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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. STRICTLY LIMITED STOCK 90 DIGITAL STORAGE $ OSCILLOSCOPES by Ian Hickman. 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