Re: Long Lasting CD and DVDR Discs

http://www.thexlab.com/faqs/opticalmedialongevity.html


How is optical media longevity estimated?
 The longevity of a product, like optical media, is estimated using Accelerated Life Tests (ALTs). ALTs are designed to estimate the life span of a given product in normal use based on its performance during rapid aging. Rapid aging simulates a lifetime's wear and tear by exposing the product to extreme levels of the stresses to which it would be subjected in regular use, resulting in product failure within a limited time period. The shortened product life span observed in the ALT is multiplied by an acceleration factor to estimate the expected life span of the product in normal use. For optical media, stresses of temperature, relative humidity, or both are typically applied as elevated levels of these stresses have been shown to accelerate the natural degradation of the dyes used in writable optical media. The International Standards Organization (ISO) currently defines two standards for testing optical media: • ISO 18921:2000 for CD-ROM media. • ISO 18927:2002 for CD-R media. Research has demonstrated that ISO 18927:2002 is applicable to estimating the life expectancy of DVD-R media and it has been applied in ALTs involving all recordable DVD formats. [2] In a typical optical-media ALT, multiple sample discs are exposed to stresses for a period of time ranging from tens to hundreds of hours. Discs are then tested for indicators of impending or actual failure, the end-of-life criteria for the test. The stress-and-test cycle is repeated until each disc meets the end-of-life criteria. Indicators of impending failure include unacceptable levels of jitter, Block Error Rate (BLER) for CDs, and Parity Inner Error (PIE) for DVDs. Uncorrectable errors — E32 errors on CDs and Parity Outer Errors (POE) on DVDs — indicate disc failure. [3] Mathematical models are used to compute acceleration factors and hence estimated product life in normal use. The model employed depends on the stresses applied in the ALT:    
   - When temperature is the only stress applied in the ALT, the Arrhenius model is employed. 
   - When temperature is combined with other stresses, such as relative humidity or exposure to sunlight, the Eyring model is employed. 
 The subject of Accelerated Life Testing is rich and complex. Readers interested in a deeper understanding of this subject should consult the following references:    
   - The "Reliability" chapter of the NIST/SEMATECH e-Handbook of Statistical Methods. 
   - ReliaSoft’s Accelerated Life Testing Reference. 
  

https://www.loc.gov/preservation/resources/rt/studyofCDlongevity.pdf
   Multiple Copies of Optical Discs It is necessary to create redundant copies of stored digital data. The integrity of all digital data is maintained by the 
production of what is termed redundant data, but which is clearly anything but redundant if the reliability of the data is important. At least three copies of each optical disc should be kept, one MASTER, stored under optimal conditions, one WORKING copy to be used for access purposes or copying, and one SAFETY copy to be stored at a different location. 
Providing suitable media is available, the MASTER and the SAFETY copy should be recorded on discs from two different producers. This is especially the case if CD-Rs or DVD-Rs are the target media, as these differ in reflective layer, dye formulation and other critical components. Such a strategy mitigates the risk associated with a particular formulation.