Disc rot.
is a phrase describing the tendency of
CD,
DVD,
Blu-ray or other optical 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.
Causes
In CDs, the reflective layer is immediately beneath a thin protective layer of
lacquer, and is also exposed 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 such as
oxygen,
sulphur, and certain 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 thousandths of an inch layer of protective lacquer is equivalent.
DVDs have a different structure from 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 less expensive, the industry has adopted aluminum reflecting layers as the standard for factory pressed optical disks.
Signs of disc rot
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.
- 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.
Variants
Laser rot
Laser rot was a disc rot phenomenon observed by some users of the
Laserdisc format in which audio and/or video quality begins to degrade over time. Laser rot is generally attributed to oxidation in the aluminum layers or inferior adhesives used to bond the discs together.
Laser rot is most noted by the appearance of multi-colored speckles appearing in the video of a laserdisc during playback. The speckles increase in volume and frequency as the disc continues to degrade. Much of the early production run LaserDiscs, titled MCA DiscoVision Discs, had severe problems with Laser rot. Many DiscoVision titles have ceased to work since their pressings in the late 1970s. The
Sony DADC plant in the USA was known for producing discs which suffer extensively from strong laser rot, especially towards the end of the LD era.
Single-sided video disks did not appear to suffer from Laser rot, while double-sided disks did. Not surprisingly, while Laser rot was quietly and selectively acknowledged within companies which either manufactured hardware or software, the existence of Laser rot was not acknowledged in general within the companies, nor outside of the companies, due mainly by their desire to limit negative press in the marketplace.
See also
- DVD-D and Flexplay, disposable optical disc formats designed to become unplayable after a limited time
