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WHY DO DVDS BECOME CORRUPTED OR UNPLAYABLE?

DVDs, they were supposed to last forever and could never get damaged enough so they would not play! That is what we were all told, and why many of us transferred our precious tapes onto them and then threw away the tapes. Fast forward 15-20 years or so and come to play a DVD, it does not work. How can this be? What can be done about it? Have I lost my videos completely? Couple this with the fact that DVD drives on new computers are disappearing fast and fewer people now have DVD players at home, then you have to question whether is a good medium to save your family memories on, or to now transfer them to today, certainly without having another backup on USB or cloud.



Adhesive Labels on the DVD


This one of the biggest problems we get with DVDs. Often DVDs had an adhesive label glued to the top of the DVD with a title on or the name of the company that transferred the footage to the disc. All well and good at the time.

Over time the adhesive and the paper shrinks, and this warps the disc so it will not play properly in any player or PC disc drive, the laser just cannot read the information. The disc probably played fine 10 years ago, but now it doesn't. In order to play the disc or extract the files it must be flat, the only way to do this is to ease the disc back into its original flat state by weight, heat, and scoring the label sometimes. Most times we can get most of the original files back, if the disc is badly warped then it is, sometimes, not possible. If you have DVDs with full size labels on the top then get them copied not before its too late.


Disc Flexing


Flexing (bending) the disc by any means, such as removing it from a jewel case or sitting on it, may harm the disc by causing stresses. The disc should be stored in its case and placed vertically, like a book, on a shelf. Long-term horizontal storage, particularly in a heated environment, can cause the disc to become permanently bowed. While the data may still be intact, the disc may not operate properly in the drive or permit the laser to follow the track. The maximum degree of flex (bend) or number of times a disc can be flexed before it incurs damage is not known. To minimize the risk of damage, it is better to avoid flexing discs.


Scratches on the Laser-Reading Side


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. These uncorrectable errors are called E32 in the Red Book for CD specifications, and PO Error in DVD specifications.

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.


Moisture


The polycarbonate substrate, or the plastic composition, that makes up most of the disc is a polymer material that is vulnerable to moisture. Any prolonged exposure to moisture resulting from a spill, humid air, or immersion allows water to become absorbed into the disc, where it may react with any of the layers. Returning the disc to a dry environment will allow the absorbed moisture or water to dissipate out of the disc over time; however, water or a water-based liquid may leave behind, within the disc, contaminants such as dyes or other dissolved minerals. If the disc has experienced no permanent damage from absorption of the liquid, it should play normally. In NIST tests, a CD totally submerged in clean water for 24 hours was found to be unreadable initially after removal and surface drying. It played normally, however, after 24 hours of drying out at approximately 70¡F and 50% relative humidity (normal room conditions).


Direct Sunlight & Heat


DVD discs can become unusable in a matter of days. If such a disc is left in an environment that allows direct sunlight and extreme heat buildup (e.g., on the dashboard of a car in summertime, or next to a heater by a window), the organic dye or phase-changing film that holds the data will degrade quickly, causing the disc to become unreadable. A disc is not protected from the effects of heat buildup if left in a case that is exposed to direct sunlight or other sources of heat. Extreme heat buildup can also cause warping of the disc.


The Disc is Not Finalised After Recording


When you write to a DVD without finalizing it, the directory information is written to a temporary area after the most recent recorded session. Players that can read unfinalized DVDs look at these areas in reverse order to get the most recent directory information. You can always add more information to unfinalized DVDs, which is why it’s done this way.

However, in order to guarantee full read-compatibility with the majority of DVD players (including DVD writers, DVD burners and DVD recorders), the directory has to be written to the standard place on the disc. This process is called “finalization” and once the disc has been or finalised it can’t be added to. Windows Media Player 10 calls the process of finalisation “closing” instead (and doesn’t give you the opportunity to bypass it).

There’s more we could say about the process of finalization, but to keep it short- for best compatibility, write everything you want to the disc, then finalize it.


Fingerprints, Smudges, Dirt, and Dust


Fingerprints, smudges, dirt, or dust on the laser reading side of the disc can disrupt laser focus on the data even more than a scratch can. Dirt or dust on the disc will block or reduce the light intensity of the laser. If severe enough, it will cause the disc drive to miss data as the disc is being read. Fingerprints, smudges, or dirt cover wide areas of data and will cause the laser beam to go out of focus or lose intensity. They will also cause widespread misreading of data along the data lines or tracks, to an extent that exceeds the error correction capability of the disc drive. Dust can also spin off into the disc drive and collect on the laser head or other internal components. Fingerprints, smudges, and dirt are easier to remove than scratches; it is simply a matter of cleaning them off.


There are other reasons why DVDs might become corrupted and unplayable but these are the most common issues.

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