March 29, 2007

CD and DVD Lifetime and Maintenance

Life of the medium

The life expectancy (LE) of optical discs depends on many factors, some controllable by the user, others not.
Factors that affect disc life expectancy include the following:
• type
• manufacturing quality
• condition of the disc before recording
• quality of the disc recording
• handling and maintenance
• environmental conditions
As noted previously, the three basic types of CD and DVD discs—ROM, R, and RW and RAM—each use a different data layer material (molded aluminum, organic dye, or phase-changing film, respectively). Deterioration of this material is the primary cause for disc degradation and, ultimately, “end of life” for the disc, assuming proper physical handling.

Environmental factors can affect the rate of disc degradation. In each of the three basic disc types, environmental forces will degrade the data layer much faster than the polycarbonate substrate layer (the clear plastic that makes up most of the disc). Because degradation
of the data layer will render the disc useless well before the polycarbonate begins to deteriorate, the relative degradation rate for the polycarbonate layer is not used for life expectancy considerations.
Physical mishandling of the disc is usually the cause of polycarbonate
layer damage. The polycarbonate may also flex or bend if stored for a long period of time in a nonvertical position.
So what is the life expectancy of a disc? First, we must define life expectancy. For most users, it means the length of time for which the disc remains usable. But that implies some acceptable amount of degradation.

How much and what type of degradation is acceptable?

With CDs and DVDs, the user does not notice early degradation because the error detection and correction capability built into the system corrects a certain number of errors. The user notices a problem only when the error correction coding is unable to fully correct the errors.One method for determining end of life for a disc is based on the number of errors on a disc before the error correction occurs. The chance of disc failure increases with the number of errors, but it is impossible to define the number of errors in a disc that will absolutely cause a performance problem (minor or catastrophic) because it depends on the number of errors left, after error correction, and their distribution within the data. When the number of errors (before error correction) on a disc increases to a certain level, the chance of disc failure, even if small, can be deemed unacceptable and thus signal the disc’s end of life.

Manufacturers tend to use this premise to estimate media longevity.
They test discs by using accelerated aging methodologies with controlled extreme temperature and humidity influences over a relatively short period of time. However, it is not always clear how a manufacturer interprets its measurements for determining a disc’s end of life. Among the manufacturers that have done testing, there is consensus that, under recommended storage conditions, CD-R, DVD-R, and DVD+R discs should have a life expectancy of 100 to 200 years or more; CD-RW, DVD-RW, DVD+RW, and DVD-RAM discs should have a life expectancy of 25 years or more. Little information
is available for CD-ROM and DVD-ROM discs (including audio and video), resulting in an increased level of uncertainty for their life expectancy. Expectations vary from 20 to 100 years for these discs.

Few, if any, life expectancy reports for these discs have been published by independent laboratories. An accelerated aging study at NIST estimated the life expectancy of one type of DVD-R for authoring
disc to be 30 years if stored at 25°C (77°F) and 50% relative humidity.
This testing for R discs is in the preliminary stages, and much more needs to be done.

Individual Disc Storage

Optical discs should be kept in individual storage containers until used and returned to those containers immediately thereafter. Typical
storage containers, as listed below, isolate and help protect discs from airborne contaminants and other foreign material. They also help buffer rapid environmental changes that can cause stresses to the disc. Cases are designed to keep surfaces of the disc from contact with the inside of the case. Only one disc should be placed on the hub (or each hub) in the case. To remove the disc, one should press down on the hub tab while holding the outer edge of the disc with the fingers and then lift up. Bending the disc while lifting it off the hub tab should be avoided.
For long-term disc storage, it may sometimes be prudent to remove
the label insert or booklet from inside the case and attach it to the outside, perhaps in a sleeve. In theory, the paper can attract moisture
and produce higher moisture content in the case. The paper may also spread moisture by contact with the disc. This recommendation is based on no specific tests of the effects of paper inside a case; it is merely a consideration—one that takes on added significance with large amounts of paper inside a disc case and higher-than-recommended
humidity conditions.
Cases commonly used for individual disc protection include the following:
• Jewel case. The jewel case, which comes in different varieties, holds one to six discs, depending on its design. It is typically a transparent
plastic case with a hinged lid, one or more plastic trays, an inlay card for labeling, and an optional booklet.
• Slimline case. As its name suggests, a slimline case is a slimmer version of the jewel case but without the tray. It comes with an inlay
card (J-card) and is primarily used for audio discs.
• Amaray case. An amaray case is a plastic case used for commercially
available prerecorded (replicated) DVD videos and games.
• Snapper case. An alternative to the amaray case, the snapper case is a plastic DVD case with a cardboard cover that is snapped shut and held in place by a plastic lip.

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. 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.


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.
To summarize, the effects of scratches versus fingerprints and smudges on the laser reading side of a disc include the following:
• Occasional fine scratches will typically not affect the focus of the laser.
• Deep scratches can affect the focus of the laser and cause errors.
• The error detection and correction coding system in the disc drive will correct many errors caused by scratches.
• Fingerprints and smudges can cause more errors than scratches and are more likely to overwhelm the error correction coding system
capability.
• Scratches in the direction of the track (tangential direction) are worse than those going from the center of the disc outward (radial direction).
• Like fingerprints and smudges, several scratches close together can also overwhelm the error correction coding system capability.

Wear from Disc Play

CDs and DVDs do not wear from friction as vinyl records or tapes do. There is no physical contact with the disc in the area that the laser
uses.
ROM Discs: The laser light will have no effect on the data or metal layer in ROM discs. In theory, it is possible for the disc to be read so many times that the cumulative effect of the laser light can eventually affect the polycarbonate. There is, however, no record of such discs having been played a sufficient number of times to incur damage from laser light. Accordingly, it is felt that any effects of the light on ROM discs is negligible. It is assumed, in fact, that the disc would likely fail much earlier from some other condition than from the effects of laser light.
R discs: In theory, R discs should have a limited number of read times (several thousand) because of the cumulative effect on the data layer from the laser light. As with ROM discs, the polycarbonate may also eventually be affected, but there is no recorded evidence of ill effects of laser light, so such effects are deemed negligible.
RW discs: RW discs, unlike the other types, can “wear-out.” CD-RW and DVD-RW discs should last for about 1,000 rewrites, and DVD-RAM discs, 100,000 times, before the rewriting capability is lost. The reading functionality of the disc should continue for a limited
number of read times after each writing. While the maximum number of read times possible after writing is unknown, it may become
fewer after each successive writing.

Cleaning

CDs or DVDs do not require routine cleaning. It is best to clean the disc only when it is absolutely necessary, specifically:
• before storing, when surface contamination is visible
• before recording, when surface contamination is visible
• before playing, to prevent surface contamination from being “flung off” while the disc is spinning in the disc drive
• when readability (playability) is impaired and surface contamination
is visible
In general, avoid using organic solvents. Harsher solvents (acetone,
benzene) will dissolve the polycarbonate and damage the disc beyond repair. Mild solvents (isopropyl alcohol, methanol), however, may be used. These mild solvents evaporate quickly and will not dissolve the polycarbonate.Other solutions that are not harmful are water-based lens cleaners or water-based detergents (with mild soap) formulated for cleaning CDs or DVDs.The polycarbonate substrate is a relatively soft and transparent type of plastic. Each time a disc is wiped, rubbed, treated with some solution, or otherwise manipulated for cleaning, that substrate, and thus the disc itself, is at risk of scratching or contamination.
If the disc needs cleaning, remember these tips:
• Use an air puffer to blow off dust.
• Use a soft cotton cloth or chamois to wipe the disc.
• Try cleaning with a dry cloth first, before using any cleaning solutions.
• Do not wipe in a direction going around the disc.
• Wipe from the center of the disc straight toward the outer edge.
• Avoid using paper products, including lens paper, to wipe the disc.
• Avoid using anything abrasive on the surface of the disc.
• If the disc has a heavy accumulation of dirt, try rinsing it with water
first.
• Use commercially available water-based detergent formulated for cleaning the surface of optical discs.
• Use isopropyl alcohol or methanol, as an alternate to water-based detergents, to clean the disc surface.


Maintaining Your Discs

Do:


1. Handle discs by the outer edge or the center hole.
2. Use a non-solvent-based felt-tip permanent marker to mark the label side of the disc.
3. Keep dirt or other foreign matter from the disc.
4. Store discs upright (book style) in plastic cases specified for CDs and DVDs.
5. Return discs to storage cases immediately after use.
6. Leave discs in their packaging (or cases) to minimize the effects of environmental changes.
7. Open a recordable disc package only when you are ready to record data on that disc.
8. Store discs in a cool, dry, dark environment in which the air is clean.
9. Remove dirt, foreign material, fingerprints, smudges, and liquids by wiping with a clean cotton fabric in a straight line from the center of the disc toward the outer edge.
10. Use CD/DVD-cleaning detergent, isopropyl alcohol, or methanol to remove stubborn dirt or material.
11. Check the disc surface before recording.

Do not:


1. Touch the surface of the disc.
2. Bend the disc.
3. Use adhesive labels.
4. Store discs horizontally for a long time (years).
5. Open a recordable optical disc package if you are not ready to record.
6. Expose discs to extreme heat or high humidity.
7. Expose discs to extremely rapid temperature or humidity changes.
8. Expose recordable discs to prolonged sunlight or other sources of ultraviolet light.
9. Write or mark in the data area of the disc (the area the laser “reads”).
10. Clean by wiping in a direction going around the disc.

For CDs especially do not:


1. Scratch the label side of the disc.
2. Use a pen, pencil, or fine-tip marker to write on the disc.
3. Write on the disc with markers that contain solvents.
4. Try to peel off or reposition a label.

3 comments:

Hawkeye said...

Thanks for this nice compilation... was quite helpful.

pittasanjay said...

thank u for the useful and important information.it will be very helpful to those who are keen on storing the information relably.

sanjay_pitta@yahoo.com

Anastasios Biris said...

HOW CAN I BACK UP MY PICTURES FREM CD? ICAN NOT OPEN IT AFTER 2 YEARS.