The DVD movie disc structure for a single-layer disc is shown in the image below. Actually, regardless of whether the DVD is a movie disc or ROM disc or audio DVD, as long as it is a read-only disc where the information is placed on the disc when it is manufactured, then the structure of the single-layered disc is as illustrated below.
First of all, the DVD is the same size and thickness as the CD. However, the pits or indents in the plastic that represent the digital information are half the size of those on a read-only CD. The spiral pattern of tracks on a DVD is closer together by a factor of about two when compared to a CD. Both of these factors allow for a greater storage capacity when compared to CD but require the use of a slightly different laser to read the disc.
Two Discs Glued Together
As for the disc itself, the DVD is actually two discs, half the thickness of a CD, glued together as shown in the DVD movie disc structure image. So, in contrast to a CD, where the pits representing the digital information and the metal reflective layer are near the top surface of the CD, a DVD has these layers sandwiched in the middle of the disc structure. This provides more physical protection for these layers because they are protected by 0.6mm of polycarbonate plastic on either side. The metal layer and plastic material used for the base of the two halves of the DVD are made of the same materials as are used in CDs. As with a CD, a DVD read-only disc has a label on the top surface of the disc which describes the disc contents.
One concern for DVD movie discs and other DVD format discs that is not an issue for CDs is the adhesive layer and how it will behave as the DVD discs age. If the adhesive degrades over time, then it might fail and cause a DVD disc to delaminate into two halves. This would ruin the disc. In addition, it is not known if the adhesive bonding the two DVD disc halves together will chemically react with and degrade the other disc components over time.
Several Information Layers Possible
One final point is that DVD movie discs and other read-only DVD discs can have one, two, or four information or data layers as shown below. The four possible structures are: single-layer single-sided disc, single-layer double-sided disc, dual-layer single-sided disc, and dual-layer double-sided disc. Note that double sided discs only have labeling on the inner hub because the laser has to read both sides of the disc and cannot do so if the label is on the disc surface. To read the other side of a double-sided disc, it has to be flipped over. The advantage of a dual-layer one-sided disc is that the second data layer is accessible without having to stop play and flip over the DVD. Read-only dual-layer discs often appear silver from the non-label side of the disc like other typical DVD read-only discs, but sometimes may also appear gold or light gold in color. This is due to the type of metal layer that is used in the disc structure.
DVD Single-Layer Double-Sided Read-Only Disc
DVD Dual-Layer Single-Sided Read-Only Disc
DVD Dual-Layer Double-Sided Read-Only Disc
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