[The following is a text description, and the picture shows their physical structure]

1.？ The data on the optical disk is represented by a series of pits pressed on the transparent plastic substrate, and the pits are distributed outward from the center in the form of spiral tracks on the pad. The depth of the pit is about 0.6 micron, the width is 0.6 micron, and the distance between adjacent spiral tracks is 1.6 micron, which is equivalent to the track density of 160000 dpi (tracks per inch), much higher than that of floppy disk (135TPI) and hard disk (hundreds of TPI). The recording area of a CD contains 15000 tracks. The total length of the spiral track is nearly 5 kilometers, and the total number of pits on it can reach more than 800 million.

On the surface, DVD and CD are very similar. But in essence, there are essential differences between them. According to various combinations of single/double-sided and single/double-layered structures, DVD can be divided into four basic physical structures: single-sided single-layered, single-sided double-layered, double-sided single-layered and double-layered structures.

A single-sided DVD disc can have one or two recording layers. Like a CD, the laser reads the data on a single-sided disc from the bottom of the disc, and the data on a double-sided DVD disc is stored on the upper and lower sides of the disc respectively.

2.？ Physical structure of DVD disc

2. 1? Single-sided single-layer SSSL (single? One-sided? Single? Layer) CD? DVD5 (D5 for short)

This refers to the common single-sided single-layer DVD disc with a total capacity of? 4.7GB, which can store and play 133 minutes of video data.

A single-sided single-layer DVD disc is formed by bonding a blank substrate and a substrate having a data recording layer. Single-layer and double-layer discs are bonded by two substrates, and the thickness of each substrate is 0.6mm, so the thickness of DVD discs is1.2 mm.

2.2? Double-sided single-layer DSSL (double-sided? One-sided? Single? Layer) CD? DVD 10 (abbreviated as D 10)

This refers to a double-sided single-layer DVD disc, which is formed by bonding 0.6mm thick substrates, with two recording layers on each side, with a total capacity of? 9.4GB, which can store about 266 minutes of video data.

There are two ways to read data from a double-sided disk:

(1) After playing the program on the first side of the CD, take the CD out of the player, turn it over and put it in the player to continue playing the program on the second side;

(2) Two reading lasers are installed in the player to read data from the upper and lower sides of the CD, or only one reading laser is installed in the player, but after reading the first side of the CD, it can automatically jump to the other side of the CD to continue playing.

If the latter scheme is adopted, it is not necessary to take out the optical disc and turn it over after reading the first side of the optical disc. But players need this feature.

2.3? Single and double SSDL (single? One-sided? Double? Layer) CD? DVD9 (D9 for short)

This refers to single-sided and double-layered DVD discs. Total capacity up to? 8.54GB, which can store about 24 1 minute video data.

There are two schemes for dual-layer optical discs. One is to put both recording layers on one substrate, and the other is a blank substrate, and then stick them together. This scheme has high technological requirements and low yield, and has not been adopted in actual production. Another scheme is to put two recording layers on the upper and lower film bases, the lower recording layer is made into a transparent layer, and the upper recording layer is made into a reflective layer, and then the two film bases are glued together. This is a common solution for DVD-9 at present.

When a laser head reads a dual-Layer optical disc, the recording layer (lower layer) that the laser beam first reaches is called Layer 0 (L0, Layer? 0), the data can be read, so it is a reflective layer. But the laser beam can read the upper layer (1 layer, that is, the reflective layer, L 1, layer? 1). Therefore, layer 0 is a translucent layer, also known as a semi-reflective layer. When reading layer 0, always read from the inner ring to the outside, and then read 1 layer after reading layer 0. There are two ways to read the 1 layer:

(1) backlight path OTP (opposite? Tracking? Path) method, that is, when reading 1 layer, it starts from the outer circle and moves from outside to inside;

②PTP (parallel? Tracking? Path) method, that is, when reading the 1 layer, it starts from the inner ring and moves from the inside out.

The spiral track of two recording layers in the forward optical path is the same as that of a single-layer optical disc, but the spiral track of 1 layer (L 1) in the reverse optical path is opposite. Therefore, the OTP dual-layer optical disc in the reverse optical path is also called RSDL(Reverse? Spiral? Double? Layer, reverse spiral double-layer disk). When making a double-layer DVD disc, you can choose whether to reverse the optical path OTP or smooth the optical path PTP, and write instructions at the end of the layer 0 data. According to this instruction, the master recording device will automatically record the second-layer master as required. When reading data, the CD-ROM also reads the data of 1 layer in the way of reverse light path or smooth light path according to this instruction. When the reading laser head reads the data of layer 0 and layer 1, its optical focus will change accordingly. When using the optical path, the laser head needs to return from the outer ring to the inner ring, and there will be a small pause when playing video programs. Therefore, manufacturers are generally willing to adopt the OTP mode of reverse optical path.

The capacity of the dual-layer disk is slightly smaller than that of the single-layer disk, because the laser beam "penetrates" the semi-reflective layer to read the upper data, and the distance between the two recording layers is very small, only 20 to 70 microns. In order to reduce the interference between the two layers, the minimum pit length of the two layers is from 0.4? Hmm? Increase to? 0.44? Hmm. In order to compensate, the reference scanning speed should be faster, 3.49? M/s (single-layer disc) increased to 3.84? Meter/second. Longer optical pits and farther apart ensure that data can be read correctly and are not easily affected by jitter. Increasing the minimum pit length means that the number of pits per turn decreases, resulting in a decrease in the data capacity of each layer.

2.4? Double-sided double-layer DSDL (double? One-sided? Double? Layer) CD? DVD 18 (abbreviated as D 18)

This refers to a double-sided and double-layered DVD disc with a total capacity of? 17GB, which can store and play video data for 482 minutes.

A double-sided dual-layer disc is formed by bonding two substrates each having two recording layers. The production of this kind of optical disc requires high production technology, which means high production cost. Therefore, unless there is a special need, the general manufacturers do not adopt the DVD- 18 format. In fact, DVD- 18 is rare in the market.