CD-ROM discs

How many layers make up a CD-ROM disc?
Typical CD-ROM discs have 3 layers however only the top and bottomlayers are visible.
Label Layer (top) - a colorful label which is typically silkscreened over the reflective coating.
Reflective Layer - a reflective coating which protects the top of the data and allows the laser to reflect back onto the data.
Data Layer (bottom) - a clear molded material in the form of 1.2mm X 120mm (4.75") disc.
Basic Production Process - Data is molded into the top surface of the Data Layer. A reflective coating is applied to the top of the disc which serves to protect the data as well provide a reflective surface for an optical reader which reads the data by focusing a tiny laser from the bottom of the disc onto the reflective material and back to the data. The final layer is typical a graphic image silkscreened (printed) onto the top of the disc, which provides extra protection for the reflective and data layers of the CD.

DVDs

What is the major differences between the CD and the DVD discs?
DVD discs have 2 separate 0.6mm x 120 discs parts which are laminated/bonded together. The two layers equal the thickness of a single layer CD-ROM disc.

What are the capacities of the various DVD formats?

• DVD-5: 4.7 GB Single Side/Single Layer
• DVD- 9: 8.5 GB Single Side/Dual Layer
• DVD-10: 9.4 GB Dual Side/Single Layer
• DVD-18: 17 GB Dual Side/Dual Layer

What are the important differences between Dual Layer and Single Layer DVD discs?

• The optical reader (laser) accesses each separate layer by re-focusing on the depth of the disc being read.
• The sensitivity and focusing ability of the optical reader (laser) becomes critical with Dual-Layer DVDs.
• The capacity of the discs.
• Dual-Layer means the player reads either both sides of a dual-layered DVD disc separately after flipping over or the player reads both layers through one side of the DVD DVD disc by focusing the optical reader(laser) and possibly reversing the direction of the read.
  

How many layers make up a typical Dual-Layer DVD disc?
DVD discs have 2 or more additional layers beyond the 3 basic layers of the CD-ROM disc.
Semi Reflective Layer (between discs) - A semi-transparent reflective layer similar to a one-way mirror is laminated/bonded between the 2 discs.
Information Layer - The information layers maybe molded into the disc or may be added as a photopolymer layer.

Which direction is data recorded onto the DVD disc in?
Data is typically recorded on the disc starting at the hub or center of the disc, and continuing toward the perimeter of the disc.

What is RSDL? How is data stored on a RSDL DVD disc?
Reverse Spiral Dual Layer allows the DVD to be recorded and played back in two directions rather than requiring the viewer to flip the DVD when after the first side has been played.

• Layer 0 - Data is recorded from hub to outside edge.
• Layer 1 - Data is recorded from the outside edge to the hub.
• A semi-transparent reflective layer similar to a one-way mirror is laminated between and separates the two layers in a RSDL DVD.

RDSL is typically used with longer length movies that do not fit on one side of DVD disc. RSDL allows a DVD movie to continue to play on a 2nd side with a less noticeable break in the movie which would occur if you had to change discs or flip the disc over.

Why do RSDL DVD discs pause or drop sound when changing layers?
As the optical reader (laser) reads the 1st layer of the DVD it is moving towards the outside edge of the disc. When it reaches the outside edge of the disc the optical reader will pause for a moment as it refocuses on the 2nd layer and then reverses direction as it begins reading the 2nd layer from outside edge back towards the center.
DVD movies using Dual-Layer recording will usually note on the jacket that there may be a pause during the layer or direction change?

Bad DVD discs are commonly misdiagnosed as driver or hardware problems
Dual-Layer discs (especially RSDL) require the bonding of several materials. RSDL discs are amongst the most difficult DVD discs tomanufacture. Manufacturing defects are not-uncommon and thesymptoms are sometimes misdiagnosed as driver or hardware issues.Commonly misnamed as "RSDL ROT", damaged areas and/ordeterioration can occur between the surfaces of the discs causingan unreadable or partially unreadable area which can cause the computer to lock up.

• A defective area typically occurs towards the outside edge or area of the disc. In some cases however, the defect may appear only as a wavy line or it may skirt the entire perimeter of the disc.
• The defective area appears as a cloudy or marble like patch usually under the surface of the disc.
• The defective areas are difficult to detect. It is best viewed under under focused lighting but can in some cases be seen under florescent lighting as well.
• Dual-Layer discs may exhibit this problem right out of the package although in many cases the problem may not occur until after a few weeks later which is why it is sometimes called "ROT" referring to a deterioration of the materials that bond the discs materials together.
  High-performance DVD drives and those designed for low power consumption will probably have more difficulties with DVD discs displaying these type of defects.
  
  Symptoms
  Data errors, frame skipping, pixelizing and more commonly a complete system lockup occurs as the optical reader (laser) moves across the affected area of the disc usually occurring as the optical reader tries to refocus on the 2nd layer of the disc. In some cases the rest of the movie will be totally unplayable depending on the length of the movie and the severity of the disc damage.
  
  Recommendation
  Because this type of defect occurs during the pressing process of a DVD it is possible that more than one copy of the title may be affected. Notifying and returning the defective DVD title to your local dealer is the recommended solution. In some cases, however, where you have tried a couple of copies of the DVD title with the same results, contacting Customer Service for the particular DVD movie label might be suggested.
  
  IMPORTANT! If one DVD titles works while others fail, or even if several titles fail, then the DVD drive is probably fine.
  

Preventative maintenance for DVD players

• DVDs are fairly resistant to fingerprints, dust, and scratches. However all can cause data errors, frame skipping, lockups and complete un-playability. Use clean discs.
• DVD players are usually not damaged by a dirty or scratched disc. If, however, a substance is thick enough to come between the disc and lens and it is large enough to hit the lens, WATCHOUT! Keep your CDs, DVDs clean and the inside of your DVD player will stay clean.
• Cracked discs should NEVER be put into a player. If the disc shatters it will probably damage the player.
• It normally won't hurt to leave a disc in the player, but leaving a running disc unattended for long periods of time is not recommended.
• Cleaning the lens on your player is not critical, however if you do, you should use a cleaning disc designed for DVD players. DVD and CD-ROM's cleaners are designed with the minor differences in lens positioning in mind.
  

Proper handling of DVDs

• Handle DVDs only at the hub or outer edge.
• Do not touch either surfaces of a DVD disc. Especially with sweaty or greasy fingers.
• Do not bend the disc when taking it out of the case or out of the drive.
• Do not to scratch the disc when placing it in the case or in the player tray.
• Always store DVDs in their protective case when not in use.
• Make sure the disc is seated properly in the player tray before you close it.
• DVDs should always be kept away from heat sources like heaters, radiators and other warm surfaces like the top of a audio tuner, cable box or bonfire. Anything that can cause heat, scratches or breaks should be avoided.
  Note: Magnets and magnetic fields do not effect CDs or DVDs.

Cleaning DVDs
Some problems encountered when playing a DVD may be corrected with a simple cleaning.

• Use a soft, lint-free cloth.
• Wipe only in a straight line between the hub and the rim.
• Data is arranged circularly on the disc. There are fewer chances of adding more damaging scratches to error correction blocks than might occur if you wipe the surface dirt around the hub or in a circular pattern.
• Never use strong cleaners, abrasives, solvents, or acids.
• Never use canned or compressed air. Very cold air can also thermally stress a disc.
• Use water, water with mild soap, or isopropyl alcohol for stubborn dirt or grime.
  Note: Let it sit for about a minute before wiping it off.
• Commercial products for CDs or DVDs discs which can provide some protection from dust, fingerprints, and scratches can be used.
  

Can scratches on DVD discs be repaired?

There are two methods of repairing scratches in DVDdiscs:

1. Fill or coat the scratch with an optical material.
2. Polish down the scratch.
   There are commercial products especially designed for CD polishing starting at around $10-$12 dollars. There is an art to polishing but if you are careful you might be able to polish out a scratch without creating new ones. Remember that several small polishing scratches may be more detrimental to the data than the problems caused by the original scratch. Polish only from the hub to the rim or in a radial direction.
   
   Commercial polishing products: Commercial polishing machines are available that can restore discs after quite a bit of abuse. Remember that the data layer on a DVD is thinner than on a CD, so the discs cannot be polished quite as often as CDs. Store that sell CDs and DVD may offer polishing services.

^-/wa