CDs and DVDs consist of the same basic materials and layers but are manufactured differently. A DVD is actually like two thin CDs glued together. A CD is read from and written to (by laser) on one side only; a DVD can be read from or written to on one or both sides, depending on how the disk was manufactured. Recordable DVDs (DVD-R, DVD-RW, DVD-RAM) can be manufactured with one re-cording layer on each side. Prerecorded DVDs (DVD-ROM) can be manufactured with one or two recorded layers on each side.

The polycarbonate substrate makes up most of the disk, including the area that is read by the laser (opposite the label side on CDs). It is present on both sides of a DVD, even a “single-sided” disk with a label on one side.the data layer of CDs and DVDs is the layer that contains the data. The data appear as marks or pits that either absorb light from the laser beam, or transmit the light back to the laser photosensor by way of the metal reflective layer. In CDs, the data and metal layers are very close to the top of the disk (label side); in DVDs, they are in the middle of the disk (see Figures 1–6). The types of data and metal layers used depend on the type of disk.

Because a DVD consists of two sides glued together (each a mere 0.6 mm thick), it can be susceptible to damage from bending and/or twisting. Also, though DVD error-correction encoding is nearly ten times better than those of audio CDs, because of DVD’s much higher compression ratio, shorter pit length and narrower tracks, severe scratches can create occasional problems in playback.

HANDLING
A good rule of thumb is to handle a DVD more carefully than a CD: Make contact only with the center hole and the outside edge of the disk. When removing a DVD from its case, press the button on the center hub and push downward. Using your other hand, gently remove the disk by its outer edge. Never remove a DVD from its package simply by prying up the outer edge of the disk.

Single-sided disks should be inserted into the player with titles/artwork facing up. Make sure the disk is seated properly inside the player before closing the tray.

Clean disk only with a damp, non-abrasive, lint-free cloth. Do not use any harsh or abrasive chemicals or cleaning agents. Wipe the disk carefully in a straight line from the inside hole to the outer edge. Never use a circular motion when cleaning a disk.

Do not stack your DVD disks. Always store the disk inside its protective case when not in use. And, of course, keep your DVD’s away from extreme heat, such as the back window of a car or the trunk area -- and keep them out of direct sunlight.

There has been some concern that DIY labels on home produced DVDs and CDs can harm them in the long term. It is not the label that causes the problem, but the glue. Some solvents can affect lacquer coatings and expose or react with the metal.that slowly eats into the disk. Research is still under way on this, but if you have a precious recording - do not label it. Use a non-solvent base marker pen to mark the label site of the disk.

 

The Rules...(also applies to CDs)

• 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 sys-tem 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.

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 contamina-tion is visible.

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 wa-ter 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.

How long should they last ?
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.

For a more details see Jim Taylor's Official DVD FAQ

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