CD-R/DVD-R reliability

The questions

Many people are using optical media for their archival needs. They are very affordable and the manufacturers claim a storage life of more than ten years, often a century. However, when such claims are made people are always interested in ensuring that the figures are right and provable (archive management cannot accept to discover in a few years that investments made for a life long storage were not up to the task).

Today, a lot of users comment about the optical discs that cannot be read either just after writing or after a few months or years (much less than anticipated). So, it is a very legitimate question to ask what really is the reliability of the CD-R or DVD-R media and if there are issues when choosing these in the perspective of very long storage. Additionaly, you should be concerned with the requirements for a good storage.

From a more personal viewpoint, I have to admit that I lost a vey large number of written CD-R discs that progressively became unreadable in much less time than expected: From a library of 600+ CD-R discs, I discovered that around 25% where showing unrecoverable errors after less than 2 years of storage. My first containment solution has been to copy everything back to hard disc drives (and it still is my recommended approach to archival, with the addition of redundant RAID storage). But I wanted to know more about the issue and try understanding the root causes and possible definitive solutions if it is possible to keep optical media in good shape.

Some answers

Unfortunately, there are not many sources of reliable (!) information on this issues (sometimes, I had the feeling that my own experience – with all its imprecision – was the most precise experiment I could collect). I tried to collect some of it and to build a set of rules that should be applicable to most cases.

Technologies

The main dye (the sensitive layer of CD-R or DVD-R discs) technologies available on the market:

phthalocyanine: A very stable dye (certainly when associated with silver/gold alloy for reflective layer) able to sustain maximum temperature/humidity/light stress. More transparent leading to a high reflectivity and an excellent compatibility with all brands of optical players.
cyanine or metal-cyanine: This dye is less stable under extreme temperature and humidity and industrial quality seems uneven.
azo or metal azo: Typically blue with a silver reflective layer, this dye is less stable than phthalocyanine. Despite its initially higher BLER rate, it has a shorter lifespan and is more sensitive to all stresses.
Formazan: Typically green Cyanine/PhthaloCyanine dye combination developed by Kodak.

Consequently, it seems useless to judge the quality of the media on its color or on the precise name of the dye compound used. The color is a poor indicator (cyanine and metal azo look about the same blue, and Formazan looks the same green as them on a gold layer) and you rarely can see the raw disc before buying it (at least if you are not buying very large quantities were buying a few more medias just for checking would be reasonable).

Even if it is clear that phtalocyanine should be favored, the differences observed between the different dye technologies seem to be small. I wouldn’t count too much on them and it is often difficult to confirm what dye has been used by the CD/DVD manufacturer.

Storage conditions

Today, it is quite clear that the end user should be very wary of the storage conditions of these optical medias. The feeling of security in front of some apparently robust media is a bad adviser. Very obviously (from what I have been reading and from my own experience), the worst factors are:

Light
Temperature

The more light brought to the media (even indirectly if the DVD-R is still in its cristal case), the more risks you are taking. One simple experiment you can do is to wait for next Summer and put one blank disc behind a glass window (inside your home or office) with a black tape crossing it (in order to cover only a part of the disc). If you wait a couple of weeks and remove the tape, you will notice that the changes to the sensitive media are so huge that the shadow of the tape is visible: What was exposed to light has been burnt and the tape-protected zone is in a much better shape. At this point, the disc is no longer usable for burning your data archives.

It appears that the worst part of the spectrum is Ultra-Violet light, but white light (sun light behind a glass window is nearly completely exempt of UV) is still a big issue.

Furthermore, the same issue appears if the CD-R is stored at a temperature exceeding the ususal 25Â°C (also known as room temperature). Just remember that if you don’t have air conditionning, it is quite common to see the temperature rise to more than 30Â°C during the Summer, even for short periods of time. This is dramatically reducing the expectations you can have from your blank medias.

Up to now, I have been discussing of blank medias, but you should keep in mind that in many ways the burnt CD-R or DVD-R is even more sensitive to these effects (light/UV aging and temperature degradation) and, once there is data on the disc, you are less tolerant to these damaging effects (instead of rendering useless a 1â‚¬ DVD-R, you will loose important data).

All in all, both unrecorded and recorded disks should be archived in clean jewel cases in a stable storage environment of 10Â°C-15Â°C and 20%-50% RH, and cautiously protected from sunlight and other radiation sources.

Humidity

Usually humidity seems to have less influence on the dye degradation with time (even though there is some influence anyway). However, there is one situation where humidity becomes a major factor: oxydizing the reflective layer. Depending on the way the disc was sealed (the way all layers of the disc were glued and the side of the stack was sealed in a water-proof and air-proof way), humidity may enter the disc from the edge. The first effect may be very visible: the reflective layer being oxydized will quickly turn from silver to grey or from golden to reddish/brownish.

If you have discs already showing this problem, you may be in luck: the CD disc is burnt starting from the center and is 99% of all cases (including CD audios) the festooned grey area is starting from the outside edge. If you are attentive to this, you can catch and correct the issue before it becomes a problem.

Mechanical storage conditions

This factor is less important than the others, but it’s a simple rule to follow: Always store your CD or DVD in vertical position. That way, the disc is suspended from the central hole. It limits the mechanical efforts and it minimizes the potential contacts between the optical surface and other materials.

Best end-of life indicators

Knowing these damaging conditions, you may want to know how to recognize bad media (not really caring where the problem comes from: bad specification, bad material, bad manufacturing, bad storage). As many of us noticed, just reading/writing the disc on a computer is not enough. Usually, the CD/DVD drives include so many error correction solutions that you are not informed of any problem until the last moment: Until when it is too late to read anything.

There comes the “Measures of CD-R longevity” paper. It is comparing a number of indicators that may be used to evaluate the quality of a media. As they indicate, most people tend to use BLER (Bit-Level Error Rate), but it has has too drawbacks: It is not easy to measure and it is a poor predictor of life expectancy since it tends to overemphasize errors that are quite easily and commonly corrected. The authors recommend the E22, E32, and BURST errors as good indicators since they tend to give an advance warning (indication that things are already gone wrong while it is still possible to read and duplicate the media).

Tools

If we agree on the good choice of media and on good indicators, there is still the issue of using the right tools in order to evaluate the quality of these DVD/CD optical medias. Most products on the consumer market do not even include the bare minimum in this aspect. But some testers could be used. My own selection would include:

Nero CD-DVD speed (part of the NERO CD-burning software), because it includes a number of disc quality criterias (C1/PIE, C2/PIF, and – with certain BenQ drives, including Benq 1640/1650/1655 – E22, E32).
Plextor’s PlexTools only works with Plextor drives (and refers to E32 errors as “CU errors”)
Kprobe, with Liteon drives
DVDInfoPro only provides PI/PO/Jitter tests, but is not freeware ($20).

References – Bibliography

Sources

Several sources of information have been used to reach this level of information.

Stability Comparison of Recordable Optical Discs – A Study of Error Rates in Harsh Conditions [PDF file, 2004] included an evaluation under less than optimal conditions. Interestingly, one of the important factors was the sustained illumination of the discs. The paper provides a detailed description of the tests used.
How Permanent is CD-R Media? Understanding CD-R’s Variables [2001] provides quite old data but is describing the exact differences between different dye technologies and the consequences on the compatibility with different CD-R writers.
Archival media for the masses [2005]
Measures of CD-R longevity provides a detailed comparison of longevity indicators for CD-R.
FAQ about CD – Frequently Asked Questions