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Scratches on Optical Media

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April 27, 2009 by admin

Scratches on the Laser-Reading Side of CDs and DVDs
Scratches generally cross data lines or tracks on the disc, and how bad (deep and wide) they are will determine the extent of interference with laser focus on the data. Small or occasional scratches will likely have little or no effect on the ability of the laser to read the disc, because the data are far enough below the surface of the disc that the laser is focused beyond the scratch. This is comparable to the effect of a light scratch on a pair of eyeglasses; it does not markedly impair vision because the viewer’s eyes are focused beyond it.

Even assuming a scratch is deep or wide enough to influence laser focus, error detection and correction coding in the disc drive can in many cases recover the misread data. However, scratches that are deep, wide, or bunched together can adversely affect the readability of the disc. These scratches can cause the laser to misread enough data to make error correction coding ineffectual.

While data errors generated from scratches that run outward from the center of the disc stand a good chance of correction by the error correction firmware, scratches running in the direction of the track, the same direction as the laser reads the disc, are more likely to cause uncorrectable errors. If scratches are deep enough to damage the data or metal layers on the reading side of a disc, the data cannot be read or repaired.

Scratches on the Label Side of CDs
Scratches on the label side of discs can be a more serious problem. Because the reflective metal layer and data layer are so close to the surface of the label side of the disc, they can be damaged very easily. A slight indentation, or pinhole in the metal from a scratch, pen, pencil, ultrafine marker, or other sharp object will destroy the reflectivity of the metal in that area on the other side (laser reading side) and the readability of the data by the laser. This type of damage cannot be repaired.

As with scratches on the laser-reading side, optical disc drives are usually able to read through minor damage easily, even if the damage is caused from the label side. The difference is that this damage is permanent. If the error detection and correction firmware in the disc drive cannot correct the data, it will not be recoverable. Scratches that do not reach through the thin protective lacquer coating should have no immediate effect but may ultimately expose the metal to moisture, air pollutants, or other adverse environmental influences.

Adhesive labels, though also somewhat vulnerable to adverse environmental influences, can provide CDs with extra protection from scratches. The extra layer on printable discs likewise offers protection.

Scratches on the Label Side of Single-Sided DVDs
Scratches on the label side of single-sided DVDs are not likely to pose a problem. The metal layer so prone to damage in CDs is in the middle of DVDs. Its location makes this layer almost impervious to surface scratches; it is in fact unlikely to be affected by any but the deepest scratches—those deep enough to reach the center of the disc where the metal and data lie.


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