A method for recording information on optical discs. Optical Discs and Optical Disc Drives

The speed and reliability of modern recorders will be the envy of any Formula 1 car. ComputerBild explains how data ends up on CDs, DVDs and Blu-ray discs.

Record music and movies on optical media– the process is as familiar as using magnetic cassettes twenty years ago, but it’s much cheaper. How do the types of media differ and how is information recorded on them?

Stamping and burning

In the industrial production of discs with music, films or games, data is recorded onto the media by stamping - a process reminiscent of the production of gramophone records. Information on disks is stored in the form of tiny indentations. Computer and consumer DVD recorders perform this task differently - they use a laser beam.

The first recordable optical media was CD-R with write-once capability. When storing data on such disks, the laser beam heats the working layer of the disc, which consists of a dye, to approximately 250 ° C, which causes a chemical reaction. Dark, opaque spots form where the laser is heated. This is where the word “burn” comes from.

In a similar way, data is transferred to DVD with the ability to write once. But no dark spots form on the surface of rewritable CDs, DVDs and Blu-ray discs. The working layer of these drives is not a dye, but a special alloy. When heated by a laser to approximately 600 °C, it transforms from a crystalline state to an amorphous one. The areas exposed to the laser are darker in color and therefore have different reflective properties.

Information carriers

Discs intended for recording at home have the same thickness (1.2 mm) and the same diameter (12 or 8 cm) as discs on which data is recorded industrially. Optical media have a multilayer structure.

Substrate. The base for the discs, which is made of polycarbonate, is a transparent, colorless and quite resistant to external influences polymer material.

Working layer. For recordable CDs and DVDs, it consists of an organic dye, and for rewritable CDs, DVDs (RW, RAM) and Blu-ray discs, it is formed by a special alloy that can change the phase state. The working layer is surrounded on both sides by an insulating substance.

Reflective layer. Aluminum, silver or gold are used to create the layer from which the laser beam is reflected.

Protective layer. Only CDs and Blu-ray discs are equipped with it. It is a hard varnish coating.

Label. A layer of varnish is applied on top of the disc - the so-called label. This layer is able to absorb moisture, so that the ink that appears on the surface of the media during printing dries quickly.

Differences between CDs, DVDs and Blu-ray Discs

These media have different characteristics. First of all, different capacities. A Blu-ray disc can hold up to 25 GB of data, a DVD can store 5 times less information, and a CD can store 35 times less. Blu-ray drives use a blue laser to read and write data. Its wavelength is approximately 1.5 times shorter than that of red laser DVD and CD drives. This allows you to record a significantly larger amount of information on an equal disk surface area.

Media formats

The following types of optical media are currently available on the market.

CD-R. Recordable CDs can hold up to 700 MB of information. There are also disks with a capacity of 800 MB, but they are not supported by all recorders and home players. Eight-centimeter miniCDs can record 210 MB of data.

CD-RW. Rewritable media has the same capacity as CD-R.

DVD-R/DVD+R. Recordable DVDs hold 4.7 GB of information. miniDVD with a diameter of 8 cm – 1.4 GB.

DVD-R DL/DVD+R DL. The DL prefix stands for Dual Layer (DVD-R) or Double Layer (DVD+R), which corresponds to two-layer media. Capacity – 8.5 GB. An eight-centimeter disk can hold up to 2.6 GB.

DVD-RW/DVD+RW. Single-layer media of this type can withstand several hundred write cycles. Like write-once DVDs, rewritable discs have a capacity of 4.7 GB, while 8 cm discs have a capacity of about 1.4 GB.

DVD-RAM. These media have the same storage capacity as single-layer DVDs. There are also double-layer discs that can hold twice as much more information. DVD-RAM can withstand up to 100 thousand write cycles, but only a few DVD players work with these discs. Data is written not on a spiral track, but in sectors on ring tracks, like on plates hard drive. Marks defining sector boundaries are clearly visible on the surface of DVD-RAM - by their presence it is easy to distinguish this type of media from others.

BD-R/BD-R DL. An abbreviation used to refer to recordable Blu-ray Discs. BD-R media has one working layer that can hold 25 GB of data. BD-R DL are equipped with two working layers, so their capacity is 2 times higher.

BD-RE/BD-RE DL. Rewritable Blu-ray discs are rated for 1,000 write cycles. They can store as much data as non-rewritable media.

"Plus and minus"

The presence of “plus” and “minus” media is a consequence of the long-standing war of formats. First the representatives computer industry relied on the “plus” format, and consumer electronics manufacturers promoted the “minus” format as the standard for recordable DVDs. Modern recorders and players support both formats.

Neither of them has clear advantages over the other. Both types of media use the same materials. Therefore, there are no significant differences between “plus” and “minus” disks from the same manufacturer.

Recording quality

The recording quality of media of the same format can vary significantly. Much depends on the recorder model used. The recording speed also plays an important role: the lower it is, the lower the number of errors and the higher the quality.

Recorder and Media Compatibility

Not every recorder is capable of recording to discs of all formats without exception. There are certain restrictions.

CD recorders. Cannot work with DVDs and Blu-ray discs.

DVD recorders. Writes CDs and DVDs, but does not support Blu-ray format.

Blu-ray recorders. They record to Blu-ray as well as any CD and DVD.

Signatures on discs

It is better to sign the media on which the information is located immediately, so as not to confuse it later. This can be done in different ways.

Printable blanks. The top side of these discs is varnished. On such a surface you can print text and images using inkjet printers and MFPs equipped with a special tray. The price of the discs is no different from regular ones.

Signature using a recorder. The recorder's support for LightScribe or Labelflash technology allows you to print single-color images and text on the surface of media specially designed for this purpose. True, the process can take up to 30 minutes, and the cost of LightScribe discs is approximately twice the cost of conventional discs. Media with Labelflash support will cost even more.

New LabelTag technology. Developed by the recorder manufacturer Lite-On and involves applying text to the working surface of the disc. This eliminates the need to use special media. However, disc space is wasted because the text is written directly onto the track. And the inscription is readable only if the areas with text contrast brightly with the empty fragments.

Signature made by hand. To do this, you need to purchase special markers with a soft, rounded tip and solvent-free ink. Other markers may corrode the disc surface and cause scratches.

Using stickers. You can print stickers on any printer. However, gluing them is not recommended, as this often leads to damage to the surface of the disk, and hence to data loss. It may happen that the label comes off during disc playback. In this case, the optical drive may be damaged.

Data storage period

Disc manufacturers often specify a storage life of 30 years or more for data on media. However, such a duration is only possible under ideal storage conditions - in a dry, cool and dark place. The recording quality must be high.

If used frequently, the service life of self-burned discs will be significantly reduced. During playback, media is exposed to high temperatures and mechanical stress. Data loss can also be caused by scratches or contamination.

Transferring information to disk

All optical media, with the exception of DVD-RAM, have a spiral-shaped track that runs from the center of the disk to the outer edge. Information is recorded onto this track using a laser beam. When burning, the laser beam forms tiny spots on the reflective layer - pits (from the English pit - pit). The areas that were not exposed to the laser are called lands (from the English land - surface). Translated into the language of the binary storage system, pita corresponds to 0, and land corresponds to 1.

When playing a disc, the information is read using a laser. Thanks to the different reflectivity of pits and lands, the drive recognizes dark and light areas of the disc. Thus, the sequence of zeros and ones that make up all physical files without exception is read from the media.

With the development of technology, there was a gradual reduction in the wavelength of the laser beam used in recorders, which made it possible to significantly improve focusing accuracy. The track has become narrower, the pits have become smaller, and a larger amount of data can be placed on an equal area of ​​the disk. The shorter the wavelength, the shorter the distance between the working layer and the laser.

Media production

Using DVD as an example, ComputerBild explains how optical media is produced and how the production of other types of discs differs.

1. To cast a plastic substrate, polycarbonate, heated to 350 °C, is fed into a mold using injection molding. A microscopic spiral track in the form of a groove (Pre-Groove) is created on the surface of the base using a matrix. This track not only records data, it also contains a signal to synchronize the recorder spindle drive. After cooling the substrate to 60 °C, a central hole is made, then the temperature is reduced to 25 °C and further processing begins. DVDs typically consist of two polycarbonate layers, each 0.6mm thick. For single-layer recordable DVDs, only one of the layers undergoes further processing, as described in steps 2–3, while for dual-layer DVDs, both layers are processed further. CDs and Blu-ray discs have only one layer 1.2 mm thick.

2. The working layer of recordable CDs and DVDs is created by centrifugation. Using a dispenser, the dye is injected onto the surface of a disk rotating at a constant speed in the area of ​​the central hole and is evenly distributed over the surface of the carrier.

3. The reflective layer is applied to the disk using ion-plasma sputtering. In a vacuum chamber, an aluminum, silver or gold plate is bombarded with charged ions, which knock out metal atoms from it - it remains on the surface of the working layer of the blank. For rewritable CDs, DVDs and Blu-ray discs, all working and reflective layers are created using ion-plasma sputtering. In four chambers, the first insulator layer, the working layer, the second insulator layer and the reflective layer are sequentially applied to the disk. When producing Blu-ray discs, these operations are performed in reverse order.

4. Two polycarbonate bases are glued together. For CDs and Blu-ray discs, instead of a second base, a varnish coating is applied, which is dried under an ultraviolet lamp. The varnish coating of Bly-ray discs is particularly durable, while DVDs do not need a protective layer of varnish.

5. At the last stage, the blanks receive a label, and an absorbent layer of varnish is applied to the printable discs.

Optical storage

Optical drives are designed to read and usually write/rewrite from optical discs. Optical disks are round and flat plates made of dense material (usually consisting of polycarbonate) with applied layers that allow information to be stored in the form of tiny pits (pits, frompit - hole, deepening). The reading process is carried out by a laser beam, which, reflected from the surface of the disk, enters a photocell, where the light is converted into an electrical signal, the magnitude of which allows the recorded information to be decoded.

The most common optical disc formats for use in personal computers are CD, DVD, Blu-ray.

CD-ROM ( Compact Disc Read Only Memory, read-only CD) a type of CD,which appeared in 1982 as a result of research by two companies - Sony and Philips. The first discs used the "Red Book" format, in which the playing time of one cassette was 74 minutes 33 seconds, which corresponds to the playing time of Beethoven's 9th Symphony, which was very popular in Japan at that time. The audio sampling frequency is 44 kHz for stereo sound and the bit depth is 16 bits. They had a capacity of 650 MB and allowed storing 75 minutes of music (starting from 200, discs with thinner recording tracks appeared, which made it possible to increase the capacity to 700 MB with recording 80 minutes of music). CD-ROM discs initially developed as an analogue of vinyl discs and were intended for recording and playing music information. They also have a single concentric track that runs from the outer edge to the inner, making many turns. The principle of reading information is optical, that is, the laser beam reads data that is recorded on an aluminum (or other type) substrate. In addition, the information is recorded on the disc, unlike a vinyl disc, in digital rather than analog form, and after reading it is decrypted and converted into sound. To protect the disc from damage, the aluminum substrate is covered with transparent plastic.

The technology for creating CD-ROM discs is as follows. First, a disk is made on which only those places where a unit of information is located are burned out, and places with zero values ​​remain unchanged. After this, a matrix is ​​made, with the help of which the blanks are stamped, a layer of metal (aluminum, silver, gold, etc.) is sprayed onto the information surface to increase the reflectivity of the laser beam, and they are coated with transparent plastic (varnish) to protect the data. When a disk is inserted into the drive, a laser beam slides along the concentric circle of the disk and by the reflected light it is determined whether it is recorded: a zero or a one.

Initially, CD-ROM discs were designed to store only music information. Due to the fact that disks use digital information rather than analog, they began to be used in computers.

Usually , storage device CD-ROM supports modes : Audio CD, Music Disc, Super Audio CD, CD-ROM (mode 1 & mode 2), CD-ROM/XA (mode 1, form 1 & form 2), Super Video CD, CD-Text, Video CD, CD -I/FMV, Photo-CD (Single & multisession), CD- i and others . The first drives could only handle certain formats, but eventually they could handle all formats. Therefore, the user does not need to know the format. As a rule, it is enough to know that there are audio, video discs and discs with programs (or text).

Next, the “Yellow Book” standard was developed, which contains a header that determines the type of disc: music or software. Music format was already well developed, and the software format was determined by each manufacturer itself. Due to the rapid development of this technology, the discrepancy in the standard could not last long, so a recommendation standard arose High Sierra, on the basis of which the ISO 9660 standard soon appeared. For this standard, the disk has a table of contents and a data area. The first track contains parameters for synchronizing the drive and disk with each other, followed by a table of contents in which the description of each file contains the direct address on the disk.

There are three types of such disks:

CD - ROM The disk is usually written in an industrial manner, and in the future it can only be read. It measures 120x1.2 mm and has a capacity of 650-879 MB. Service life 10-50 years. These discs are often supplied with computer devices and contain software, there are music CDs, etc.

CD - R The disc has the same characteristics as a CD-ROM, but allows information to be written to them once.

CD - RW the disc has the same characteristics as a CD-ROM, but allows you not only to write information on them, but also to write more of it, also to erase previously recorded data and write new ones.

To work with them, CD drives were used, which have several types:

CD- ROMthe drive can only read CD disks. One of the most important characteristics of this device is reading speed information. Normal (single) speed corresponds to the speed of reading audio discs, which is 150 kb/sec. Then came CD-ROMs with 2, 4, 6, 8, 10, 12, 16, 24, 32, 36, 40, 52 times the speed. The data transfer rate is accordingly a multiple of 150 kb/sec. For example, for a 40x drive it will be equal to 40x150 = 6,000 Kb/sec, and here the maximum speed is indicated, which is equal to or lower for different types of drives, which depends on the manufacturer. The six-speed drive allows for video output at frame rates of 25 frames per second or higher, which is fast enough for on-screen viewing. Discs for use with this device are sometimes also called compact discs (this concept also includes CD-R, CD-RW discs) or CD-ROM discs (Compact Disk; see figure below).

CD - R drive is a write-once optical drive. It allows you to read CD-ROM, CD-R, CD-RW discs, but also allows you to write CD-R discs once. This drive has the characteristic of not only reading disks, but also for writing. For example, the read speed is 40 times, and the write speed is 6 times.

In such devices, a laser beam burns grooves on the surface of the disk, while the areas that reflect light are called “lands”, and the non-reflected areas are called “pits”. The combination of these sections makes it possible to encode information in a two-bit representation.

For various reasons, in practice, when recording, it is impossible to achieve an ideal location of the burnt grooves, and during playback, sound defects and jitter appear, which is called “jitter”. To a certain extent, you can get rid of such unwanted distortions by using a special Audio Master mode, when the burned grooves are forcibly increased in length. This mode is used in cases where you need to improve the quality of the recorded sound.

Typically recording is done at constant angular velocity (CAV). However, when the rotation speed changes several times (x2, x4, x8, etc.), the recording pauses and so-called “connection points” are formed, which deteriorates the quality of the recording. In such cases, a buffer underrun protection called SafeBurn is used. As a rule, it turns on only when the disk rotation speed changes, and the recording mode with constant angular velocity (CAV) is mainly used. This method of improving the quality of playback audio is called Zone Constant Line Velocity (Z-CLV) recording.

A very interesting feature on some devices for recording digital information on discs is the ability to burn text onto the surface of a laser disc, be it a list of music files or your data. For this, the DiscT2 mode is used, in which any text is typed that is worthy of reproduction on the surface of a music or other type of disc created by oneself.

CD - RW (Compact Disc-ReWritable) storage device is a reusable optical storage device. It allows you to read CD-ROM, CD-R, CD-RW discs, write CD-R discs once, but also write and re-write, as well as rewrite previously recorded CD-RW discs. This drive has the ability not only to read discs, but also to write them. For example, the read speed is 40 times, and the write speed is 6 times. There could also be a speed of additional recording.

A CD-RW device works on a different principle, that is, when writing to them, the beam does not burn out, but transforms the substrate into an amorphous state, which allows you to establish a different reflective effect. Therefore, they can write data multiple times. However, discs dissipate information worse than standard CD-ROM discs, so they cannot always be read on standard media.

The more capabilities a device has, the more limitations it has. The simpler the discs, the greater the reflective effect they have. CD-ROM discs have the best reflective effect, which can be read in CD-ROM, CD-R and CD-RW drives.

CD-RW format discs have even less reflectivity and may not be readable on all older CD-ROMs and CD-R drives (old drives). It is quite difficult to say definitely which drives will be readable and which will not, since it depends on the device model. Currently, CD-R compact discs are sold on which information can be recorded. If after recording there is still free space, then you can write additional information to the disk, and so on. CD-RW discs allow you not only to record information, but also to delete unnecessary data, that is, write data repeatedly and are somewhat more expensive than CD-R discs.

In 1996 they appeared DVD -discs(Digital Versatile Disc - digital universal disk, originally stood for Digital video Disc - digital video disk. Now it is not decrypted in any way), which had a capacity of 4.7 Gigabytes due to compaction of recording tracks, that is, 7 times more than the capacity of CD-ROM disks. This is the most common type of disc, which is single-layer and single-sided. However, there are discs that have two layers on one side and have a capacity of 8.5-8.7 Gigabytes (they may be called DVD 9, the number means rounded capacity), there are discs with one layer, but with recording on two sides, with a capacity of 9.4 Gigabytes (they may be called DVD 10), double-layer and double-sided with a capacity of 17.08 Gigabytes (they may be called DVD 18). Double-layer discs have two translucent layers with powerful beam focusing, allowing information to be read from either the first or second layer. Higher data density is achieved by reducing the disk area per bit and using compression techniques. But in practice, the most common are single-sided, single-layer ones.

After the creation of a single DVD standard for recording video films on them, the whole world was divided into six zones so that films recorded in one zone could not be read in others. Therefore, an old DVD drive may have a pictogram showing an image globe with numbers indicating which zones this drive works with or ALL (all) - to work with disks in all zones. Modern DVD drives do not have such a partition.

The information on the disks is located in sectors that contain data and 882 bytes for the error correction code, which increases the reliability of reading information, since in case of failures the values ​​​​are calculated using the correction code. If there are bad sectors, the reading speed slows down and repeated reading occurs, and so on up to a certain number of attempts. As a result, either the code will be read, or a message will appear on the screen stating that it is impossible to read information from of this disk, after which it switches again to maximum speed.

Unlike CDs, DVDs have their own file system UDF or for ISO -9660 data. Data is stored in 2048 byte sectors. Disks can be DVD-video, DVD-audio, DVD-Data and mixed types.

Discs DVD - ROM just like CD-ROMs are read-only. They have already been recorded somewhere and are sold with the recorded information.

The standard for recording on disk was developed in two ways, one standard called MMCD was developed by Philips and Sony, the second called Super Disc by Toshiba and several others. Therefore, two formats for recording data arose - DVD -R and DVD +R. These formats are close to each other, however, the plus format is better to use, since it takes less time to rewrite and the recorded data has fewer errors. Accordingly, there are two formats of rewritable discs DVD -RW and DVD +RW.

Write-once discs that have a double layer on one surface are designated by DL symbols, for example, DVD -R DL and DVD +R DL. They have a capacity of up to 8.5 gigabytes.

To work with DVDs, DVD drives are used, which have several types:

DVD - ROM The drive can only read both DVDs and CDs. One of the most important characteristics of this device is reading speed information. The multiplicity per unit is taken as 1.32 MB/sec, which is 9 times faster speed CD. They have different speeds reading CD and DVD discs, which is indicated in the device manual.

DVD - R drive is a write-once optical drive. It allows you to read CD-ROM, CD-R, CD-RW discs, all types of DVD discs, and also allows you to write CD-R discs and DVD +R and DVD-R discs once. This drive has the ability not only to read discs, but also to write them. For example, the reading speed is 40 times, and the writing speed is 6 times, and the speed is indicated separately for CDs and DVDs and, accordingly, separately for DVD -R and DVD +R disks.

DVD - RW storage device is a reusable optical storage device. It allows you to read and burn all types of CD and DVD discs. Reading and writing speeds are indicated separately for CDs, DVD -R, DVD +R, DVD +R DL, DVD -R DL, DVD +RW, DVD -RW, DVD +RW DL, DVD -RW DL, that is, those operations , which the drive can conduct. Here it is also better to use the plus format, since the minus format requires you to first erase the information and then write it down, and the plus format allows you to rewrite data in real mode time.

Standard Blu - ray Disc (BD ) (blue ray- blue beam and disc- disk; writing blue instead of blue- intentional)was developed by the BDA consortium, released in 2006. U this standard there was a competitor - HD DVD from Toshiba, however, this company abandoned further support for HD discs in 2008 after the “format war”. The information reading speed (single speed) is 4.5 Mb/s. Increasing the volume of recorded information is achieved by using a laser beam in the blue-violet range with a shorter length of 405 nm, at the same time as CD and DVD drives They use red and infrared lasers with wavelengths of 650 nm and 780 nm.

A single-layer disk can store 25 gigabytes, a double-layer disk can store 50 gigabytes, a three-layer disk can store 100 gigabytes, and a four-layer disk can store 128 gigabytes. A disk can have more layers. So in 2008, 20-layer disks with a capacity of 500 gigabytes were demonstrated.

Currently available are BD-ROM read-only, BD-R write-once and BD-RE write-once. There are also double-layer disks with DL symbols in the name with a capacity of up to 50 gigabytes.

Drives for these disks are Blu - Ray Read-only discs that allow you to read and write all types of CDs and DVDs, as well as read-only BDs. Respectively Blu - Ray RE allow you not only to read, but also to write all types of CDs, DVDs and BD discs (single-layer, for multi-layer you need to read the instructions).

To insert a CD or DVD into the drive, first press the button on the front panel of the drive (picture below). At the same time, a tray is pulled out of the drive, into which you need to place the disk in a special recess for it with the working surface on which the data is located, down, or with the pattern facing up. Then press the button again, and the tray slides into the drive housing. Now you can work with the disk. The tray has a second recess for disks, approximately half the diameter and currently very rarely used (they are often shown in detective and science fiction films).

For normal operation, the drive must be in a horizontal position. There is a drive that can work in a vertical position. In this case, the disk is inserted into the slot by hand, after which a special mechanism holds it and inserts it into the drive.

The optical drive has an emergency eject hole for the tray if it does not eject. To do this, you need to insert a thin rod, for example, a straightened paper clip, and press on it. In addition, there may be a button to skip to the next song for audio CDs. A configuration switch can be installed at the back, it is advisable to install a Slave, and there is also a connector for testing the drive by the manufacturer. Some drives may come with microphones, headphones, or sound cards.

For boot disk need to:

Turn on the computer;

Press the tray open button and it will slide out;

Place the disc with the print side up on the tray;

Press the tray open button again. The tray slides in, after which you can start working.

Do not pull the tray out or in by hand. It is not advisable to keep the tray open for a long time when there is no work; you should not place foreign objects on the tray, for example, placing a cup of coffee; you should not put pressure on the tray when placing a disc.

When there is no operation, the drive goes into energy saving mode, and the drive noise stops. When a read command is received, the drive starts working automatically.

The production of a disk occurs as follows: first, a disk is made, which is called the “mother”, then a working copy is stamped - the “father”, then others are pressed on its basis.

Basic drive characteristics:

Type: interior or external. The internal drive is inserted into the system unit. The external one has a rectangular body, connects to a parallel port (in old computers), USB (in modern ones) and has a wire connected to the mains. There is also an external option for laptop computers, connected using a PCMCIA connector;

- baud rate(Data Transfer Rate, DTR), respectively indicated as two-speed, four-, thirty-two, etc.;

- buffer memory capacity(Buffer Memory). Cache memory is a RAM chip that is located on the drive board. They provide benefits, so the larger the volume, the better;

- average time between breakdowns(Mean Time Between Failure, MTBF). This characteristic available in many devices, but not described everywhere;

- interface type or bus to which it is connected;

- average access time(Access Time, AT). It is larger for CD-ROM drives than for hard drives, which is determined by fundamental differences in the design of the drive, and differs tens of times, and the greater the multiplicity, the shorter the access time. So, for a 4x drive it is approximately 150, and for a 32x it is 80 ms. This value can be found in the device passport;

- error rate(Error Time);

- list of supported formats.

There may also be other parameters, such as noise and vibration levels. In addition, when purchasing, you need to see whether the tray moves smoothly and whether it is firmly held open.

The latest BIOS versions allow you to boot your computer from CD and DVD drives. A CD-ROM disc at the beginning of the track has a service area, which contains information for synchronizing the drive and disk, then the volume table of contents (VTOC), which contains data about the organization of directories and files on the disk, then data and a label end of the volume. Thus, knowing the path and file name, you can use the table to find the file’s location on the disk and directly position the head to read data, which reduces search time and read operations.

Connects device using two cables: power and information. There are three types of drives: those connected to the SCSI bus, to the IDE bus or to the SATA connector. It is better to have a drive that connects to the IDE connector, if the motherboard supports it. Since there are usually few SATA connectors, and if you need to install several optical or hard drives, there may be a problem with the availability of a free connector.

The connection to just such a bus is described below. Optical drives can be connected along with a hard drive. The data cable consists of 40 cores (shown in the figure above) and has three plugs. One is connected to the hard drive controller (on older boards) or directly to the motherboard (see also the description of the boards and hard drive). The second to the optical drive and the third to the hard drive. Do not forget that the edge of the cable, marked in red, when connecting the plug, should be located near the markings 1, 2, which indicate the first cores of the wire, the opposite end - near the numbers 33 and 34. The second power cable should be connected to the marking indicated on top of the plug, that is, red (5v), black, black and yellow.

If you have a sound card, to listen to sound from music discs, you must connect a third cord consisting of four wires. One end connects to the sound card, the other to the drive. They are marked with the symbols R and L. The wire coming from the sound card with the symbol R must match the R on the drive. The figure below shows the back of the drive, which has connectors for connecting wires.

The procedure for installing a new optical drive is similar to installing a drive for floppy disks. If Windows 9x is installed, a corresponding message about finding a new device will appear on the screen. IN Windows system the operating system itself recognizes new devices, including an optical drive.

When working with disks, you must do following rules:

Do not touch the work surface, otherwise greasy fingerprints may remain on it;

Take the disk by the outer edges, you can take it by the edges of the central hole;

Clean the disc from the center of the disc to the outer edge with a soft, dry cloth. Do not use strong solvents such as acetone, detergents, antistatic aerosols;

Store discs in a special box or disc sleeve;

Do not bend the disc;

Do not write on the working surface of the disc;

When storing the disc, avoid exposing it to sunlight or strong heat, which may cause the disc to warp.

Disks may have defects that prevent data from being read. If there is a displacement of the concentric tracks relative to the center of the disk, then such a disk will be difficult to read, and such a defect cannot be detected by eye. Reducing the disk rotation speed may help, for example, try doing this on a slower drive. If the disk is warped, sometimes noticeable to the eye, then reducing the rotation speed can also help to read such disks.

If there are specks on the disk, then, depending on their location and size, it is sometimes possible to use such a disk. Scratches that go from the edge to the center are often harmless, but scratches that run along the edge may prevent data from being read. Therefore, you need to wipe the disc from the center to the edge. To check the disk, special test programs. When installing, use an interrupt (IRQ) - 7 and higher, base addresses 300h to 340h, DMA1. CDs are quite reliable, however, if there are cracks on the CD, it is recommended to make a copy of the disc, as new cracks may appear in the future and the information on the disc will be unreadable.

Installing the drive. to install this device, need to:

Turn off computer;

Remove the protective cover of the system unit;

Insert the drive into the guides of the system unit. After installation, be sure to tighten the screws on the sides of the device. Sometimes, in order to reach with a screwdriver and tighten the screws, you may need to remove other devices. After this, connect the wires as described above, install the protective cover, turn on the computer and check the operation of the drive.

The technical installation of an optical drive is similar to installing hard disk.

If the tray does not extend, the reason may be that the drive is tightly secured with screws inside the system unit, which causes the drive to become skewed. The sound during CD overclocking is not a sign of malfunction. After installing the optical drive for testing, you can try copying some files from the optical drive to your hard drive. You should not disassemble the drive yourself. The drive should not be exposed to rain or in a damp place.

Unlike an NMD, an optical disk has only one physical track in the form of a continuous spiral, running from the inner diameter to the outer one. But a physical track can be divided into several logical ones. If for NMD it is possible to record on different tracks, then recording on optical discs occurs sequentially in a spiral.

The area on an optical CD that contains data is called IPformation Area(information area). This section starts with a diameter of 44 mm, ends 2 ... 3 mm before the edge of the disk and contains the following three zones (in the order of their placement from the center of the disk):

1) input directory zone (Lead-in Zope);
2) data zone (Data Zope), which contains data written to the disk;
3) zone of the source directory (Lead-out Zope) with an end-of-disk mark.

At the end external track and at the beginning of the inner track is placed middle zone (MiddleZope), which does not contain data. This area is used to allow the laser beam to change focus to read data from the internal track.

The input directory area in CD discs contains the TOC content (Table of Contents), recording addresses, number of titles, total recording time, volume and name of the disc. The data area of ​​a CD has the following data structure. The basic data unit of a CD is frame (frate), which contains 24 coded bytes, one control byte and eight bytes for error correction. The frame is preceded by 24 bits, any of which has a fixed value (pattern) and three merge bits. When the laser is brought to the frame, the beginning of the frame is determined by the template; 98 frames form a sector, the smallest addressable unit of data on a CD. The sector contains 3234 encoded bytes (2352 information bytes and 882 error correction and control bytes). Of the 2352 bytes, user information can occupy 2048 (in mode “1”) or 2336 bytes (in mode “2”). This organization of data recording on CDs and the use of error correction algorithms allows for high-quality reading of information with a probability of error per bit of 10-10.

Generalized structure of an optical disk drive

The simplified structure of the GCD is shown in Fig. 3.
Several recording methods are used for GCD: ablative - by burning holes in the opaque medium of the medium; by using local change reflectance coefficient of the environment; transferring the storage medium from the crystalline phase to the amorphous one and vice versa; transformation of the magnetic state of the structure; changing the color of the local area. The first two methods are used for “non-erasable” recording, and the rest are used for repeated rewriting of information on the GCD.

When recording, a beam of a semiconductor laser diode, driven by the recording data through a collimator, mirror and objective lens, burns a hole in the information layer of the disk. The presence of a hole corresponds to the entry “1”. During reading, an uncontrolled laser beam (received from a beam splitter) enters the working surface through another beam splitter, a mirror and a lens.
In reading mode, the mirror moves. Light from the laser passes through a polarization-separation prism, hits a polarizing filter (the light is polarized in a certain plane), and then is focused on the surface of the optical disk. If a laser beam hits the flat surface (lands) of a CD-ROM or DVD-ROM disc, the light is reflected almost entirely. If the light hits the recess (pits), then most of the light is scattered. The reflected light enters the photodiode through the beam splitter, the signal from which is processed electronic circuits reading. Precise alignment of the beam on the track is ensured by the track servo unit, focusing by the focusing servo unit, and a constant number of revolutions by the disk rotation servo unit (see Fig. 3).

Based on the capabilities of recording discs, optical disc drives are divided into drives with the ability to both read and write discs (recordable drives) and read-only drives , and by disk type - to CD drives And DVD drives.

Figure 4

The optical drive consists of: main functional units:

- boot device;
- disk drive;
- optical block;
- track drive;

- data encoding-decoding block;
- automatic control systems;
- audio unit;
- connectors.

Boot device There are two types of disk drives: container (caddy) And tray. In the first type of floppy drive, the disk is placed in a plastic container and inserted into the drive (this container performs the same functions as a 3.5-inch floppy disk container). In the second type of drive, the disc is placed on a tray (Fig. 3), which is pulled out after pressing the Eject button. After pressing the button again or lightly pressing the tray, it is pushed into the drive (pop-up mechanism).

On the front panel of the drive, in addition, there are: a device operation indicator (busy, light bulb), a jack for connecting headphones or a stereo system (for listening to audio CDs), and a sound volume control (also for audio CDs).
There is also a hole with which you can remove the CD even in an emergency, for example, if the Eject button does not work. You need to insert a thin rod 2-3 cm into this hole, then the tray will slide out.

Optical block contains the optical drive system shown in Fig. 4. To read data from CDs, a laser with a wavelength of 780 nm (in the infrared range) is used, and for reading from DVDs, a laser with a wavelength of 650 nm (red) is used. Therefore, DVD drives that read from CDs and DVDs typically contain two lasers with separate optical systems or a common optical system (switchable between one or the other laser).

Figure 5. Optical drive design: 1 - tray; 2 - track drive; 3 - interface control bus; 4 - optical system; 5 - floppy drive; 6 - disk

To write data to the disk, a separate recording laser is used, which operates in a pulsed mode with variable power (to “burn” the disk, change the phase state from crystalline to amorphous, and return to the crystalline state). Typically, the recording and reading lasers have a common optical system. The surface of the optical disk moves relative to the laser head at a constant linear speed, and the angular speed varies depending on the radial position of the head. Thus, reading of internal tracks is carried out with an increased, and external - with a reduced number of revolutions. The servomotor, upon command from the internal microprocessor of the drive, moves the reflecting mirror. This allows the laser beam to be precisely positioned on the track. The beam penetrates the protective layer of plastic and hits the reflective layer of aluminum, silver or gold on the surface of the disc. When it hits the protrusion, it is reflected onto the detector and passes through a prism, which deflects it onto a light-sensitive diode. If the beam hits the pit (pit), it is scattered, and only a small part of the radiation is reflected back and reaches the photosensitive diode. On the diode, light pulses are converted into electrical ones; bright radiation is converted to “1”, weak radiation – to “0”. Thus, the pits are perceived by the drive as logical zeros, and the smooth surface as logical ones. Note that the pits formed by the laser beam are very small in size. Approximately 30-40 depressions correspond to the thickness of a human hair, which is approximately 50 microns.

Drive track according to commands that come from the built-in microprocessor, it moves the optical system to the desired track on the disk for writing or reading. During recording, the laser performs the desired operation (“burning” or phase change) according to the commands of the encoding-decoding unit, and during reading, the beam reflected from the disk hits the photodetector, the signals from which enter the encoding-decoding unit and the automatic tracking system.

Encoding-decoding block is a signal handler that is written to or read from the disk. It consists of an encoder, a decoder, a random access memory and a control controller. The encoding device prepares data for recording on disk, performing character conversions according to EFM (for CD disks) or RLL (2.10) (for DVD disks) encodings and adds clock signals and service information to the data. The decoder extracts data from the digital stream, restoring its original form. The random access memory device performs the function of a buffer memory, and the controller manages error correction modes for data recorded in all formats supported by the drive.

When writing or reading data from a disk, disturbances in the positioning of the laser beam may occur due to radial beats of the disk. To avoid data loss, optical drives use automatic tracking system using control signals. There are several ways to isolate these signals. However, the most common is the method in which the laser beam after the first lens (see Fig. 1.) enters the diffraction gratings, where it is split into three beams, one of which is used to read data, and the other two are used by the track tracking system. Pulse signals from the photodetector they enter the amplifier of the automatic control system, where the tracking error signals are separated and the reading signal is corrected.

Audio block The optical drive was inherited from CD players. It converts audio data from digital to analog. After amplification, this data is transmitted either to external device, or on headphones.

Next to the computer interface connector and the power connector, optical drives also have connectors for connecting to a sound card or audio unit of the motherboard.

On the front panel of the drive, in addition, there are: indicator device operation (busy, light bulb), nest for connecting headphones or a stereo system (for listening to audio CDs), volume control sound (also for audio CD).
It is also provided hole, with which you can remove the CD even in an emergency, for example, if the Eject button does not work. You need to insert some thin stick into this hole about 2-3 cm, then the tray will slide out.

Optical Disc and Drive Specifications

Optical disc characterized by its type(CD or DVD) and capacity. The capacity of a disk is determined by its type and size (diameter). Optical discs are available in 120 mm (4.7 inch) and 80 mm (3.1 inch) diameters.
CDs measuring 120 mm have a capacity of 650 or 700 MB (depending on the recording mode), and discs measuring 80 mm - capacity 185 MB. Sometimes used to record data optical Business Cards(оtісalbusiness сards) - rectangular CD-R disks measuring 80 x 61 mm and a capacity of 50 MB.

Optical drives have the following main characteristics:

Compatibility;
- data transfer speed;
- average access time;
- cache memory capacity;
- error rate;
- reliability;
- device type;
- interface type;
- list of supported formats;
- audio path parameters.

All optical disc drives have a 5.25-inch form factor and are compatible in disc sizes, that is, they can read both 120 mm and 80 mm discs, as well as optical business cards (for reading discs of the latter two types, there is a provision in the drive tray special recess).

Compatibility of different types of drives means the ability to read and write other types of disks.
Data transfer rate - This is the maximum speed at which data is exchanged between the drive and the computer. This is the most important characteristic of an optical drive, which is almost always given along with the model name, and it is not specified by the number of megabytes per second, as for other external memory devices, but by the increase factor relative to the base speed. The first CO drives had a data transfer rate of 150 kB/s, just like CD players. This basic speed is clearly insufficient for reading, for example, video data. Therefore, the rotation speeds of disk drives and, accordingly, data transfer rates began to increase (at first twofold). Such drives (with a data transfer rate of 300 kB/s) became known as 2x drives. Subsequently, drive speeds increased even more. Now drives have a maximum transfer speed of 54x (16.2 MB/s) and higher (up to 76x).

For DVD drives Due to the higher data density and higher rotation speed, a value of 1x corresponds to a data transfer rate of 1.32 MB/s, that is, a 1x DVD drive is approximately equivalent to a 9x CD drive. The maximum value for DVD drives is now 16x or 21.13 MB/s.

The figures given are valid for reading data. For data recording, maximum data transfer rates are lower and are now equal to 40x for recording CD-R drives, 24x for CD-RW drives and from 2x to 8x for DVD drives. Typically, for CD-RW drives, both the write speed and the read speed are indicated separately, and for DVD record drives, the write speed of DVDs, the write speed of CDs, the read speeds of DVDs and CDs.

For recording optical discs (R or RW), the maximum permissible recording speed or range of permissible speeds (for example, 24x or 1x-24x) is usually indicated.

Average access time - This is the time (in milliseconds) that the drive needs to remain on the media with the necessary data. Obviously, working on the internal areas of the disk requires less access time than reading information from external areas. Therefore, the drive data sheet lists the average access time as the average value for performing several data reads from different (randomly selected) sections of the disk. The average access time for CD-ROM drives is 100 ... 200 ms, and for new DVD drives it is 40 ... 250 ms.

Cache memory capacity - This is the capacity of the optical drive's random access memory, which is used to increase the speed of access to data recorded on the media (buffer memory). If you use special driver programs to control the drive, you can pre-write the contents of the disk into the cache memory. Then accessing a fragment of the requested data is much faster. Cache capacity modern devices- from 64 to 2.048 MB.

The drive buffer is a memory for short-term storage of data after it is read from the CD-ROM, but before it is sent to the controller board and then to the CPU. This buffering makes it possible disk device transfer data to the processor in small portions, rather than taking up its time by slowly sending a constant stream of data. Important characteristic floppy drive is buffer fill level, which affects the playback quality of animated images and videos. This value is defined as the ratio of the number of data blocks transferred to the buffer from the drive and stored in it until the start of their output to system bus, to the total number of blocks that the buffer can hold. Too much fill may cause delays in output from the buffer to the bus; On the other hand, a buffer that is too low will require more attention from the processor. Both of these situations lead to jumps and stuttering during playback.

On those who write CD-ROM drives Buffer memory is very important because it ensures that information flows smoothly onto the CD-R or CD-RW. This allows for more reliable recording, since you cannot allow a track to stop recording, otherwise the entire disc may be damaged.

The reading quality level is characterized error rate(Error Rate). This parameter displays the optical drive's ability to correct read/write errors. Typically the error rate is 10-10...10-12. The error rate is an estimate of the probability of an information bit being corrupted when it is read. If the drive reads data from a dirty or scratched area of ​​the disk, it records a group of bad bits. If the error cannot be eliminated due to the excessiveness of the noise-resistant code (used during writing and reading), then the drive reduces the data reading speed and repeats it many times. If the error correction mechanism cannot cope with the failure, then the message “Sector not found” appears on the computer monitor. not found). If the failure is resolved, the drive switches to the maximum data reading speed.

Reliability optical drives, expressed in terms of MTBF (mean time between failures - MeanTimeBetweenFailure), is 50...125 thousand hours, which is almost an order of magnitude longer than the obsolescence period of the device.

Hot stamped discs (CD-ROM and DVD-ROM) provide up to 10,000 cycles of error-free data reading. DVD-RAM discs can be rewritten up to 100,000 times.

By device type Optical drives, like other external storage devices, can be either internal or external.

Connecting CD-ROM drives. The first connection method is based on the fact that one IDE interface channel can support two built-in devices. CD-ROM drive connected to the I/O board via an IDE interface along with a hard drive using the master/slave principle. However, in this case, the speed of data exchange with the hard drive is reduced. One way to solve this problem is to connect CD-ROM devices to different channels of the same EIDE interface or to two different IDE controllers. If the CD-ROM has a SCSI interface, then it is connected to the SCSI controller accordingly. It is also possible to connect CD-ROM drives via the sound card controller. Also, we should not forget that modern motherboards can contain built-in SCSI and IDE controllers, which generally eliminates the need for an additional I/O card to connect CD-ROM drives.

Connecting audio channels. Almost everyone CD-ROM drive It has a built-in digital-to-analog converter (DAC), as well as an output jack for outputting stereo signals. If there is audio information on the CD, the DAC converts it into analog form and sends the signal to the headphone jack, as well as to the audio output connectors of the drive, from which the signal in turn goes to the amplifier and sound system directly or through sound card. The advantage of the active output is that the audio signal from the CD-ROM is additionally processed by the sound card.
An important characteristic of a CO-ROM drive is scroll supported by him formats for recording data on CDs. You can write data to optical discs using different formats X. To record data such as documents, programs use the CD-ROM (ISO) and DVD-ROM (ISO) formats, as well as CO-DOM (UDF) and DVD-ROM (UDF).

CD-ROM formats (1S0) and DVD -ROM (IS0), sometimes called CD-ROM and DVD-ROM formats, they are defined in the IS0 9660 standard. This standard specifies three format levels. The level 1 format defines the names of the recorded files as MS DOS file names, that is, file names can contain up to eight characters using the specified three extension characters. The files to be written must occupy several sectors one after another (non-fragmented recording). Level 2 format allows the use long names files, and the level 3 format additionally allows files to be written on several sections of the disk (fragmented recording) in batch mode. In order to be able to record long names defined in the Windows operating system, the company Mісrosofі expanded the ISO 9660 Level 1 format with the Joliet specification. Varieties of the IS0 9660 format are the CD-ROM (Boot) and DVD-ROM (Boot) formats, in which a special section (in addition to the content) is written to the disk (at the beginning of the disk), which allows the optical disk to be used as a boot disk.

If the ISO 9660 standard is intended to provide compatibility between CD-ROM and DVD-ROM discs used in different computer systems, then the UDF format (UniversalDiskFormat - Universal Disc Format) is designed for compatibility between read-only optical discs (ROM) and recordable optical discs (R or RW) in different operating systems. This format, like IS0 9660, allows long file names and data recording. Data is written to an optical disc in small portions using the Packet Writing mode (for CDs) or Incremental Writing mode (for DVDs).
Audio CD formats And Audio DVD used for recording music. These are two different formats. Format Audio DVD provides better quality music recording.

Video CD (VCD) and VideoDVD formats used for recording films. These are also two different formats with different additional features(for example, regarding the choice of language for dubbing a film). Format VideoDVD Provides better quality movie recording. High-quality movie playback on CDs is ensured by the Super Video format. CD. To record movies in Video DVD format onto CO discs, use the mini-DVD format.

These formats are the most common, since they allow you to record music and films, as well as text data, graphic data and programs, on optical discs. These formats are supported by most disk drives and optical disc writing programs. There are also other formats, mostly designed for types of data that are used much less frequently, for example, Photo CD from Kodak, Record and play back high-quality digital photos. Both photos and music can be recorded in IS0 9660 or UDF formats as regular graphics or sound files.

A new, promising format is the format Mt. (Mount) Rainier, also known as format EasyWrite. This format writes data to an optical disk in the same way as it does to a floppy disk. Using this format, an optical disk can be accessed (read or written) from any application without calling special optical disk read-write programs.

CD formats are sometimes named by the color of the cover of the books in which these formats are described. So, the very first format CD - Audio CD is described in the “red” book. The “yellow” book describes the CD-ROM (IS0) format, the “orange” book describes the CD-R and CD-RW formats, the “green” book describes the CD-ROM format CD-I(now almost never used), in “blue” - Enhanced CD and in “white” - Video CD.

Optical discs are a popular storage medium. Most users are only familiar with CD and DVDs, in fact, there are many more types of disks. The Land of Soviets will tell you what there are types of disks, and will help you understand their diversity.

Types of CDs

CDs or compact discs, were originally intended for recording and playing music, but are now used to store almost any computer information. Writing and reading disk information is carried out using a laser. CD thickness - 1.2 mm, diameter - 120 mm, capacity - 650 or 700 MB (corresponding to 74 or 80 minutes of sound). Exist mini CD with a diameter of 80 mm, but their capacity is smaller - 190-200 MB (21 minutes of sound). Mini CD can be read on any media except car radio. There are curly cds of various shapes, they are produced mainly for commercial purposes. Such discs are not recommended for use in computer drives, because high speed rotation they may burst.

CD discs can be divided into CD-ROM, CD-R and CD-RW. This division is determined by the ability to write information to the disk and the purpose of the disk. Information on disk CD-ROM recorded by the manufacturer, it cannot be changed or deleted, you can only read the data. To disks CD-R(they are sometimes also called “blanks”) you can record your information, but it will be impossible to erase or change it. If there is free space left on the disk, and you enabled the option to add information when recording, you can add files to the disk. Discs CD-RW support deleting and rewriting information, but such discs will not be readable by all drives.

Types of DVDs

DVDs allow you to store more information than CDs due to the use of a laser with a shorter wavelength. DVD capacity standard size(120 mm) can range from 4.7 GB to 17 GB, and the capacity of mini DVD (80 mm) is 1.6 GB.

Depending on the capacity of DVD, the following types of discs are distinguished:

• DVD-5— single-layer, single-sided disk, capacity — 4.7 GB
• DVD-9— double-layer single-sided disk, capacity — 8.5 GB
• DVD-10— single-layer double-sided disk, capacity — 9.4 GB
• DVD-14- double-sided disc, double-layer on one side and single-layer on the other, capacity - 13.24 GB
• DVD-18- double-layer, double-sided disc, capacity - 17.1 GB

Dual-layer discs contain two layers of information on one side and are marked with the abbreviation DL. A double-sided disk is actually two disks glued together with non-working surfaces. Naturally, the thickness of such a disc is controlled to match the thickness of a conventional single-layer DVD.

Where possible, recording, overwriting and deleting DVD information Disks, like CDs, are divided into ROM, R and RW. But additionally, the following types of disks are distinguished:

• DVD-R for general, DVD-R(G)- a recordable disc intended for home use.
• DVD-R for authoring, DVD-R(A)— a one-time recordable disc for professional purposes.
• DVD-RW- rewritable disc. You can overwrite or erase information up to 1000 times. But you cannot erase part of the information, you can only erase the disk completely and completely rewrite it.
• DVD-RAM use phase change technology. They can be rewritten up to 100,000 times and have a theoretical service life of up to 30 years. But they are expensive, are produced mainly in special cartridges and are not supported by most drives and players.
• DVD+RW are based on CD-RW technology and support rewriting information up to 1000 times. This format appeared later than DVD-RW.
• DVD+R- A recordable disc similar to DVD-R.

It is clear that no single drive or player fully supports all DVD formats. Most modern drives support both the DVD-R(W) and DVD+R(W) formats. But older drives and home players released before the advent of the DVD+R(W) format will only read DVD-R(W) discs. There are "super multi" drives that support all types of discs, including DVD-RAM.

Other types of disks

Standing apart are the so-called Dual Discs. These discs combine CD and DVD formats. On one surface of such a disk music is recorded in CD format, and on the other - five-channel sound, video, menus, subtitles, images, etc. in DVD format.

HD DVDs (DVDs) high density) can have a capacity of up to 15 GB, and double-layer ones - up to 30 GB. Their main competitor is BD, Blu-ray Disc holds from 23 to 66 GB depending on the number of layers. A prototype of a four-layer disk with a capacity of 100 GB has been announced, and it is also planned to release ten-layer disks with a capacity of up to 320 GB.

The confrontation between BD and HD DVD is called the “fight of formats.” But leading film studios abandoned the use of HD DVD in favor of BD discs, so the release and support of the HD DVD format was officially discontinued.

So, there are many types of optical discs. You should choose a disk for recording information based on its capacity, the ability to rewrite information and the model of your drive or home player. Knowing the main types of disks, you will never get confused in their rich assortment.

External memory

Optical discs

Optical (laser) discs are currently the most popular storage media. They use the optical principle of recording and reading information using a laser beam.

Information on a laser disk is recorded on a single spiral-shaped track, starting from the center of the disk and containing alternating sections of depressions and protrusions with varying reflectivity.

When reading information from optical disks, a laser beam installed in the disk drive falls on the surface of the rotating disk and is reflected. Since the surface of the optical disk has areas with different reflection coefficients, the reflected beam also changes its intensity (logical 0 or 1). The reflected light pulses are then converted into electrical pulses using photocells.

In the process of recording information on optical discs, various technologies are used: from simple stamping to changing the reflectivity of areas of the disc surface using a powerful laser.

There are two types of optical discs:

CDs (CD - Compact Disk, CD), on which up to 700 MB of information can be recorded;

DVDs (DVD - Digital Versatile Disk, digital universal disk), which have a significantly larger information capacity (4.7 GB), since the optical tracks on them are thinner and are placed more densely.
DVDs can be double-layered (8.5 GB capacity), with both layers having a reflective surface that carries information.
In addition, the information capacity of DVDs can be further doubled (up to 17 GB), since information can be recorded on two sides.

Currently (2006), optical discs (HP DVD and Blu-Ray) have entered the market, the information capacity of which is 3-5 times greater than the information capacity of DVDs due to the use of a blue laser with a wavelength of 405 nanometers.

Optical disk drives are divided into three types:

• No recording option- CD-ROM and DVD-ROM
  (ROM - Read Only Memory, read-only memory).
  On CD-R discs OM and DVD-ROM stores information that was recorded on them during the manufacturing process. It is impossible to write new information to them.
• Write once and read many times -
  CD-R and DVD±R (R - recordable, recordable).
  On CD-R and DVD±R discs, information can be written, but only once. Data is written to the disk using a high-power laser beam, which destroys the organic dye of the recording layer and changes its reflective properties. By controlling the laser power, alternating dark and light spots are obtained on the recording layer, which, when read, are interpreted as logical 0 and 1.
• Rewriteable- CD-RW and DVD±RW
  (RW - Rewritable, rewritable). On CD-RW and DVD±RW discs, information can be written and erased many times.
  The recording layer is made of a special alloy, which can be heated into two different stable states of aggregation, which are characterized by different degrees of transparency. When recording (erasing), the laser beam heats a section of the track and transfers it to one of these states.
  When reading, the laser beam has less power and does not change the state of the recording layer, and alternating areas with different transparency are interpreted as logical 0 and 1.

Key characteristics of optical drives:

disk capacity (CD - up to 700 MB, DVD - up to 17 GB)

data transfer speed from the storage medium to RAM - measured in fractions of the speed
150 KB/sec for CD drives (the first CD drives had this speed of reading information) and
1.3 MB/sec for DVD drives (This was the reading speed of the first DVD drives)

Currently, 52-speed CD drives are widely used - up to 7.8 MB/sec.
CD-RW discs are written at a lower speed (for example, 32x).
Therefore, CD drives are marked with three numbers “read speed X CD-R write speed X CD-RW write speed” (for example, “52x52x32”).
DVD drives are also marked with three numbers (for example, "16x8x6"

access time - the time required to search for information on a disk, measured in milliseconds (for CD 80-400ms).

If the storage rules are observed (stored in cases in an upright position) and used (without causing scratches or contamination), optical media can retain information for decades.

Additional information about disk structure

The industrially produced disc consists of three layers. An information pattern is applied to the base of the disc, created from transparent plastic by stamping. For stamping, there is a special prototype matrix for the future disk, which extrudes tracks on the surface. Next, a reflective metal layer is sprayed onto the base, and then a protective layer of thin film or special varnish is applied on top. Various drawings and inscriptions are often applied to this layer. Information is read from the working side of the disk through a transparent base.

Recordable and rewritable CDs have an additional layer. For such discs, the base does not have an information pattern, but between the base and the reflective layer there is a recording layer, which can change under the influence of high temperature. When recording, the laser heats up specified areas of the recording layer, creating an information pattern.

A DVD disc may have two recording layers. If one of them is performed using standard technology, then the other is translucent, applied lower than the first and has a transparency of about 40%. To read double-layer discs, complex optical heads with variable focal length are used. The laser beam, passing through the translucent layer, is first focused on the inner information layer, and after reading it is refocused on the outer layer.