Basic types of CDs
Currently the following main types of CDs are used:
• a readable compact disk or compact disk with read-only memory (Compact Disk Read Only Memory - CD-ROM), manufactured by industrial stamping, which contains information and is intended for multiple reading;
• Recordable CD (Compact Disk Recordable - CD-R), allowing the user to perform a single (multi-session) data record, then use the disk for multiple reading;
• rewritable CD (Compact Disk Re-writable - CD-RW), designed for multiple writing and reading;
• Digital Versatile Disk (DVD), designed for high-quality playback of video, audio and digital data. It differs from other drives with high density recording.
• Blu-ray Disc, BD (from English, blue ray - blue ray and disc - disc, writing instead of blue - intentional) used to record high density and store digital data, including high-frequency video.
The physical structure of the CD is created in the process of its manufacturing and is determined by the number of layers and their arrangement. Therefore, consider the manufacturing technology and features of the main types of CDs.
CD-ROM discs. The physical structure of such a disk (along the path) is shown in Fig. 6.8, a. In mass production, compact discs are manufactured by stamping or pressing. First, an original disk is created, consisting of a glass substrate with a recording layer applied to it. In the process of recording data on the original disc using sophisticated precision equipment, the relief of its recording layer changes. Received
Fig. 6.8. Physical structures of CD-ROM (a), CD-R < strong> (b) and CD-RW ( in )
the relief from the original disc is transferred to the metal matrix. The manufactured matrix is used for mass production of CD-ROM compact discs with polycarbonate base. To create a reflective surface, a thin (0.05-0.1 μm) layer of aluminum is deposited on the stamped disc. To protect the aluminum layer from oxidation, a layer of acrylic lacquer curing under the influence of ultraviolet rays is applied. The text of the label is applied to the surface of the disc by screen printing.
CD-R discs. The main technological stages of the disc manufacturing process are:
• Pressing the molten polycarbonate mass using a molding matrix, which creates a polycarbonate substrate;
• Forming a spiral primary groove (Pre-groove) or a groove in the form of a protrusion on the disk using a matrix. On the laser side, this groove has a wavy form (the deviation is 0.03 μm with a distance between the grooves of 1.6 μm). The groove deviations from the centerline serve as additional information that is read by the drive. The signal generated by the oscillation of the track is called the absolute time of the original ATIP track (Absolute Time In Pre-groove);
• application of a uniform layer of organic dye by the method of centrifugation;
• creating a reflective layer of gold with high corrosion resistance and having the greatest possible reflectivity;
• coating the surface of the disc with acrylic lacquer to protect the reflective layer and the dye layer;
• application of a silk-screened ink layer, which is used for identification and additional protection of the disc.
As follows from Fig. 6.8, b, which shows the section of the CD-R across the track, its structure contains more layers than the CD-ROM (there was a recording layer and a layer with original grooves).
In the recording mode, a laser beam with a wavelength of 780 nm of increased power is used. The laser operates in a pulsed mode. The organic dye layer is heated to a temperature of 250-300 ° C. The dye burns out and becomes opaque, so the recording process is often called burning out. The burned part of the dye corresponds to the level of logical zero, the saved part - to the level of the logical unit. The burnt areas are not restored, so CD-Rs are called single-write media.
In the reading mode, a laser beam of reduced intensity is used. If the burned areas are hit, the beam is not reflected: a logical zero is read, in other areas, a logical unit is read.
CD-RW discs. The main manufacturing processes of the manufacturing process:
• creating a polycarbonate substrate;
• the formatting of a spiral groove of a wavy shape, the oscillations of which determine the positioning information;
• application of the first layer of the dielectric;
• deposition of a recording layer with a change in the phase of the state, which is an alloy of silver, indium, antimony and tellurium (Ag-In-Sb-Te), having a polycrystalline structure with a reflectivity of 20% and having the ability of phase transformations (from an amorphous state in the crystalline and vice versa);
• application of the second dielectric layer;
• application of a reflective layer of aluminum;
• application of acrylic lacquer as a protective coating.
In contrast to CD-R, CD-RW discs have dielectric layers (Figure 6.8, c), located above and below the recording layer. They are designed to protect the polycarbonate substrate and the reflective layer from overheating in the recording mode. In addition, the CD-RW uses an recording layer that has properties that can overwrite information multiple times.
During recording, the laser can work in two modes:
• In the P-recording mode, the laser beam heats the recording layer material to a temperature of 500-700 ° C, which leads to its melting. In the liquid state, the alloy molecules begin to move freely, the material loses the crystal structure, goes into an amorphous state, after which the reflectivity decreases to 5% in the solidified state;
• In the P-erase mode, the layer of active material is heated to a temperature of 200 ° C, at which material softens. When the material is cooled, its transition from amorphous to crystalline state takes place. Reflectivity is restored to 20%. Despite the name of the mode, there is no erasure.
Due to the possibility of changing the reflectivity on CD-RW discs, the technology of direct data rewriting is applied. During the rewriting process, a permanently switched on laser generates pulses of different power, creating regions of an amorphous and polycrystalline structure. The structure of the regions is completely independent of their previous state. Disc media can be overwritten 1000 times or more.
DVDs. The digital versatile DVD is a high-capacity CD. Capacity increase is achieved due to:
• reduce the stroke length of the signalogram by 2.25 times (0.9-0.4 μm) and the distance between the tracks is 2.16 times (1.6-0.74 μm);
• increase the surface of the disk to write data (8.6-8.76 mm2);
• Reduce sectors from 2048/2352 to 2048/2064 bytes (their number increases);
• using more efficient modulation methods and error correction code;
• Use two sides of a disc to write data and two recording layers on each side.
The DVD disc has the same dimensions as a regular CD. The only track, consisting of depressions and areas, starts on the inside of the disk and spirals out to the outside. The length of the track exceeds 11 km. DVD-disks are one- and two-sided, with one or two recording layers on each side. DVD technology is similar to that of conventional CDs, except that the layers and sides of the discs are stamped from individual polycarbonate blanks. In Fig. 6.9 there are four known types of DVD-ROM drive:
Fig. 6.9. Physical structures of four types of DVD
• one-sided single-layer DVD-5 disc of 4.7 GB capacity, consisting of two interconnected substrates (upper empty, lower one with a single recording layer);
• a one-sided, two-layer, 8.5 GB DVD-9 disc consisting of two interconnected substrates (the top is empty, the bottom is with two recording layers);
• Two-sided single-layer DVD-10 disk of 9.4 GB capacity, consisting of two stamped substrates connected to each other (upper and lower with one recording layer);
• Two-sided, dual-layer DVD 17 with a capacity of 17.1 GB, consisting of two interconnected substrates (upper and lower with two recording layers);
The DVD standard meets the requirements for video playback and data storage. The level of image quality and sound stored in the DVD format is very close to the level of professional studio videos. High sound quality is provided by using 16-, 20- or 24-bit linear pulse code modulation with a sampling frequency of 48 or 96 kHz, which corresponds to sound recording in a frequency band of 22 or 44 kHz, respectively.
Reading of audio information in DVD format is performed at a speed of 384 Kb/s, which allows you to organize multi-channel audio.
Using MPEG-2 compression, a full-length video (up to 135 min.) with three channels of quality audio and four subtitle channels can be placed on a DVD-ROM. For disks with a capacity of 9.4 and 17 GB, the playback time of the information recorded on them is 4.5 and 8 hours, respectively.
The working wavelength of the laser radiation was reduced from 0.78 microns to 0.63-0.65 microns, which made it possible to reduce the dimensions of the recesses (strokes) of recording by almost two times, and the distance between the recording tracks was from 1.6 to 0 , 74 μm.
When reading information, the laser beam first focuses on the first layer (the first layer is read), and then on the second layer (the second layer is read). For a two-sided disc, DVD drives with two independent reading systems are preferred.
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