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Published: Friday, 7/9/2004

DVD rot legend bends the truth

Urban legends are stories that often begin with a grain of truth. Each retelling, however, embroiders the facts with a few more fictional threads. The stories spread fast and far, winding up as crazy quilts that exaggerate the facts.

They rarely die. Instead they wind up on Internet sites. The yarns are just believable enough to trick people who encounter them for the first time. Then the tall tale gets into circulation again.

The DVD rot myth is following that pattern.

It s making the rounds once again worrying people who own movies in DVD format and use DVDs for long-term storage of precious digital images and other data.

Rohan Byrnes, an Australian engineer, discovered the grain of truth in 2002. Some of his DVDs showed signs of corruption. The discs wouldn t load, skipped during playback, or stuck. Oddly, there were no surface scratches usually responsible for those problems.

Mr. Byrnes analyzed the discs, and found signs of corrosion in the reflective aluminum layer sealed inside the clear polycarbonate plastic discs. DVDs, like CDs, store data in a pattern of tiny pits written into the plastic. A DVD reader shines a laser beam through the plastic and it reflects off an aluminum coating inside the pits.

Corrosion darkened the pits, making them unreadable. It apparently resulted from problems during the DVD manufacture. Mr. Byrnes found corrosion in only a handful of DVDs, and the problem apparently was limited to DVDs manufactured before 2001.

However, DVD rot still gets blame when discs go bad. That s bad, because it diverts attention from the real threat to those expensive movies and irreplaceable digital photos: Mishandling by the consumer.

DVDs may look just like high-capacity CDs, but internal differences make DVDs more delicate.

For instance, DVDs are made from two pieces of plastic bonded together, rather than the single piece in CDs. That makes the internal layers, which hold data, more vulnerable to damage from stress when a DVD is bent.

DVDs also pack data into smaller pits and smaller tracks, the circular patterns winding around the disc. Unwind the data tracks from a typical DVD, and they would stretch for 30 miles.

Even tiny surface scratches can disrupt smooth reading of data. The disc must stay absolutely flat for the increasable precise tracking needed to read the data. Repeated bending or flexing can warp a disc, throw off the tracking, and make the DVD unplayable.

The bottom line: Handle those DVDs with great care, even more than you use for CDs.

Avoid any contact that could scratch the surface.

Avoid storing DVDs in CD jewel cases, or other cases that fit too tightly at the hub, and make you bend the disc during removal. Each bend flexes and stresses the internal coatings. Removal should be effortless. Some DVD cases have a central rosette that releases the hub when pressed.

Never leave a DVD in direct sunlight, which can damage the interior coatings



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