CD rot

CD rot (or DVD rot) is a phrase describing the tendency of CD or DVD disks to become unreadable due to physical or chemical deterioration. The causes of this effect vary from oxidation of the reflective layer, to physical scuffing and abrasion of disk surfaces or edges (including visible scratches), to other kinds of reactions with contaminants, to ultra-violet light damage and de-bonding of the adhesive used to stick the layers of the disc together.

In CDs, the reflective layer is immediately beneath a thin protective layer of lacquer, and is also exposed (again protected by a layer of lacquer) at the edge of the disk. The lacquer protecting the edge of an optical disk can usually be seen without magnification. It is rarely uniformly thick; thickness variations are usually visible. The reflective layer is typically aluminum, which reacts easily with several commonly encountered chemicals (eg, oxygen, sulphur, some ions carried by condensed water, ...). In ordinary use, a surface layer of aluminum oxide is formed very quickly when an aluminum surface is exposed to the atmosphere; it serves as passivation for the bulk aluminum with regard to many (but not all) contaminants. CD reflective layers are so thin that this passivation is less effective. In the case of CD-R and CD-RW media, the materials used in the reflecting layer are more complex than a simple aluminum layer, but also can present problems if contaminated. The thin (10-20 thousands of an inch) layer of protective lacquer is equivalent.

DVDs have a different structure than CDs, using a plastic disc over the reflecting layer. This means that a scratch on either surface of a DVD is not as likely to reach the reflective layer and expose it to environmental contamination and perhaps to cause corrosion, perhaps progressive corrosion. Blu-ray disks are more like CDs in that the reflective layer is at a disk surface, though in Blu-ray disks, it is the reading surface, not the label surface. Blu-ray disk producers have developed several surface coatings which are intended to reduce susceptibility to scratches and abrasion. Each type of optical disk thus has different susceptibility to contamination and corrosion of its reflecting layer; furthermore, the writeable and re-writable versions of each optical disk type are somewhat different as well. Finally, disks made with gold as the reflecting layer are considerably less vulnerable to corrosion problems, though no less susceptible to physical damage to that layer. Because they are expensive, the industry has adopted aluminum reflecting layers as the standard for factory pressed optical disks.

Types

On CDs, the rot becomes visually noticeable in two ways:

# When the CD is held up to a strong light, light shines through several pin-prick sized holes. [http://news.bbc.co.uk/1/hi/entertainment/music/3940669.stm BBC: Is music safe on compact disc?] ]
# Discoloration of the disc, which looks like a coffee stain on the disc (see also CD bronzing).

In audio CDs, the rot leads to decreased audio quality, chatter, scrambled audio, and static.

A Philips press officer has declared CD rot to be an isolated problem affecting only an "absolute minority" of cases. PDO has offered to replace any discs affected by CD bronzing if supplied with the defective disk and proof of purchase. However, according to the website of one of the affected record companies, Hyperion, PDO's helpline was discontinued in 2006 after a change of ownership, and defective CDs are now no longer replaced by the manufacturer, even though some of the affected record labels continue to offer replacements. [ [http://www.hyperion-records.co.uk/bronzed.asp Bronzing CD's ] ]

ee also

*Laser rot
*CD bronzing

References

External links

* [http://www.macobserver.com/article/2004/05/06.4.shtml Mac Observer article]
* [http://www.classical.net/music/guide/society/krs/excerpt3.html "CD Bronzing" article, with PDO replacement information, at Classical.net] - How a manufacturing problem can cause disk quality degradation.
* [http://www.slais.ubc.ca/people/students/student-projects/C_Hill/hill_libr516/print.htm "Using CDs for Data Storage"] article, with extensive footnoting. Much of the information contained here contradicts central tenets in this wiki entry.
* [http://www.smh.com.au/articles/2003/01/31/1043804519345.html A bad case of DVD rot eats into movie collections]
* [http://www.avsforum.com/avs-vb/showthread.php?t=857067 Blu-ray rot on copies of "The Prestige"] - AVS Forums