How does a laser beam read or write a disc? CD-R - Compact Disk Recordable - a disk with a single record and multiple reading. Session of Practical Magic in Video CD

Attention! Not all drives support reading/writing "raw" subchannel data. Make sure the "Profile Options" of the Clone CD is set to "read subchannels from data tracks" and the checkbox "do not restore subchannel data" is unchecked. Otherwise, you won't succeed.

Appendix: Drive Reliability Testing

Conclusion

The main example of the operation of semiconductor lasers is magneto-optical drive(MO).

MO drive is built on the combination of magnetic and optical principle information storage. Information is written using a laser beam and a magnetic field, and reading is done using a laser alone.

In the process of writing to an MO disk, the laser beam heats certain points on the disk, and under the influence of temperature, the polarity reversal resistance for the heated point drops sharply, which allows the magnetic field to change the polarity of the point. After the end of heating, the resistance increases again, but the polarity of the heated point remains in accordance with the magnetic field applied to it at the time of heating. In the currently available MO storage devices, two cycles are used to write information, an erase cycle and a write cycle. During the erasing process, the magnetic field has the same polarity, corresponding to binary zeros. The laser beam sequentially heats the entire erasable area and thus writes a sequence of zeros to the disk. In the write cycle, the polarity of the magnetic field is reversed, which corresponds to a binary unit. In this cycle, the laser beam is turned on only in those areas that should contain binary ones, and leaving areas with binary zeros unchanged.

In the process of reading from the MO disk, the Kerr effect is used, which consists in changing the plane of polarization of the reflected laser beam, depending on the direction of the magnetic field of the reflecting element. The reflecting element in this case is a point on the surface of the disk magnetized during recording, corresponding to one bit of stored information. When reading, a laser beam of low intensity is used, which does not lead to heating of the read area, thus, when reading, the stored information is not destroyed.

This method, unlike the usual one used in optical discs, does not deform the surface of the disc and allows re-recording without additional equipment. This method also has an advantage over traditional magnetic recording in terms of reliability. Since magnetization reversal of disk sections is possible only under the action of high temperature, then the probability of accidental magnetization reversal is very low, in contrast to traditional magnetic recording, which can be lost due to random magnetic fields.

The scope of MO disks is determined by its high performance in terms of reliability, volume and turnover. The MO disk is needed for tasks that require a large disk space, these are tasks such as CAD, audio image processing. However, the low speed of data access does not make it possible to use MO disks for tasks with critical system reactivity. Therefore, the use of MO disks in such tasks is reduced to storing temporary or backup information on them. For MO disks, a very advantageous use is the backup copy hard disks or databases. Unlike tape drives traditionally used for these purposes, storing backup information on MO disks significantly increases the speed of data recovery after a failure. This is because MO disks are random access devices, which allows you to recover only data that has been found to have failed. In addition, with this recovery method, there is no need to completely stop the system before full recovery data. These advantages, combined with high reliability of information storage, make the use of MO disks for backup profitable, although more expensive compared to streamers.

The use of MO disks is also advisable when working with large volumes of private information. The easy replacement of disks allows you to use them only during work, without worrying about protecting your computer during non-working hours, data can be stored in a separate, secure location. The same property makes MO discs indispensable in situations where it is necessary to transport large volumes from place to place, for example, from work to home and back.

The main prospects for the development of MO disks are primarily associated with an increase in the speed of data recording. Slow speed is determined primarily by the two-pass write algorithm. In this algorithm, zeros and ones are written in different passes, due to the fact that the magnetic field that determines the direction of polarization of specific points on the disk cannot change its direction quickly enough.

Most real alternative two-pass recording is a technology based on phase change. Such a system has already been implemented by some manufacturers. There are several other developments in this direction related to polymer dyes and modulations of the magnetic field and laser radiation power.

Technology based on the change of the phase state is based on the ability of a substance to move from a crystalline state to an amorphous state. It is enough to illuminate a certain point on the surface of the disk with a laser beam of a certain power, as the substance at this point passes into an amorphous state. This changes the reflectivity of the disk at that point. Writing information is much faster, but this process deforms the disk surface, which limits the number of rewriting cycles.

Polymer dye-based technology also allows re-writing. With this technology, the surface of the disk is covered with two layers of polymers, each of which is sensitive to light of a certain frequency. For recording, a frequency is used that is ignored by the upper layer, but causes a reaction in the lower one. At the point of incidence of the beam, the lower layer swells and forms a bulge that affects the reflective properties of the disk surface. For erasing, a different frequency is used, to which only the upper layer of the polymer reacts, during the reaction the bulge is smoothed out. This method, like the previous one, has a limited number of write cycles, since the surface is deformed during writing.

Currently, technology is already being developed that allows you to change the polarity of the magnetic field to the opposite in just a few nanoseconds. This will make it possible to change the magnetic field synchronously with the arrival of data for recording. There is also a technology based on the modulation of laser radiation. In this technology, the drive operates in three modes - a low-intensity read mode, a medium-intensity write mode, and a high-intensity write mode. Modulating the intensity of the laser beam requires a more complex disc structure, and supplementing the drive mechanism with an initialization magnet placed in front of the bias magnet and having opposite polarity. In the simplest case, the disk has two working layers - initializing and recording. The initializing layer is made of such a material that the initializing magnet can change its polarity without additional laser action. During the recording process, the initializing layer is written with zeros, and when exposed to a medium-intensity laser beam, the recording layer is magnetized by the initializing one, when exposed to a high-intensity beam, the recording layer is magnetized in accordance with the polarity of the bias magnet. Thus, data recording can take place in one pass, when the laser power is switched.

Of course, MO disks are promising and rapidly developing devices that can solve emerging problems with large amounts of information. But their further development depends not only on the technology of recording on them, but also on progress in the field of other storage media. And if no more is invented effective method storage of information, MO disks may take a dominant role.

On laser, or optical, discs, information is recorded due to the different reflectivity of individual sections of such a disc. All optical discs are similar in that the media (disk) is always separate from the drive, which is a standard device in a computer. Unlike hard disks or flash drives, there are much fewer hardware problems with laser discs, and they are much easier to solve - by simply replacing the drive. Physical location data on laser disc is strictly standardized, and information about all standards is publicly available, although many specifications have been created.

Types of media and technologies

The first laser discs were created in 1980 by Sony and Philips to record sound. These discs (CD-DA) were played on home players. Since appearance and the geometric dimensions of any laser discs remain unchanged. The disk is a polycarbonate plate with a diameter of 120 mm and a thickness of 1.2 mm, in the center of which there is a hole with a diameter of 15 mm. The disk has a spiral track starting in the central part and going to the periphery. Initially, there were only discs that were replicated industrially from specially manufactured matrices, but subsequently technologies were developed that made it possible to record laser discs on computer CD-R drives and then CD-RW

At the beginning of the 21st century, DVD standards were developed, which should gradually replace CD. These discs differ from CDs in their increased track density by several times, and a laser with a shorter wavelength is used to read and write them. Double-sided (Double-Sided - DS) and double-layer (Double Layer - DL) discs have appeared, which contain two reflective layers and have almost doubled, compared with regular discs, capacity. Recent developments, the Blu-Ray and HD-DVD standards, have made it possible to further increase the amount of data stored on a laser disc, although the principle of recording has remained almost the same. Great importance attached backward compatibility standards and formats so that more modern drives can work with older discs.

On factory-made, or stamped, discs, the track is formed by alternating depressions and protrusions extruded onto the surface of the plate during the stamping process of the disc. A thin reflective aluminum layer is subsequently deposited onto this surface. Since the projections and depressions reflect the laser beam differently, it becomes possible to read the resulting pattern.

On recordable and rewritable discs ("blanks"), both surfaces of the plate are completely smooth, and the recording and reading of information are associated with a change in the physicochemical characteristics of a thin recordable layer deposited on the upper side of the plate (Fig. 5.1). The recordable layer in write-once discs (CD-R or DVD-R) consists of an organic dye that changes irreversibly under the influence of a powerful laser beam, and in rewritable discs (CD-RW or DVD-RW) it is formed by a special alloy film that can change its reflectivity depending on the conditions of heating and cooling. One way or another, the physical quality of the recording entirely depends on the quality of the disc itself and the characteristics of the drive on which the recording was made: speed, focusing accuracy and beam power.

In all cases, a number of protective layers are applied to the upper, farthest from the laser, surface of the disk, which protects the reflective layer from damage. Although the protective layers are quite strong, the disk is much more vulnerable on this side than on the substrate side. Rewritable discs are especially unprotected - active layer close in its properties to liquid crystals and reacts even to slight pressure or bending of the disk.

From the center to the periphery, the disk is divided into several concentric regions, or zones (Fig. 5.2). The diameter of each area is strictly standardized:

Rice. 5.2. Laser disc zones

The landing area, or fixation, does not contain any data and lies on the drive spindle. Roughness and dirt in this area can affect the balance and runout of the disc as it rotates;

The Power Calibration Area (PC A) is present only on recordable discs and is used for trial recording and automatic adjustment of the recording laser power depending on the individual characteristics of the disc and drive;

The Program Memory Area (PMA) also exists only on recordable discs. It pre-records a temporary table of contents (Table of Content - TOC). At the end of the recording session, this information is rewritten to the zero track;

Zero track (Lead-in) contains the table of contents of the disc or recording session. The table of contents includes the starting addresses and lengths of all tracks, the total length of the data area, and information about each of the recording sessions. If a disc is recorded in several sessions, a separate zero track is created for each of the sessions. Standard size zero track - 4500 sectors, or about 9.2 MB of data;

The data area contains useful data. This is the main part of the disk;

The end zone (Lead-Out) serves as a marker for the end of the recording session. If the disc is written in one session, the size of the target zone is 6750 sectors. If the disc was recorded in several sessions, each subsequent session creates its own end zone with a size of 2250 sectors.

Information when written to a CD is many times redundant. This is necessary to correct possible errors. Although it is believed that the capacity CD-ROM is about 700 MB, in fact, such a disk carries about 2.5 GB of information!

The spiral track is divided into sectors, the length of one CD-ROM sector is 17.33 mm, and on standard disk fits up to 333,000 sectors. For a DVD disc, the standard number of sectors is 2298496 (single layer DVD, DVD-R(W)) or 2295104 (single layer DVD+R(W)). Each sector consists of 98 blocks, or frames (frames). The frame contains 33 bytes of information, of which 24 bytes carry useful data, 1 byte contains service information, and 8 bytes are used for parity and error correction. These 8 bytes contain the so-called Reed-Solomon code, calculated from 24 usable bytes. Thus, the sector size is 3234 bytes, of which 882 bytes are redundant. From them, the drive firmware is able to recreate the true values ​​of the remaining 2352 bytes in case of errors. Moreover, out of the remaining 2352 bytes, 304 bytes are reserved for sync codes, identification bits, ECC error correction code, and EDC error detection and correction code. As a result, 2048 bytes are useful in one sector.

To minimize the impact of scratches and other physical defects, cross-block interleaving between adjacent sectors is used. Due to this, any limited defect is likely to affect blocks belonging to different sectors and will not end up on two or three consecutive blocks. In this case, error correction can be very effective.

Physically, sequences of "dark" and "light" areas, obtained as a result of EFM modulation, are recorded on the disk. Eight-to-Fourteen Modulation is another layer designed to provide redundancy and data integrity. Instead of each byte, i.e. 8 bits, a sequence of 14 binary values ​​(bits) is written. Three merge bits are added to these 14 bits, and the length of the sequence increases to 17 bits. A 24-bit sync number is added to the beginning of each block.

The algorithms schematically described here are standard and embedded in the firmware of any drive. In the process of reading the disk, the drive firmware performs error correction if necessary and shows through the interface already clean sectors of 2048 bytes each.

Optical disc drives

The design of any laser disc drive has not changed much since the 20th century (Fig. 5.3). All significant differences between CD or DVD drives, reading or writing, are only in lasers, sensors and optical elements. Of course, the support of new standards also required new error correction algorithms embedded in the firmware of disk drives.

Rice. 5.3. Laser disc drive scheme

The disc rotates on the spindle axis. The rotation frequency can reach up to 12,000 rpm. Under the disk, a carriage moves along guides, on which a miniature semiconductor laser, a system of lenses, prisms and mirrors, as well as a photocell receiver are fixed. Modern combination drives may have multiple lasers. The laser beam passes through the optical system, is focused on the lower surface of the rotating disk, is reflected from it, and again enters the receiver through the same lenses and prisms. The receiver converts the light beam into electrical signals that are sent to preamplifier and further into the electronic circuit of the drive.

The top lens is focusing. It is mounted on very light suspensions and can move slightly relative to the rest of the optical system. The position of this lens is controlled by complex automation, so the beam must always be accurately focused on the reflective layer of the CD. By moving the carriage, the laser beam can be directed to any part of the disk.

According to the CD standard, the track width is about 0.6 µm, the distance between adjacent tracks is about 1.6 µm. Each element of the track (a depression or a platform, or a section that differs in reflectivity from the adjacent one on the disc being recorded) must have a length of 0.9 to 3.3 microns. For DVD, these dimensions are much smaller. The difference in the reflectivity of the "dark" and "light" areas is quite small and amounts to no more than a few tens of percent. When reading, the laser disc drive picks up fairly slight fluctuations in the brightness of the reflected beam. When the laser beam is focused on the reflective layer of the disk, the spot it creates should roughly correspond to the geometric dimensions of the tracks. If the spot is larger, the fluctuations in the brightness of the reflected beam become even smaller, and deviations in positioning exacerbate the situation.

The laser power, focusing accuracy, response speed of the focusing system, as well as the degree of vibration and beating of the disc are different for different models drives. In addition, to work specific instance device makes a negative contribution to the wear of bearings and guides, as well as the aging of suspensions.

This explains the well-known case when a disk is read normally on one drive, on the other it is read, but uncertainly, and on the third it is not read at all with an error message. Paradoxically, it is not at all necessary that a disc will be best read on the same drive it was written on! The variety of parameters, both the disks themselves and the drives, is quite large. It's not even worth talking about cheap discs from unknown manufacturers and the percentage of defects among them. There are also initially unsuccessful drive models.

Drive quality is a very vague concept. It includes the thoroughness and accuracy of manufacturing and assembly of mechanics and optics, design features, including mechanisms for balancing and compensating for backlash, the properties of the laser emitter, as well as the features of the microprogram.

The behavior of the drive during unstable reading of problematic disks depends on the firmware. In general, the lower the speed, the greater the chance of successfully reading a disk with poor optical characteristics. When a large number of errors occur, the drive must step down the read speed until the read becomes stable, but this mechanism is implemented differently in different drives. The lower the disk rotation speed, the simpler the requirements for its quality. Practice shows that the quality of a CD or DVD drive can be indirectly judged by the ratio of plastic / metal, that is, by the weight of the device and its price. In this case, we are talking about prices for models of one generation.

Plextor drives are well known. They cost more than double or triple the average price for common drives, but they are stable and durable. In addition, some LG drive models have the ability to read even a heavily scratched disc or the poorest quality disc. Teas drives were also characterized by stable reading, however, release models after 2006, for some reason, began to cause criticism. Experienced computer users, whose occupation often has to extract data from unstable disks, usually choose for a long time, and then carefully use the drive. Sometimes such a drive is connected to a computer only to read a problematic disk, and the rest of the time it is physically disconnected to avoid unnecessary wear.