The market of optical media — writable CDs, DVDs and Blu-Ray discs — may see a decline, but that doesn’t mean these media will be history any time soon. They are invaluable for archiving multimedia content, and medical and administrative data. Cloud storage is fine, but there are problems with ownership, copyright, and for how long the data remains stored. These issues are irrelevant to users of optical media. They worry more about longevity.
Long life for any information storage medium is always a combination of manufacturers’ and users’ responsibilities. Several manufacturers of recordable A-brand optical media products, such as Kodak and Verbatim share information that helps users of CD/DVD/Blu-Ray technology understand all aspects of media longevity.
Exceptional data life is one of Kodak’s design criteria, determining the choice of all materials used in making Kodak writable CDs and DVDs, particularly the recording layer. Repeated tests show that 95% of Kodak writable CDs will have a data lifetime of greater than 200 years if stored in the dark at 25°C, 40% relative humidity (RH). Stored in an office or home environment, the lifetime should be 100 years or more. The same applies to Verbatim’s Archival Gold and Medical archival media.
Compared with other digital storage media (e.g. magnetic tape or hard disks), optical media have much longer life expectancies. Accelerated-aging tests, which speed up the reactions of decay, can determine the rate at which slow chemical changes can make discs unreadable. When tested this way, Kodak’s writable media stored under archival conditions showed a life expectancy of around 200 years. By contrast, magnetic tape storage media will only last a few decades, and cloud storage is even a more uncertain path. While this is a powerful advantage for optical data storage, there are other important aspects to consider.
Data stored on CDs, DVDs and Blu-Ray discs is encoded in digital form. The data must be read by a reading device and converted into music, text, images, and so on by software applications. Because the real goal is access to the discs’ contents the survival of playback hardware devices and software formats is key. Migration of digital data from one storage medium or software standard to another is essential.
Because migrating data requires effort and expense, there is a risk that it will be neglected. History teaches that many things are preserved only by accident, merely because they were tough enough to endure until they could be valued once more by new owners. Optical media will physically survive long enough to be rediscovered by new generations, but the reading devices will have long gone. Therefore, it is prudent to create redundant copies of stored digital data. If images or data, for example, are recorded on film as input or output from digitization, the film serves as a collection back-up. Having two copies in different forms and in different physical locations increases the chances that the information will survive.
In addition to redundancy, a key aspect of archiving information on writable optical media is the principle of master copies and derivatives. DVD, CD and Blu-Ray discs can be copied over and over again without loss of information or quality. This means, for example, that multiple departments of an organization can have and use their own discs, all for very low cost. These copies, used for different purposes, are the derivatives.
Someone should have responsibility for the creation and physical care of a master. This master copy should be stored properly, be physically secure, and be handled only to make derivative copies. Such a strategy will help to ensure both long life and economical access to stored data for many years.
How long and under which circumstances?
Leaving aside scratches, fires, floods, and concentrating on the slow chemical changes that determine the inherent life expectancy of a disc that uses chemicals to enable recording, extensive accelerated-aging tests suggest that A-brand writable CD/DVD/Blu-Ray products, will not reach a maximum Block Error Rate (BLERmax) of 50 for a period of around 200 years when kept in the dark at moderate storage conditions. This long potential life expectancy is only possible with archival media that use a special dye as the one used in Kodak Gold and Professional DMD, and Verbatim’s “professional” grade products.
Considering that BLERmax 50 is still not an unreadable level of error, there is a chance these media will survive for even longer periods of time. Similar research by the 3M Company showed that CD-ROM products made by them will not attain a block error rate of 50 per second for more than 100 years in moderate storage conditions.
Accelerated aging is subject to uncertainties, but it does rest on firm scientific footing. Behind the data is the simple assumption that raising the temperature causes the reactions of decay to happen faster — so fast, in fact, that they occur within a few months, rather than decades. The science of reaction rates is called kinetics, and the lifetime predictions are based on well-established principles of that branch of chemical science. Because there is so much practical experience with the laws of kinetics, lifetime predictions based on them are approximately correct.
Read about the test I conducted on Verbatim and Kodak archival media.