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Magnetic Tape Storage and HandlingA Guide for Libraries and ArchivesDr. John W.C. Van Bogart
National Media Laboratory
June 1995 |
Magnetic tape consists of a thin layer capable of recording a magneticsignal supported by a thicker film backing. The magnetic layer, or top coat, consists of a magnetic pigment suspended within a polymer binder. As its name implies, the binder holds the magnetic particles together and to the tape backing. The structure of the top coat of a magnetic tape is similar to the structure of Jell-O that contains fruit - the pigment (fruit) is suspended in and held together by the binder (Jell-O). The top coat, or magnetic layer, is responsible for recording and storing the magnetic signals written to it.
Figure 1. Diagram of a Tape Reel A schematic of a tape reel showing the principal components. Tape is wound around the hub of a tape reel forming a tape pack. The tape pack is protected from damage and disruption by flanges on the reel.
The binder also has the function of providing a smooth surface to facilitate transportation of the tape through the recording system during the record and playback processes. Without the binder, the tape surface would be very rough, like sandpaper. Other components are added to the binder to help transport the tape and facilitate information playback. A lubricant is added to the binder to reduce friction, which reduces the tension needed to transport the tape through the recorder and also reduces tape wear. A head cleaning agent is added to the binder to reduce the occurrence of head clogs that result in dropouts. Carbon black is also added to reduce static charges, which attract debris to the tape.
The backing film, or substrate, is needed to support the magnetic recording layer, which is too thin and weak to be a stand-alone film layer. In some tape systems, a back coat is applied to the backside of the tape substrate layer. A back coat reduces tape friction, dissipates static charge, and reduces tape distortion by providing a more uniform tape pack wind on the tape reel (Figure 1). A schematic diagram of a magnetic tape construction is shown in Figure 2.
Figure 2. Cross Section of
Magnetic Tape
Magnetic particles are held together with a
binder coated on a film substrate. Lubricant and other agents (not shown) may also be included in the top coat layer. A back coat may also be added to control friction and static charges. The structure of the top coat is analogous to that of Jell-O filled with grapes where the grapes represented the
magnetic particles and the Jell-O represented the
binder.
All three tape components - magnetic particle, binder, and backing - are potential sources of failure for a magnetic tape medium. The Magnetic-Media Industries Association of Japan (MIAJ) has concluded that the shelf life of magnetic tape under normal conditions is controlled by the binderrather than the magnetic particles ("DDS Specs Drive DAT Reliability," Computer Technology Review, 13 (5), May 1993: 30). In this instance, the shelf life would refer both to the life of recorded as well as unrecorded media; the life of the binder is independent of whether or not the tape has ever been recorded.
2.1 Binder Degradation
The binder is responsible for holding the magnetic particles on the tape and facilitating tape transport. If the binder loses integrity - through softening, embrittlement, loss of cohesiveness, or loss of lubrication - the tape may become unplayable. Sticky tape and sticky shed are commonly used terms to describe the phenomenon associated with deterioration of themagnetic tape binder.
The binder polymers used in magnetic tape constructions are subject to a chemical process known as hydrolysis. In this process, long molecules are broken apart by a reaction with water to produce shorter molecules. The shorter molecules do not impart the same degree of integrity to the bindersystem as do the longer molecules. As in a wool sweater, if enough individual yarns are cut, the sweater will eventually fall apart.
Specifically, it is the polyester linkages in the commonly used polyester polyurethane-based binder systems that undergo scission (are broken) by water molecules. Water must be present for the hydrolysis reaction to occur. Furthermore, the more water that is present, the more likely it is that polyester chains will be broken. The binder polymer will absorb water from the air. It will absorb more water in a high humidity environment than a low humidity one. This process is analogous to that observed for open bags of crackers, potato chips, and breakfast cereals: They will loose their crunch quickly on humid, summer days (80 to 90% RH) as they absorb high amounts of moisture from the air. In the winter, however, indoor humidities generally can be lower (10 to 20% RH), less moisture is absorbed from the air, and the snacks never seem to get as stale.
Binder hydrolysis can lead to a sticky tape phenomenon characterized by a softer than normal binder coating, higher friction, and/or gummy tape surface residues. A sticky tape can exhibit sticky shed, produce head clogs, result in stick slip playback, and in extreme cases, seize and stop in the tape transport. Tape binder debris resulting from binder deterioration will result in head clogs that will produce dropouts on a VHS tape when played back. The sticky tape syndrome will result in the squealing of audio tapes as the tape very rapidly sticks to and releases from the playback head.
Procedures such as tape baking can temporarily improve binder integrity, allowing sticky tapes to be played and data recovered. The Ampex Recording Media Corporation reports that treating a sticky tape at 122° F (50° C) for three days will sufficiently firm up the binder coating so that the tape can be played. The effect of the treatment is temporary, and it is recommended that the information on the treated tape be transcribed to new tape within one to two weeks. Tape baking should not be considered a universal panacea for the treatment of sticky tapes. The tape baking procedure was developed for a specific type of degradation phenomenon on specific tape types - hydrolysis of reel-to-reel audio tapes and computer tapes. For other kinds of degradation on other tape types, tape baking may actually cause more damage. Expert advice is recommended.
Lubricant Loss
Lubricants are normally added to the binder to reduce the friction of themagnetic topcoat layer of the tape. Lower friction will facilitate tape transport through the recorder and reduce tape wear. In a VHS recorder, where the tape is wrapped around a rapidly rotating head, low friction is also important as it prevents overheating of the tape. The surface of amagnetic tape is actually quite porous. In some tapes, a liquid lubricant is added to the binder and will reside in these pores, similar to water absorbed in a wet sponge. When the tape passes over a head or a tape guide, lubricant is squeezed out onto the tape surface, providing a slippery interface between the tape and the guide pin. After passing by the guide pin, the excess lubricant on the surface of the tape is absorbed back into the surface of the tape. The phenomenon is similar to that observed when the surface of a wet sponge is gently pressed and released - water is exuded to the surface when the sponge is pressed and is reabsorbed when the pressure is released.
Over time, the level of lubricant in the tape decreases. Lubricants are partially consumed every time the tape is played. This is all part of their job as lubricants - they are consumed and worn down sacrificially to protect the tape. Some of the lubricant will migrate from the tape to the guide pins and heads of the recorder each time the tape is played.
Lubricant levels decrease over time even in unplayed, archived tape as a result of evaporation and degradation. The lubricants used in some tapes are oily liquids that are volatile and slowly evaporate away over time. Some lubricants are also subject to degradation by hydrolysis and oxidation, just like the binder polymer, and will lose their essential lubrication properties with time.
The information stored on severely degraded magnetic tapes can be recovered, in specific instances, after relubrication of the tapes. By significantly reducing the friction of the magnetic coating with the addition of lubricant, tapes can be made to play back. Prior to relubrication, the tape may have seized in the tape transport as a result of high friction, or the magnetic coating may have been readily torn off the tape backing by a high speed tape head. Relubrication of tapes must be done carefully by experienced individuals. If a tape is over-lubricated, the excess lubricant on the surface of the tape will act as debris and increase the head-to-tape spacing, causing signal losses and dropouts.
2.2 Magnetic Particle Instabilities
The magnetic particle, or pigment (the terminology is a carryover from paint and coatings technology), is responsible for storing recorded information magnetically as changes in the direction of the magnetism of local particles. If there is any change in the magnetic properties of the pigment, recorded signals can be irretrievably lost. The magneticremanence characterizes the pigment's ability to retain a magnetic field. It refers to the amount of signal that remains after a recording process. The strength of the signal recorded on a tape magnetically is directly related to the magnetic remanence of the pigment. Thus, a decrease in the magneticremanence of the pigment over time can result in a lowered output signal and potential information loss.
The coercivity characterizes the pigment's ability to resist demagnetization. It refers to the strength of the magnetic field that must be applied to a magnetic particle in order to coerce it to change the direction of its magnetic field. Demagnetization of a tape can result from an externally applied field, such as that produced by a hand-held metal detector at an airport security check point. A magnetic tape with a lower coercivity is more susceptible to demagnetization and signal loss.
Magnetic pigments differ in their stability - some particles retain theirmagnetic properties longer than others. Thus, some tapes will retain information, which is stored magnetically, longer than others. Iron oxide and cobalt-modified iron oxide pigments are the most stable pigment types of those used in audio and videotapes. These pigments are generally used in the lower grade audio and low to high grade VHS/Beta videotapeformulations. The low coercivity of these pigments disallows their use in high grade audio formulations.
Metal particulate (MP) and chromium dioxide (CrO2) pigments provide a higher tape signal output and permit higher recording frequencies than the iron oxide pigments, but are not as stable as the iron oxide pigments. A decrease in signal output of two decibel (dB) may be observed over the lifetime of metal particle and chromium dioxide based tapes. However, even with these losses, the output signal will still be better than a comparable iron oxide based tape. A loss in signal will manifest itself as a reduction in the clarity and volume of a sound recording and in the loss of hue and reduction in saturation for a video recording. Chromium dioxide is used in medium to high grade audio tape and some high grade VHS/Beta video tape. Metal particulate is used in high grade audio and 8mm video tape. Metal particulate is also used in most digital audio and digital video tape formulations. The type of pigment used in the audio or video tape formulations is normally indicated in the product literature that comes with the tape. This information can also be obtained from the manufacturer via the toll-free number provided on the literature that accompanies the tape cassette or reel.
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. However, the rate of deterioration can be slowed by storing the tapes in cooler temperatures. The level of humidity has little direct effect on the deterioration of magnetic pigments. However, by-products of binderdeterioration can accelerate the rate of pigment deterioration, so lower humidity would also be preferred to minimize the degradation of themagnetic pigment.
Metal evaporated (ME) video tapes are prevalent in the 8mm video formats. These tapes require no binder polymer, as the entire magneticlayer consists of a single, homogeneous metal alloy layer that is evaporated onto the tape substrate. These tapes have chemical stabilities similar to those of metal particle tapes. However, because the magneticcoating on an ME tape is much thinner than the corresponding layer on an MP tape, they are generally not as durable and do not hold up well in repeated play or freeze-frame video applications.
2.3 Substrate Deformation
The tape backing, or substrate, supports the magnetic layer for transportation through the recorder. Since the early 1960s, audio tapes andvideotapes have used an oriented polyester (also known as polyethylene terephthalate, PET, or DuPont Mylar®) film as a tape substrate material. Polyester has been shown, both experimentally and in practice, to be chemically stable. Polyester films are highly resistant to oxidation and hydrolysis. In archival situations, the polyester tape backing will chemically outlast the binder polymer. The problem with polyester backedvideotapes is that excessive tape pack stresses, aging, and poor wind quality can result in distortions and subsequent mistracking when the tapes are played.
The best way to reduce the degree of tape backing distortion is to storemagnetic media in an environment that does not vary much in temperature or humidity. Each time the temperature or humidity changes, the tape pack will undergo expansion or contraction. These dimensional changes can increase the stresses in the tape pack that can cause permanent distortion of the tape backing. Distortion of a VHS tape backing will show up as mistracking when the tape is played.
Tape backing deformation can also arise if the tape experiences nonlinear deformation as a result of nonuniform tape pack stresses. This normally results if the tape pack wind quality is poor as indicated by popped strands of tape - one to several strands of tape protruding from the edge of a wound roll of tape. Methods of controlling the quality of the tape pack wind are discussed in the Ampex Guide to the Care and Handling of MagneticTape that appears in the Appendix.
Older tapes used other backing materials. In the 1940s and 1950s, acetate (cellulose acetate, cellulose triacetate) film was used as an audio tape backing. This is the same material used in some older movie film. In general, if light can be seen coming through the tape windings when the reel is held up to a light, it is an acetate based magnetic tape. This substrate is subject to hydrolysis and is not as stable as polyester film. However, more stable vinyl binder systems were used during this time period. Thus, the life of tapes produced during this period can be limited by the degradation of the backing rather than the binder. Degradation of the backing in these tapes is indicated by the presence of the vinegar syndrome, where a faint odor of vinegar (acetic acid) can be detected coming from the tapes. In the advanced stages of degradation, themagnetic tape will become brittle and break easily if bent too sharply or tugged. The backing also shrinks as it decomposes, resulting in a change in the length of the recording. Any tape on an acetate backing should be stored in a low-temperature, low-humidity archive to reduce the rate of deterioration of the acetate tape backing.
Acetete film has also been used as a base film for photographic film, cinema film, and microfilm. The "IPI Storage Guide for Acetate Film" has been prepared by the Image Permanence Institute, Rochester Institute of Technology, Post Office Box 9887, Rochester, New York, 14623-0887, Phone: 716-475-5199, as an aid in preserving still and motion picture film collections on acetate base films. The comments in that guide are equally appropriate for acetate based magnetic recording tape. In general, lower storage temperatures and relative humidities are recommended to increase the time to onset of the vinegar syndrome. Tapes having the vinegar syndrome should be stored separately to prevent the contamination of other archive materials by acetic acid. After the onset of the vinegar syndrome, acetate films degrade at an accelerated rate. Tapes that have been stable for fifty years may degrade to the point of being unplayable in just a few years. Any valuable tape showing vinegar syndrome should be transcribed as soon as possible.
Prior to cellulose acetate, paper was used as a tape backing material. Audio recordings of this type are very rare and should be stored in a tape archive. Although generally stable, these backings are very fragile and subject to tearing or breaking on playback. For this reason, particular care should be taken to ensure that the playback recorder is very well maintained.
2.4 Format Issues
Helical versus Longitudinal Scan Recording
The susceptibility of the recording to loss as a result of dimensional changes in the backing is dependent on recording format. Videotape, which uses a helical scan recording format, is more sensitive to disproportionate dimensional changes in the backing than analog audio tape, which uses longitudinal recording.
Helical (Figure 3). Tracks are recorded diagonally on a helical scan tape at small scan angles. When the dimensions of the backing change disproportionately, the track angle will change for a helical scan recording. The scan angle for the record/playback head is fixed. If the angle that the recorded tracks make to the edge of the tape do not correspond with the scan angle of the head, mistracking and information loss can occur.
Distortion of a helical scan videotape can result in two types of mistracking - trapezoidal and curvature (Figure 4). In trapezoidal mistracking, the tracks remain linear, but the track angle changes so that the playback head, which is always at a fixed angle to the tape, cannot follow them. Curvature mistracking can be a more serious type of deformation where the recorded tracks become curved as a result of nonlinear deformation of the tape backing. Mistracking will result in a video image where some or all of the screen is snowy or distorted. For example, in the case of trapezoidal mistracking, the upper portion of the TV screen may appear normal, whereas the lower portion of the screen may be all static. The appearance on the screen will be similar to the playback of a good tape where the tracking adjustment control has been purposely misadjusted.
Figure 3. Helical Scan Recording A moving tape wraps 180° around a cylindrical drum rotating at high speeds; the rotating head is oriented at a slight angle to the tape so that the tracks written by the tiny record head embedded in the surface of the rotating drum run diagonally across the tape from one side to the other.
Figure 4. Types of Mistracking for Helical Scan Recording Trapezoidal error occurs when the angle of the recorded track does not agree with the scan angle of the playback head. Curvature error occurs when the tape has deformed nonlinearly. The playback signal corresponds to that for a single helical scan.
Longitudinal (Figure 5). In a longitudinal tape system, the heads are arranged along a fixed head stack - one head per track - and the tracks will always remain parallel to the edge of the tape. Mistracking is not as great a problem in longitudinal recording for this reason.
Distortion of a longitudinal audio recording tape will appear as a temporary muffling, change in pitch, or loss of the audio track. Distortion of the tape backing can impart a slight curve to the normally linear tape. When the distorted portion of tape passes over the playback head, the recorded tracks can move out of alignment with the head gap, causing a temporary reduction in sound volume and quality.
Figure 5. Longitudinal Recording A moving tape passes across a stationary record head. The recorded tracks are parallel to the edge of the tape and run the full length of the tape. A nine-track tape is shown.
Analog versus Digital Storage
Some comments concerning the archival stability of analog versus digitalmaterials may be instructive. In an analog recording, the signal recorded on the audio or videotape is a representation of the signal originally heard or seen by the microphone or video camera. The volume of a sound recording or the intensity of the color of a video image is directly related to the strength of the magnetic signal recorded on the tape. In a digital recording the audio or video source signal is digitized - the signal is sampled at specific points in time and converted to a number that reflects the intensity of the signal at the time of sampling (analog-to-digital conversion). These numbers, in binary form, are written to the tape, rather than the analog signal. On playback, the numbers are read and used to reconstruct a signal that is representative of the original signal (digital-to-analog conversion).
The chief advantage of an analog recording for archival purposes is that the deterioration over time is gradual and discernible. This allows the tape to be transcribed before it reaches a point where the recording quality has degraded to an unusable level. Even in instances of severe tape degradation, where sound or video quality is severely compromised by tape squealing or a high rate of dropouts, some portion of the original recording will still be perceptible. A digitally recorded tape will show little, if any, deterioration in quality up to the time of catastrophic failure when large sections of recorded information will be completely missing. None of the original material will be detectable in these missing sections.
The chief advantage of a digital recording is that copies of the original tape can be made without any loss in recording quality. A copy of a digital tape can be made that is truly identical to the original source tape. When an analog tape is copied, the original information signal is actually copied along with any tape noise inherent in the tape and any electronic noise inherent in the recording device. This will be written to a new tape that also has its own level of inherent tape noise. Therefore, the noise level on the dubbed copy will always be greater than that on the original tape, or the sound quality of the original recording will be altered as it is filtered to reduce noise. The presence of noise in the recording will make the recorded information less distinct to see or hear. (Recording engineers refer to a signal-to-noise ratio, which defines the quality of the recording with a higher value being better.) Digital tape recordings are virtually unaffected by tape noise, even though digital tapes are not noise free. In digital recording, binary numbers (comprised entirely of ones and zeros) are read from and written to the tape. The ones and zeros are easily distinguished from the background noise. In an analog recording, the recorder cannot distinguish between the recorded signal and the tape noise so that both are read and reproduced on playback. In addition, digital recordings usually have an error correction system that uses redundant bits to reconstruct areas of lost signal.
Analog recording continuously records the complete signal heard or seen by the recording microphone or video camera. However, distortion in both recording and playback will vary with the quality of the electronic components used. In digital recording, the source signal is quantized to a fixed number of allowed signal levels. For example, a video image quantized at 8-bits/color would only allow for 256 distinct colors to be reproduced, whereas an analog image would allow an infinite number of colors. By increasing the number of bits/color used, the number of color levels that can be reproduced will increase (see bit in the Glossary for more detail). For example, an image quantized at 24-bits/color will allow 16,777,216 distinct colors. With digital recording, higher quality video images require greater storage volumes. Some audiophiles with highly trained ears claim that they can hear limitations in a digital CD audio recording (16-bit quantization permitting 65,536 distinct sound levels and a maximum frequency of 22 kHz) when compared to an analog recording of the same sound source.
Analog tape recordings do not require expensive equipment for recording and playing. Digital audio and video equipment which records high frequencies at high speeds and performs the complex tasks of analog-to-digital and digital-to-analog conversion and error correction is relatively expensive.
2.5 Magnetic Tape Recorders
This document is primarily concerned with tape media, not tape recorders. However, in discussing what can go wrong with media, recorders must be mentioned. Audio and video recorders must be maintained in excellent condition in order to produce high quality recordings and to prevent damage to tapes on playback. Dirty recorders can ruin tape by distributing debris across the surface of the tape and scratching the tape. Recorders that are not mechanically aligned can tear and stretch tape, produce poor tape packs, and write poorly placed tracks. Recorders that are poorly aligned electrically can cause signal problems that will result in inferior playback. Follow the manufacturer's instructions for good recorder maintenance in order to protect recordings.
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