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Disc rot is a phrase describing the tendency of CD or DVD or other optical discs 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 disc 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 adhere the layers of the disc together.
In CDs, the reflective layer is immediately beneath a thin protective layer of lacquer, and is also exposed at the edge of the disc. The lacquer protecting the edge of an optical disc 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 0.25-0.5mm 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. Each type of optical disc thus has different susceptibility to contamination and corrosion of its reflecting layer; furthermore, the writable and re-writable versions of each optical disc type are somewhat different as well. Finally, discs 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 discs.
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 scrambled or skipped audio or even the inability to play a track.
Laser rot is the appearance of video and audio artifacts during the playback of Laserdiscs, and their progressive worsening over time. It is most commonly attributed to oxidation in the aluminum layers by poor quality adhesives used to bond the disc halves together. Single-sided video discs did not appear to suffer from laser rot while double-sided discs did. The name Laser Rot is a misnomer; the disc degradation does not involve the player's laser.
Laser rot was indicated by the appearance of multi-colored speckles appearing in the video output of a laserdisc during playback. The speckles increased in volume and frequency as the disc continued to degrade. Much of the early production run of MCA DiscoVision Discs had severe laser rot. Also, in the 1990s, LaserDiscs manufactured by Sony's DADC plant in Terre Haute, Indiana were plagued by laser rot.
- Data rot, a similar concept
- CD bronzing
- DVD-D and Flexplay, disposable optical disc formats designed to become unplayable after a limited time
- BBC: Is music safe on compact disc?
- Bob Niland. Widescreen Review Laserdisc Magic 1998. p. 119.
- Mac Observer article
- "CD Bronzing" article, with PDO replacement information, at Classical.net - How a manufacturing problem can cause disc quality degradation
- "Using CDs for Data Storage" article, with extensive footnoting.
- A bad case of DVD rot eats into movie collections
- Blu-ray rot on copies of The Prestige at AVS Forums