Number 214 - March 2001

Do Compact Discs Degrade?
by Bob Starrett, from Adaptec's mailing list: Submitted by Carl Tenning" ctenning@hotmail.com
The answer to this question is easy and apparent: of course compact discs degrade. As with every other material, eventually time, temperature variations and the elements will take their toll, on the disc as a whole, as well as each element that comprises that disc. The real question is, how does disc degradation affect readability?     According to the Optical Storage Technology Association (OSTA), CD-R discs have a long lifespan:     "Lifetime estimation of any storage medium is a very complex and statistics-based process. Improper testing or measurement procedures can seriously impact and compromise lifetime estimates. ANSI Committee IT-9 is currently developing comprehensive guidelines for CD-ROM and CD-R life estimation. The CD-R media manufacturers have performed extensive media longevity studies using these industry-defined tests and mathematical modeling techniques, with results claiming longevity from 70 years to over 200 years. The primary caveat is how you handle and store the media. With proper handling and storage, your CD-Rs will outlive you."     It is one thing to say that discs will last that long. But while we grow old waiting to find out, we need to take some precautions and not rely stiffly on numbers such as these.     How long can you store a disc before you need to possibly reburn it to preserve your data? And is a CD really the best place to store important data for a long period of time? A comprehensive study of this topic is best left to someone or some company that has the deep pockets to perform it. The resources necessary to burn, test, age and retest media of different dyes, reflective layers and coatings are enormous.     In the meantime, the best we can do is use existing surveys and anecdotal evidence as a guide. While somewhat old, two studies are also available that provide a little insight into how the media manufacturers look at the longevity issue.The studies are rather technical, but if you want to look at them you can find them here:     TDK - www.cd-info.com/ CDIC/ Technology/ CD-R/ Media/ TDK.html     Kodak - www.cd-info.com/ CDIC/ Technology/ CD-R/ Media/ Kodak.html     US News and World Report also has an interesting article with a useful chart comparing the lifetimes of several types of storage media. It draws the startling conclusion that unless you buy certain high-quality brands, your CD-R discs may last as little as two years! But note that the National Media Lab took issue with the manner in which its data was used to produce the chart and does not agree with the conclusions drawn. www.usnews.com/ usnews/ issue/ 980216/ 16digi.htm     It sure would be nice to conduct our own study, but let's do the best we can with what we have on hand.	Let's look at some really old discs and see if they are readable. In my personal collection, I found three CD-R discs that were at least 10 years old. These discs have not gone through any aging process or artificial stimulus. They have had a nice home in a jewel case, have made several trips to California and back, been joggled, of course, but otherwise they should be in good shape. They are clean discs without scratches, the kind that go into a jewel case right after recording and stay there.     Of the three, one was unreadable in four different models of Plextor drives, while it worked fine in a Kenwood 72X TrueX CD-ROM drive. But the fact that the Plextors could not read it and the Kenwood could, caused me to make a copy of it off the Kenwood, because this ten-year old disc contains data that I may need in the future. I feel safer now with the original and a backup that the Plextor drives are happy to deal with.     When will I check it again? Probably in a couple of years. Does this mean that you have to check all of you discs every so often to make sure they are still readable? Well, not everything, but do check your most valuable discs at least every year or two. Hopefully, you don't have so many that this becomes an undue burden. Just try to save the checking and re-recording for truly valuable discs of irreplaceable material, rather than copies of popular audio CDs, for example.     The other two discs were both readable in a professional CD tester and the Plextor drives, as well as the Kenwood drive. One had an error rate in the CD tester comparable to a freshly recorded CD-R disc, and the other had error rates higher than I would like, but still well within the maximum error rate considered safe for CD-ROM discs.     These discs were not tested for errors when they were made, so we do not know how the current error rates compare to the original and therefore cannot really judge degradation. But we do know that out of three ten-year-old recorded discs, one was on the edge, another was middling, and a third was just fine after all that time. Obviously too small a sample to draw any conclusions from, but the best we can do with what we have. One additional thing to note is that CD-R media has improved in the interim, at least discs from quality vendors have. There are also a lot of no-name brands now that may be worse than our 10-year-old discs.     If your job is to manage a large collection of stored data on CD-R for a company, then you have a rather large task ahead of you setting up protocols and procedures for archival disc testing and re-recording. But for most of us, the time spent checking key discs will be minimal.     So the controversy over the actual life expectancy of CD-R discs continues and our quick anecdotal testing does not solve the mystery. Just be careful with crucial data, use good quality media, and store and treat it correctly. Chances are that you have at least 10 years before you need to start worrying.
Number 214 - March 2001