Figures not yet available
All forms of high-density information storage require tender-loving-care if it is to survive centuries--or even decades. Books, films, CD's and tapes must all be stored in a COOL and DRY environment if they are to be preserved. The ideal archival medium for high-density data storage has not been discovered yet!
Magnetic tape is as good an archival medium as any that we know. There are many mis-conceptions about magnetic tape, and this paper will shed light on some of these.
Magnetic tape recording was first developed in Germany in 1934 but did not achieve world recognition until Ampex introduced the first professional quality audio recorder in 1948. Eight years later, Ampex introduced the first practical videotape recorder--in 1956. These two inventions have played a critical part in the technological revolution we are experiencing today.
When you think of videotape preservation, you should be thinking about the tape recorder as well as the tape. The tape recorder must be kept clean and properly aligned so it will not damage tape. Also, if you plan to keep tapes for many years after the format loses popularity, you should obtain the service manuals and a stock of critical spare parts and/or spare recorders. Another alternative is to transfer the old format to a newer and better format.
If you follow my advice, you will find that magnetic tape is a good archival medium.
The term "format" is used to describe the physical dimensions of the recording on the tape and also such things as the cassette size, the width of the tape and the speed in which the tape is recorded and played. The original videotape format was the Ampex 2" quad. The enclosed videotape format chart includes only the most popular videotape formats. There have been many formats that were introduced but never became popular.
Betacam-SP is a good archival format and cost $12,000 to $25,000 per new recorder. Cheaper versions are available but not recommended for archival use. Type C is also a good archival format but it is much more expensive then Betacam-SP and also is no longer in production. Both Betacam-SP and Type C are analog.
Figure 2 illustrates the four categories of videotape formats. I will not describe Composite and Component other than to say that Composite is the combination of sync, black/white video and color video signals and uses only one cable. A Component TV system keeps the signals separate and requires two or three cables.
The term Analog is used to describe the special FM method used to record the information onto the tape. Analog has the advantage of being inexpensive but the disadvantage of being very susceptable to variations in the head/tape interface.
Digital is the best archival medium because digital is the answer to two of the main archival problems:
With digital, the unprocessed off-tape data error can be measured--which is a measure of the quality of the recording. This takes only a minute or two and can be done periodically--such as annually. The error rate can be tracked over the years and if the error rate ever reaches a pre-established questionable level, then the tape can be copied. During the copying process, the error correction circuits are used and the copy will be a clone of the original. I have used two D2 recorders to make 40 generations and could barely tell the difference between the original and the 40th.
With digital, a section of the tape can be severely damaged and have little, or no, effect on the picture. The reason for this incredible feat is that the data is recorded in binary form (zeros and ones) and the incoming video is shuffled and distributed over a large area of the tape. These two factors make it easy for the decoding electronics to reassemble the original video picture--even if a large amount of the data is missing.
I do not recommend using the consumer VHS for important archival material. This analog format was developed to make videotape recording available to everyone and was not designed with long life in mind. If you must use VHS, then use industrial VHS VCR's which are more rugged then consumer models. The S-VHS format is a much better format than the VHS--especially if you use a playback machine with automatic tracking. I consider the S-VHS to be a good archival format, but not as good as the Betacam-SP.
Two important points to remember about formats are:
Regardless of what format is used, the following are the most common tape problems:
The sticky tape/powder problem can be temporarily relieved by baking the tape for at least 8 hours at 55°C (130°F) and an extreme case may require 18-24 hours. A convection oven is recommended for this procedure. This heating process makes the tape usable for a few weeks and can be repeated many times. I recommend copying any tapes that develop this problem because their long-term durability is questionable. The second problem, binder degradation, can sometimes be reversed by storing the tapes in a cold and dry environment for a couple of weeks. The third problem of tape damage is usually caused by one edge of the tape being curled and is the result of an improperly aligned tape transport. A severe case of edge damage, pleating, or creasing is usually difficult to play, but I have developed a method of correcting the problem so that the tape is at least playable.
To minimize the risk of developing tape problems, I recommend storing tape at a humidity of around 25% RH and at a temperature less than 22°C (700°F). Around 5°C (40°F) is the best long-term storage temperature if you are really interested in permanence. I developed the enclosed chart (Figure 3) 1 to illustrate what is meant by a cool and dry environment--which is the area to the lower-left. Humidity variation should be less than ±5% RH and a temperature variation of less than ±2°C.
The primary enemy of tape life is high humidity. Several studies have indicated that 20% RH to 30% RH is necessary to prevent the deterioration of the tape binder. Installing a dryer in your air conditioner system is one possible way of controlling humidity. If the tape storage room is well insulated, a relatively small dryer can be used. The room should be able to maintain the proper environment for about two days--in case of a major power failure. The room must be fireproof and not contain wood boxes, cardboard boxes, or wood shelving. If an overhead sprinkler system is used, design the shelves so that sprinkler water will not contact any of the tapes. Also, don't store tapes on the floor.
An alternative to storing tapes in a humidity controlled environment, is to seal each tape in two quick-seal plastic bags. Thick freezer quick-seal bags will be adequate--provided the room temperature is kept fairly constant. Before sealing a tape in a bag, expose it to a very dry environment for a few days. This can be done at a time of the year when the room is dry or you can use a small temperature/humidity chamber. This will remove moisture in the tape which would be harmful if trapped in the bag with the tape. (In both cases, remove the tape from its container before conditioning it.) Use a couple of rubber bands to hold the two bags in place so the cassette label is easier to read.
Environment: 25% RH and as near 5°C as possible.
Storage area: Well insulated, fireproof, no sprinkler system, and restricted access.
Environment: Below 50% RH and below 22°C
Storage area: Restricted access
Environment: Below 50% RH and below 22°C
Storage area: Controlled access but not necessary to be restricted.
I would like to clarify some mis-conceptions about the permanence of magnetic recordings. Tests indicate that the magnetic recording itself should last hundreds of years. It is very difficult to erase a tape unless you press the Record button on the machine. The Record function was designed to erase the tape! I recommend breaking off the cassette record tab and also disabling the Record function on your archive tape machines. The record/erase circuits can be disabled by a key-lockout so only the manager of the Archive can record a tape.
X-Rays from airport security systems will not harm tapes. But, the electric motor under the conveyor belt is a potential hazard, so pack the tapes so they are at least 1/2 inch inside the package. Any magnet, transformer or electric motor has to be nearly in contact with the tape to have an effect, and, a 1/2 inch spacing is sufficient. The magnetic fields of a TV set are not large enough to erase a tape placed on top the set. In summary, tape erasure is not a problem provided you eliminate the possibility of recording over archival material.
When shipping tapes, I recommend packing them in a well insulated package to reduce the temperature and humidity variations encountered during shipment. Also, ship by overnight express mail to eliminate the possibility of the package being exposed to high heat or severe cold while sitting on a loading dock.
Tests indicate that Metal Particle tape life expectancy is comparable with iron oxide tape. The main brand tape manufacturers understand that the magnetic particles must be properly coated during the manufacturing process so the metal particle is never exposed to oxygen. A properly made metal particle tape has a magnetic life expectancy of about 100 years if stored at 22°C/70°F and about 1,500 years if stored at 5°C/40°F (see figure 4)2.
Low temperature storage also means you never have to rewind your archived tapes! That's because the tape pack stresses decrease as the temperature decreases. Figure 53 illustrates that a tape stored at 5°C/40°F requires rewinding about every 100 years! The problem with low temperature storage is that the tape should be wound in an environment similiar to that in which it is to be stored. This can be done by locating the tape winding machine inside the low temperature storage room. Also, when tapes are taken out of cold storage, they must be warmed up for several hours in a room with low humidity--to prevent condensation.
So far, there is no reliable method that will predict the life of tapes. An American National Standards Institute (ANSI) committee is currently considering development of a method for estimating Life Expectancy for tapes. Twenty years is a reasonable life expectancy, and a hundred years is possible for most tapes if they are stored at a low temperature of around 5°C (40°F) and a humidity of around 25% RH.
Most archived tapes are poly(ethylene terephthalate) (PET) basefilm and the magnetic particle is bound to the basefilm by a polyester urethane binder. The most common cause of long-term tape degradation is the hydrolysis of the polyester urethane binder. For most tapes, the hydrolysis is stable around 25% RH. This hydolysis process was not clearly understood until Edward Cuddihy did some extensive tests at the Jet Propulsion Laborary (JPL). He first published his results in 1976, but in 1982, he collaborated with Dr. Neal Bertram at Ampex and published a more thorough paper4.
Since then, most tape manufacturers have been investigating alternative binders. Dr. David Davies of Ampex, published a paper in September 19935 which discusses metal particle tape stability and also the progress being made in binder stability. Figure 6 from Davies' report is enclosed here as figure 6. It illustrates the progress made in binder stability in recent years.
The information contained in this report is based on experience, theoretical investigations, and accelerated tests. By following the recommendations made in this report, the life of the information recorded on tape will be increased, but there is no guarantee that all information recorded on the tape will be permanent.
1 J. Wheeler, "Long-Term Storage of Videotape," SMPTE Journal, June 1983.
2 Y. Okazaki, K. Hara, T. Kawashima, A. Sato, and T. Hirano; SONY Corporation Sendai Technology Center; "Estimating the Archival Life of Metal Particle Tape"; IEEE Transactions on Magnetics, Sept 1992.
3 N. Bertram and A. Eshel, "Recording Media Archival Attributes (Magnetics),"Contract # F30602-78-C-0181 AFSC Rome Air Defense Center, 1979.
4 N. Bertram and E. Cuddihy, "Kinetics of the Humid Aging of Magnetic Recording Tape," IEEE Transactions on Magnetics, MAG-18, Sept 1982.
5 D. Davies, "Stability of Metal Particle Tapes", National Media Lab Technical Article MI-0002, Sept 1993.
Timestamp: Sunday, 23-Nov-2008 15:20:23 PST
Retrieved: Thursday, 19-Jul-2018 15:41:59 GMT