisation of the
tape. This effect is greatest after the
first playback, with later losses progressively smaller. The final value
is between 85‑90% of the originally
recorded signal amplitude.
In addition to the stability of the
magnetisation of the tape, the magnetic particles themselves must be
chemically stable if tape degradation
is not to occur. Pure metal iron pigments or metal powders can oxidise
readily if exposed to air and moisture.
To prevent this happening, the iron
particles are protected with a passiv
ating coating of iron oxide, aluminium oxide and silicon dioxide. But as
time passes, oxidation of the particles
still results in a decrease in magnetic
Print‑through or echo
Another problem affecting stored
magnetic tape is that of print‑through.
This is where the magnetic fields from
one layer of tape influence the tightly
packed adjacent layers. The occurrence of print‑through (sometimes
called “echo effect”) is dependent on
the magnetic material, the recording
wavelength, tape thickness and storage temperature.
Fig.2 shows the effects of wavelength and tape thickness on the
occurrence of print‑through. A C90
compact cassette has maximum printthrough at frequencies of 4kHz and
63Hz, with the effect on high‑speed
studio tape even more severe.
The way the tape is wound and the
temperature at which it is stored will
14 Silicon Chip
Fig.4: tape behaviour is very much affected by the temperature and relative
humidity. (Emtec Magnetics.)
Fig.5: airborne dirt assumes huge proportions when examined in the size
context of the audio tape system. (Emtec Magnetics).
processes will cause drop‑outs on
a VHS tape being played back and
squealing of sticky audio tapes. This
degradation process can sometimes
be temporarily overcome if the tapes
are baked in an oven to drive off the
excess moisture. The tapes should
then be quickly transcribed before
they revert to their old ways!
The lubricant in magnetic tapes is
contained within pores in the surface
of the tape. When the tape passes
over the head or guide, lubricant is
squeezed out, thus easing the passage
of the tape. Excess lubricant is absorbed back into the tape once it has
moved on, although some lubricant is
always sacrificed each time the tape is
played. Lubricant is left on the head
and guide pins and some evaporates
into the atmosphere. When lubricant
levels are very low, the tape can be
restored by being re‑lubricated.
Acetate base
remanence.
Chromium dioxide particles are
also subject to oxidation, converting to
the more stable oxide forms which are
non‑magnetic. Thus, magnetic reman
ence of these tapes also decreases with
time. “Metal” tapes do not use conventional binder technology. Instead, they
have a continuous, thin layer of metal
alloy deposited onto a base film. This
alloy is subject to oxidation and over
time, again a decrease in magnetic
remanence can be observed. Polluting
gases in the atmosphere can act as a
catalyst for this oxidisation.
As Fig.3 shows, magnetic tape uses
a base film to provide the physical
strength, and a binding agent is used
to tie the magnetic particles to the base
film. The binding agent serves several
other purposes.
First, it provides a smooth surface
for the tape which would otherwise
be as rough as sandpaper. Second
and third, it acts as a lubricant and
head-cleaning agent and finally, the
binder contains carbon black to reduce static charges that would otherwise attract debris to the tape.
Binder polymers are subject to
hydrolysis, whereby the polyester
linkages in the commonly used poly
urethane‑based binder systems are
broken down through reaction to
water in the air. This process can also
release organic acids which accelerate
the rate of hydrolytic decomposition.
The acids also attack and degrade
the magnetic particles. Tape binder
debris released through hydrolytic
Audio tapes manufactured in the
1940s and 1950s used acetate as the
base. One way of determining if a
tape is of this type is to hold it up to
the light. If light can be seen coming
through the tape windings, an acetate
base has been used. Degradation of the
backing of these tapes can be indicated
by an odour of vinegar and the tape
may become brittle and break easily
if bent sharply or tugged. Note that
Shelf life
The shelf life of magnetic tape is
controlled by the durability of the
binder, rather than the magnetic particles or the base film.
Binders can deteriorate through
softening, embrittlement, loss of cohesion or loss of lubrication.
Fig.6: signal strength loss, as the distance between the tape surface and the
head increases, depends on the frequency concerned and the tape format.
(Emtec Magnetics.)
JUNE 1999 15
Fig.7: the safe storage
conditions for magnetic
tape require a temperature
around 20°C and a relative
humidity of about 40%.
Note the danger zone: high
humidity and high
temperature. (John Van
Bogart)
tapes of this kind can degrade quite
suddenly, with 50‑year old tapes
becoming unplayable over just a few
years. Tapes degraded to the “vinegar”
stage should be stored separately to
tapes still in good condition.
Since the 1960s, audio and video
tapes have used oriented polyester
(polyethylene terephthalate or Mylar)
as a base. This material is generally
very stable, with the tape backing
outlasting the binder in practical situations. The only problems with poly
ester‑backed videotapes is that excessive tape winding stress can result
in distortions and subsequent tape
mistracking and that large changes in
temperature can cause the backing to
become dimensionally unstable.
As one expert put it “the conditions
which are good for you are also good
for magnetic tape storage”. This means
that temperatures of about 18‑21°C
and a relative humidity of no more
than 40‑50% will give the longest
tape life. Fig.4 shows the behaviour
of magnetic tape across a range of
temperature and relative humidity.
If you live in an area with high
humidity, there is only one way to
ensure that your tapes are not exposed
to the risk of fungus. That is to store
them in sealed containers along with
a small bag of silica gel. From time
to time, you need to check the silica
gel so see if it has turned pink. If so,
it needs to be baked in an oven set to
a low temperature, to drive off all the
Humidity and fungus
All of the above factors can cause
tapes to eventually become unplay
able but a much more aggressive factor
is often the cause of short tape life and
that is humidity.
Most parts of the Australian coastline are subject to high humidity for
much of the summer and all year
round in the tropics. On the east
coast of Australia, anywhere north
of Wollongong can experience relative humidities of 80% or more for
weeks or months at a time, during
wet weather. This can quickly destroy
tapes because it promotes the growth
of fungus.
Once fungus gets a hold, the tape
quickly deteriorates and it can be
thrown out. Just a few months of high
humidity can destroy your tapes, particularly if you live near the seaside.
Preserving magnetic tapes
16 Silicon Chip
troy
High humidity can quickly des
the
tes
mo
pro
it
e
tapes becaus
gets
growth of fungus. Once fungus
erio
det
a hold, the tape quickly
t
Jus
.
out
n
ow
rates and it can be thr
can
y
idit
hum
h
hig
a few months of
y if
destroy your tapes, particularl
e.
sid
you live near the sea
moisture so that it can again become
hygroscopic; ie, water absorbent.
Temperature and humidity are important but magnetic tapes should also
be stored and played back in a clean
environment. Fig.5 shows the relative
size of some common pollutants in
proportion to the tape and magnetic
head. As can be seen, particles that
are very small to the human eye are
significantly larger when considered
within the context of magnetic tape
recording technology!
Tapes should be stored and transported on edge with the weight of
the tape being supported by the hub.
Tapes positioned on their sides place
pressure on the edge of the wound
reels which can cause edge distortion
and damage.
The more frequently a tape is
played, the shorter will be its life. In
addition to the magnetic deterioration
discussed above, the cassette mechanism is subjected to wear and tear and
may fail structurally before the tape
itself wears out. Ideally, cassette tapes
should never be ejected mid‑program
– if there is a problem with the ejection that causes damage to the tape,
it is better if this occurs at either the
beginning or end of the tape.
The winding of the tape should
always be carried out on equipment
that maintains a constant tension at
low speed.
Library wind mode
Libraries storing archival magnetic tapes use special equipment that
provides a “Library Wind Mode” to
give a predetermined winding speed
and tension. In normal domestic use,
equipment should be maintained in
mechanically good condition.
Tapes should be stored in a “tail
out” condition, with the program end
on the outside of the spool. Rewinding
the tape prior to playing will then
reduce print‑through. Some experts
recommend tape “refreshing” on a
periodic basis.
This means that the tape should be
unspooled and rewound at intervals
of about three years to redistribute
tape stress and prevent tape pack slip.
Care should be taken that the surface of the tape does not become contaminated by fingerprints or dust. Any
surface contamination that increases
the distance from the playing head
to the tape results in a reduction in
signal strength.
The frequencies which are most
affected are dependent on the distance
and the format of the tape recording.
The potential for information loss
is far greater on formats where the
information density is highest. Fig.6
shows this.
Storage temperatures
Storage at temperatures over 23°C
increases tape pack tightness, resulting in distortion of the tape backing
and an increase in permanent drop-
outs as wound‑in debris is forced into
the magnetic layer. This deformation
of the magnetic layer can also affect
adjoining windings.
Layer‑to‑layer adhesion can also
occur if the tape is stored at higher
temperatures. Fig.7 shows the recommended storage conditions for
magnetic tapes.
When tapes are transferred from
cool, dry storage conditions to normal
environments, time should be given
for the tape to “acclimatise”. A compact audio cassette should be allowed
an hour for temperature and six hours
for relative humidity stabilisation. A
VHS/Beta cassette can take up to four
hours to become temperature stable
and as long as eight days to settle in
relative humidity.
How long do they last?
So how long will your magnetic
tapes last, given good care? Magnetic
tape is certainly not as stable as film or
paper. Properly cared for, non‑acidic
paper can last for centuries but manufacturers’ data sheets indicate that the
life expectancy for video and audio
tapes is about 30 years.
However, accelerated aging tests
have indicated that high grade VHS
video tape may have a life of only
10 years if stored in conditions with
a relative humidity of 50% and a
temperature of 22°C. This improves
to 30 years at 30% relative humidity
and 18°C. The very highest estimates
of VHS tape life are in the region of
40 years, while the lowest are closer
to five years.
And as we have said, if you have
high humidity, the life of a tape may
only be a few months!
The life expectancy of digital data
tapes is more related to the long‑term
availability of hardware on which to
play the tapes back than any other factor! With digital recording technology
changing every 5‑10 years, storing appropriate playback equipment should
be seriously considered if the taped
information isn’t regularly dubbed
to new media as the old becomes
obsolete.
If the playback equipment remains
available and the tapes have been
stored correctly, digital tapes have
been shown to have a long life.
One US government agency recently transcribed 20,000 10‑year‑old 3480
tape cartridges. Only two cartridges
SC
had unrecoverable errors.
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