• Optical discs in storage and use. What is an optical disc? Compact disc, laser and other optical disc device

    Optical drives

    Optical drives are designed to read and, as a rule, write / rewrite from optical discs. Optical discs are round and flat plates made of a dense material (usually made of polycarbonate) with applied layers that allow storing information in the form of tiny pits (pit, frompit - pit, deepening). The reading process is carried out by a laser beam, which is reflected from the surface of the disc and enters a photocell, where the light is converted into electrical signal, the value of which allows decoding the recorded information.

    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 a study of 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, very popular at that time in Japan. The sampling frequency of the signal sound is 44 kHz for stereo sound and the bit depth is 16 bits. They had a capacity of 650 MB and allowed to store 75 minutes of music (starting from the 200s, discs with thinner recording tracks appeared, which made it possible to increase the capacity to 700 MB with 80 minutes of music recorded). CD-ROM discs were originally developed as an analogue of vinyl discs and were intended for recording and playing musical 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, information is recorded on a disc, unlike a vinyl disc, in digital rather than analog form, and after reading it is decrypted and converted into sound. To protect the disk 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 the information unit is located are burned out, and the place with zero values ​​\u200b\u200bis left unchanged. After that, a matrix is ​​produced, with the help of which blanks are stamped, a layer of metal (aluminum, silver, gold, etc.) is deposited on the information surface to increase the reflectivity of the laser beam, and they are coated with transparent plastic (lacquer) to protect data. When a disk is inserted into the drive, a laser beam slides along the concentric circle of the disk and the reflected light determines whether it is written zero or one.

    Originally, CD-ROM discs were designed to store only music information. Due to the fact that the disks use digital information, and not 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 work only with certain formats, but eventually with all formats. Therefore, the user does not need to know the format. As a rule, it is sufficient to know that there are audio, video, and program (or text) discs.

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

    There are three types of such discs:

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

    CD - R a disc has the same characteristics as a CD-ROM, but allows you to write information to them once.

    CD - RW the disk has the same characteristics as the CD-ROM, but allows not only to write information to them, but also to add it, also to erase previously recorded data and write new ones.

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

    CD- ROMdrive only reads 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 CDs, which is 150 kb / s. Then came CD-ROMs with 2x, 4x, 6x, 8x, 10x, 12x, 16x, 24x, 32x, 36x, 40x, 52x. The data rate is accordingly a multiple of 150 kbps. For example, for a 40x drive, it will be equal to 40x150 = 6,000 Kb / s, and here the maximum speed is indicated, which is equal or lower for different types of drives, depending on the manufacturer. The 6x speed drive enables video output at frame rates of 25 frames per second or higher, which is sufficient for on-screen viewing. Disks for working with this device are sometimes also called CDs (this term also includes CD-Rs, CD-RWs) or CD-ROMs (Compact Disk - compact disc; 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 40x and the write speed is 6x.

    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-reflective areas are called "pits". The combination of these sections and allows you to encode information in a two-bit representation.

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

    Typically, recording is done at a constant angular velocity (CAV). However, when the rotation speed changes by several times (x2, x4, x8, etc.), the recording stops and so-called “connection points” are formed, which degrades the recording quality. In such cases buffer underrun protection called SafeBurn is used. As a rule, it turns on only at the moment of changing the speed of rotation of the disk, and the recording mode with constant angular velocity (CAV) is mainly used. This method of enhancing the quality of the reproduced sound is called constant line velocity (Z-CLV) zone recording.

    It is very interesting that some devices for recording digital information on discs have the ability to burn text on the surface of a laser disc, whether it is a list music files or your data. To do this, use the DiscT2 mode, in which any text is typed that is worthy of reproduction on the surface of a musical or other type of disc created by oneself.

    CD - RW (Compact Disc-ReWritable) a rewritable optical drive. It allows you to read CD-ROM , CD-R , CD-RW discs, burn CD-R discs once, but also burn and overwrite, as well as rewrite previously recorded CD-RW discs. This drive has the characteristic of not only reading disks, but also for writing. For example, the read speed is 40x and the write speed is 6x. Could also be the speed of rewriting.

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

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

    CD-RW format discs have even lower reflectivity and may not be readable on all old CD-ROM and CD-R drives (on old drives). It is rather difficult to say for certain which drives will be readable and which will not be, since it depends on the device model. CD-R CDs are currently being sold on which information can be written. If, after recording, the disc is left with free space, then information can be added to the disk, and so on. CD-RW discs allow you not only to write information, but also to delete unnecessary data, that is, to write data repeatedly and are somewhat more expensive than CD-R discs.

    In 1996 there were DVD -discs(Digital Versatile Disc - digital versatile disc, originally stood for Digital video Disc - digital video disc. Now it is not decrypted in any way), which had a capacity of 4.7 GB due to the compression of tracks with recording, that is, 7 times more than the capacity of CD-ROM discs. This is the most common type of discs that are single layer and single sided. However, there are discs that have two layers on one side and they have a capacity of 8.5-8.7 GB (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 GB (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 you to read information either from the first or from the second layer. Higher data density is achieved by reducing the area on disk per bit and applying compression techniques. But in practice, one-sided, single-layer ones are the most common.

    After creating a single DVD standard for recording videos on them, the whole world was divided into six zones so that movies recorded for one zone could not be read in others. Therefore, an old DVD drive may have an icon showing a globe with numbers indicating which zones this drive works with or ALL (all) - to work with discs in all zones. Modern DVD drives do not have this partitioning.

    Information on disks is located in sectors that contain data and 882 bytes for an error correction code, which makes it possible to increase the reliability of reading information, since in case of failures values ​​​​are calculated according to the correction code. In the presence of bad sectors, the reading speed slows down and re-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 about the impossibility of reading information from this disk, after which the switch to top speed.

    Unlike CD discs, DVD discs have their own UDF or ISO-9660 file system for data. Data is stored in 2048 byte sectors. There can be DVD-Video, DVD-Audio, DVD-Data and mixed discs.

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

    The standard for writing to a disc 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 denoted by the symbols DL , for example, DVD -R DL and DVD +R DL . They have a capacity of up to 8.5 gigabytes.

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

    DVD - ROM The drive only reads 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 / s, which is 9 times faster than the speed of CD. They have different speeds reading CDs and DVDs, which is indicated in the manual for the device.

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

    DVD - RW a rewritable optical drive. It allows you to read all kinds of CDs and DVDs and burn them. Read and write 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 that the drive can hold. 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, and the plus format allows you to overwrite data in real time.

    Standard blu - ray Disc (BD ) (blue ray- blue beam and disc- disk; writing blue instead of blue- deliberate)was developed by the BDA consortium and released in 2006. At this standard There was a competitor - Toshiba HD DVD, however, this company refused further support for HD discs in 2008 after the "format war". Reading speed (single speed) is 4.5 Mb/s. The increase in the amount of recorded information is carried out by using a laser beam in the blue-violet range with a shorter wavelength of 405 nm, while CD and DVD drives use red and infrared lasers with a wavelength of 650 nm and 780 nm.

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

    Read-once BD-ROM, write-once BD-R, and rewritable BD-RE are currently available. There are also dual-layer discs with the symbols DL in the name with a capacity of up to 50 gigabytes.

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

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


    The actuator must be in a horizontal position for normal operation. 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, insert a thin rod, such as 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 desirable to install a Slave, and there is also a connector for testing the drive by the manufacturer. Some drives may come with microphones, headphones, sound cards.

    For disk loading need to:

    Turn on the computer;

    Press the tray open button, while it extends;

    Place the disc label 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 in and out manually. It is undesirable to keep the tray open for a long time when there is no work, do not put foreign objects on the tray, for example, put a cup of coffee, do not put pressure on the tray when the disc is placed.

    If there is no operation, the drive enters the power saving mode, and the noise of the drive stops. When a read command is received, the drive starts working automatically.

    The manufacture of the disk is 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.

    Main drive characteristics:

    Type: interior or external. The internal drive is inserted into the system unit. The external one has a rectangular case, connects to a parallel port (in older 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(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;

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

    - interface type or the bus to which it is connected;

    - average access time(Access Time, AT). It is greater for CD-ROM drives than for hard drives, which is determined by fundamental differences in the design of the drive, and differs by dozens of times, and the greater the multiplicity, the shorter the access time. So, for a 4x drive, it is approximately equal to 150, and for 32 - 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 the level of noise, vibration. In addition, when buying, you need to look at whether the tray moves smoothly and whether it is firmly held open.

    BIOS latest versions allows you to boot your computer from CDs and DVDs. A CD-ROM disc at the beginning of a track has a service area that contains information for synchronizing the drive and the disc, then the Volume Table of Contents (VTOC), which contains information about the organization of directories and files on the disc, then data and a label end of volume. Thus, knowing the path and file name, you can use the table to find the location of the file on the disk and carry out direct positioning of the head for reading data, which reduces the time of searching and reading operations.

    connects device using two cables: power and information. There are three types of drives: connected to the SCSI bus, to the IDE bus or SATA connector. It is better to have a drive connected 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 drives for hard drives, then there may be a problem with the presence of a free connector.

    The connection to such a bus is described below. Optical drives can be connected together 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 disk controller (on older boards) or directly to the motherboard (see also the description of the boards and the 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 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 must be connected to the marking indicated above the plug, i.e. red (5v), black, black and yellow.

    If you have a sound card, to listen to sound from music discs, you need to 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 R symbol must match the R on the drive. The figure below shows the rear side of the drive, which has connectors for connecting wires.


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

    When working with disks, you must perform 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 sprays;

    Store discs in a disc case or sleeve;

    Do not bend the disc;

    Do not write on the working surface of the disc;

    When storing a disc, avoid direct sunlight and extreme heat, which may warp the disc.

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

    If there are specks on the disc, then, depending on their location and size, it is sometimes possible to use such a disc. Scratches that run from the edge to the center are often not dangerous, while scratches that run along the edge can 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 are used. When installing, use an interrupt (IRQ) - 7 and higher, base addresses 300h to 340h, DMA1. CDs are quite reliable, however, if the CD has cracks, 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 not be readable.

    Drive Installation. To install this device, you need:

    Turn off computer;

    Remove protective cover system block;

    Insert the drive into the guides of the system unit. Be sure to fasten the screws on the sides of the device after installation. Sometimes, in order to get to the screwdriver and tighten the screws, it may be necessary to remove other devices. After that, connect the wires as described above and install the protective cover, turn on the computer and check the operation of the drive.

    Technical installation optical drive similar to installing a hard drive.

    If the tray does not slide out, then the cause may be the hard fixing of the drive with screws inside the system unit, in which the drive was skewed. A sound during CD acceleration is not a malfunction. After installing the optical drive for the test, you can try to copy some of the files from the optical drive to the hard drive. Do not disassemble the drive yourself. The storage device must not be exposed to rain or be kept in a damp place.

    What can be a carrier of information? That on which everything that we need to remember can be preserved, for human memory is short-lived. Our ancestors left important data on the ground, and on stone, and on wood, and on clay until paper appeared. It turned out to be a material that meets the most important requirements for a data carrier. It was light, durable, easy to record and compact.

    These requirements are met by modern storage media - optical(these are CDs or laserdiscs). True, at the transitional stage (since the beginning of the 20th century), between paper and disks, magnetic tape helped us a lot. But her days are over. To date, the most convenient and reliable receptacle and storage of information are disks.

    And how to put the information on the disk? The concept of "recording a cassette" has been known to us for more than a dozen years. We are also talking about disks. Only this process has become much easier and cheaper.

    Today we will talk about optical storage media: device, recording technology, main differences.

    CD-R became the very first recordable optical media. They had the ability to record only once. The data were stored when the working layer was heated by a laser, causing its chemical reaction (at t? = 250°C). At this point, dark spots are formed in places of heating. That's where the concept of "burn" came from. DVD-R discs are burned in a similar way.

    The situation is slightly different with CD, DVD and Blu-ray discs that have an overwrite function. Such dark dots do not form on their surface, because. the working layer is not a dye, but a special alloy, which is heated by a laser up to 600°C. Then, the areas of the disk surface that fell under the laser beam become darker and more reflective.

    At the moment, in addition to CD discs, which can be considered pioneers in a number of optical media, discs such as DVD and Blu-ray have appeared. These types of discs are different from each other. For example, capacity. A Blu-ray disc can hold data up to 25 GB, a DVD disc can hold up to 5 GB, and a CD disc can hold up to 700 MB in total. The next difference is the way data is read and written to Blu-ray drives. The blue laser is responsible for this process, the wavelength of which is one and a half times less than that of the red laser CD or DVD drives. That is why on the surface of Blu-ray discs, equal in area to discs of other types, you can record information many times larger.

    laser disc formats

    The three types of laser discs listed above can also be classified according to their formats:

    1. CD-R, CD-RW discs are the same in size (up to 700; sometimes 800 MB, but such discs are not readable by all devices). The only difference is that CD-R is a one-time recordable disc, while CD-RW is reusable.

    2. DVD-R, DVD+R, and DVD-RW format discs differ only in the ability to rewrite DVD-RW discs multiple times, but otherwise the parameters are the same. 4.7 GB is the size of a standard DVD and 1.4 GB is the size of an 8 cm DVD.

    3. DVD-R DL, DVD+R DL are double-layer discs that can hold 8.5 GB of information.

    4. Formats BD-R - Blu-ray discs are single-layer, 25 GB and BD-R DL - Blu-ray discs are double-layer, 2 times larger.

    5. Formats BD-RE, BD-RE DL Blu-ray discs - rewritable, up to 1000 times.

    Discs with "+" and "-" signs are a relic of format disputes. Initially, it was believed that "+" (for example, DVD + R) is the leader for the computer industry, and "-" (DVD-R) is the quality standard for consumer electronics. Now almost all equipment easily recognizes discs of both formats. None of them have clear advantages over each other. The materials for their production are also identical.

    what are optical discs

    The disc itself, which is used at home to record information, is no different in size from commercially produced discs. The structure of all optical media is multilayer.

    • The basis of each is the substrate. It is made of polycarbonate, a material resistant to various external environmental influences. This material is transparent and colorless.
    • Next comes the working layer. For recordable and rewritable discs, it differs in its composition. For the former, it is an organic dye, for the latter, a special alloy that changes the phase state.
    • Then comes the reflective layer. It serves to reflect the laser beam, and may include aluminum, gold or silver.
    • Fourth - protective layer. The protective layer, which is a hard varnish, covers only CDs and Blu-ray discs.
    • The last layer is the label. This is the name of the top layer of varnish that can quickly absorb moisture. It is thanks to him that all the ink that falls on the surface of the disc during the printing process dries quickly.
    the process of transferring information to disk

    Now a drop of scientific theory. All optical storage media have a spiral track running from the very center to the edge of the disk. It is along this track that the laser beam records information. The spots formed during the "burning" of the laser beam are called "pits". Areas of the surface that remain untouched are called "lands". In binary language, 0 is pit and 1 is land. When the disc starts playing, the laser reads all the information from it.

    "Pits" and "lands" have different reflectivity, therefore, the drive easily distinguishes all dark and light areas of the disk. And this is the very sequence of ones and zeros inherent in all physical files. Gradually, it became possible to increase the accuracy of focusing due to the development of technologies that have achieved a reduction in the wavelength of a laser beam. Now a much larger amount of information can be placed on the same area of ​​the disk as before. the distance between the laser and the working layer directly depends on the wavelength. Shorter wave means shorter distance.

    disc burning methods

      Recording in the industrial production of discs is called stamping. In this way, discs with recording of music, movies, computer games are produced in large quantities. All the information that gets on the disk during stamping is a lot of tiny depressions. Something similar happened when gramophone records were made.

    • Recording a disc in domestic conditions occurs with the help of a laser beam. It is also called "burning" or "cutting".
    organization of the recording process on optical media

    Stage 1. Media type recognition. We loaded the disc and wait until the recorder gives out information about the appropriate recording speed and the most optimal power of the laser beam.

    Stage 2. The recording management program queries the recorder about the type of media being used, the amount of free space, and the speed at which the disc should be burned.

    Stage 3. We indicate all the necessary data requested by the program, and make a list of files that require writing to disk.

    Stage 4. The program transfers all the data to the recorder and monitors the entire “burning” process.

    Stage 5 The recorder sets the power of the laser beam and starts the recording process.

    Even with media of the same format, the quality of the recording can be drastically different. In order for the recording quality to be high, you should pay attention to the speed specified in the recording. There is a "golden rule" - fewer errors at lower speed and vice versa. The recorder itself, namely, its model, also plays a significant role.

    signature on optical discs

    It is advisable to immediately sign the disk on which some information appeared, in order to avoid confusion. This can be done in different ways:

    • printing text on blanks, the surface of which is varnished and allows you to print texts and images using an MFP with a special tray.
    • with the help of a recorder, with the support of special technologies that perform the application of text and a monochrome image on a special surface. The cost of such disks can be 2 times higher than the cost of simple disks;
    • a signature made independently by hand (with a special marker);
    • LabelTag technology - the text is applied directly to the disk work surface. The inscription may not always be well read;
    • labels printed separately on any of the printers. Their use is not welcome, because. they can damage the surface of the disc, come off at the time of its playback.
    duration of storage of optical storage media

    On the labels of new discs, you can see a period that indicates how long you can save data on this medium. Sometimes this figure corresponds to 30 years. In reality, such a period is practically impossible. During its existence, the disk can be subjected to various influences and damage. If it was recorded at home, then its shelf life is reduced even more. Only ideal storage conditions will keep all the data on the disks safe and sound.

    Unlike NMD optical disc, has only one physical track in the form of a continuous spiral, going from the inner diameter to the outer. But a physical track can be split into several logical tracks. 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 Ipfortatioop 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 order of their placement from the center of the disk):

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

    At the end of the outer track and at the beginning of the inner track is placed middle zone (MiddleZone), which contains no data. This zone is used to allow the laser beam to change focus to read data from the inner track.

    The input directory area in CD discs contains the contents of TOC (Table of Contents), recording addresses, number of titles, total recording time, volume and title of the disc. The data area of ​​a CD has the following data structure. The basic unit of data for a CD is frame (frate), which contains 24 encoded bytes, one control byte and eight error correction bytes. The frame is preceded by 24 bits, each 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 address unit of data on a CD. The sector contains 3234 encoded bytes (2352 data bytes and 882 error correction and control bytes). Out of 2352 bytes, the user information can occupy 2048 (in mode "1") or 2336 bytes (in mode "2"). Such an organization of data recording on CDs and the use of error correction algorithms makes it possible to ensure high-quality reading of information with an error probability of 10-10 per bit.


    Generalized structure of an optical disc drive

    A simplified GCD structure is shown in fig. 3.
    For GCD, several recording methods are used: ablative - by burning holes in an opaque media medium; by means of a local change in the reflection coefficient of the medium; transfer of 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 - for multiple overwriting of information on the GCD.

    When recording, a beam of a semiconductor laser diode controlled by the recording data through a collimator, a mirror and an objective lens burns a hole in the information layer of the disc. The presence of a hole corresponds to the entry "1". When reading, the unguided laser beam (derived from the beam splitter) enters the work surface through another beam splitter, a mirror and a lens.
    In reading mode, the mirror moves. The light from the laser passes through a polarizing-separating prism, hits a polarizing filter (in this case, the light is polarized in a certain plane), and then is focused on the surface of the optical disk. If the laser beam hits the flat surface (lands) of a CD-ROM or DVD-ROM disc, the light is reflected almost entirely. If the light enters at the recess (pts), then most of the light is scattered. The reflected light through the beam splitter enters the photodiode, the signal from which is processed electronic circuits readings. Precise alignment of the beam on the track is provided by the track servo, focusing by the focusing servo, and a constant number of revolutions by the disk rotation servo (see Fig. 3).

    Optical disc drives are divided into disc drives with the ability to both read and write discs. (recording drives) and read-only drives , and by types of disks - on CD drives And DVD drives.

    Figure 4

    An optical drive consists of main functional units:

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

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

    boot device There are two types of drives: container (caddy) And tray (tray). In the first type of container loading drive, the disc is placed in a plastic container and inserted into the drive (this container performs the same function as the 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 Ejest button. After pressing the button again or lightly pressing the tray, it is inserted into the drive (pop-up mechanism).

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

    Optical block contains the optical drive system shown in fig. 4. A 780nm (infrared) laser is used to read data from CDs, and a 650nm (red) laser is used to read DVDs. Therefore, DVD drives that read from CDs and DVDs typically contain two lasers with separate optical systems or a common optical system (switchable to one or the second laser).


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

    To write data to the disk, a separate recording laser is used, which operates in a pulsed mode with variable power (for "burning" the disk, changing the phase state from crystalline to amorphous and returning to the crystalline state). Typically, the write and read 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 velocity varies depending on the radial position of the head. Thus, the reading of the inner tracks is carried out with an increased, and the outer - with a reduced number of revolutions. The servomotor, on command from the drive's internal microprocessor, moves the reflecting mirror. This allows precise positioning of the laser beam on the track. The beam penetrates the protective layer of plastic and strikes a reflective layer of aluminum, silver or gold on the surface of the disc. When it hits the ledge, it is reflected on the detector and passes through the prism, which deflects it to the photosensitive diode. If the beam enters the hole (pit), it is scattered, and only a small part of the radiation is reflected back and reaches the light-sensitive diode. On the diode, light pulses are converted into electric-bright radiation is converted to “1”, weak - 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.

    track drive on commands that come from the built-in microprocessor, 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) at the commands of the encoding-decoding unit, and during reading, the beam reflected from the disk hits a photodetector, the signals from which enter the encoding-decoding unit and the automatic tracking system.

    Block encoding-decoding is a handler for signals written to or read from disk. It consists of an encoder, a decoder, a random access memory and a control controller. The encoding device prepares data for writing to a disc by performing character conversions according to EFM (for CDs) or RLL (2.10) (for DVDs) encodings and adds clock signals and service information to the data. The decoder extracts data from the digital stream, restoring their original form. The random access memory acts as a buffer memory, and the controller manages the error correction modes for data written in all formats supported by the drive.

    During writing or reading data from a disc, there may be irregularities in the positioning of the laser beam due to the radial beats of the disc. In order 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 the data, and the other two are used by the track tracking system. The pulse signals from the photodetector are fed to the amplifier of the automatic control system, where the tracking error signals are separated and the reading signal is corrected.

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

    Next to the computer interface connector and the power connector, optical drives also have connectors for connecting to a sound card or audio block 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 CDs).
    It is also provided hole, with which you can eject the CD even in an emergency, for example, even if the Eject button does not work. It is necessary to insert some thin stick about 2-3 cm into this hole, then the tray will slide out.

    Characteristics of optical discs and drives

    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 inches) and 80 mm (3.1 inches) diameters.
    120mm CDs have a capacity of 650MB or 700MB (depending on the recording mode), while 80mm CDs - capacity 185 MB. sometimes used to record data. optical business cards (ortiсalbusiness сards) - CD-R discs are rectangular in size 80 x 61 mm and have a capacity of 50 MB.

    Optical drives have the following main characteristics:

    Compatibility;
    - data transfer rate;
    - 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 (the last two types of discs are read in the drive tray special hole).

    Compatibility different types disk drives means the ability to read and write other types of discs.
    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 set by the number of megabytes per second, as for other external memory devices, but by an increase factor relative to the base speed. The first CO drives had a data transfer rate of 150 kb/s, as did CD players. This basic speed is obviously insufficient for reading video data, for example. Therefore, the speed of rotation of disk drives and, accordingly, the data transfer rate began to increase (at first, twice). Such drives (with a data transfer rate of 300 kb / s) became known as 2x drives. In the future, the speed of disk drives increased even more. Drives now have a maximum transfer rate of 54x (16.2 MB/s) and higher (up to 76x).

    For DVD drives due to higher data density and top speed A 1x value corresponds to a data transfer rate of 1.32 MB/s, which means that a 1x DVD drive is approximately the same as 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 writing data, maximum data transfer rates are lower and are now 40x for CD-R burners, 24x for CD-RWs, and 2x to 8x for DVDs. Typically, for CD-RW drives, both write and read speeds are specified separately, and for DVD burners, DVD burn speed, CD burn speed, DVD read speed, and CD read speed.

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

    Average access time - this is the time (in milliseconds) the drive needs to stay on the storage medium for the desired data. Obviously, work on the inner parts of the disk requires less access time than reading information from the outer parts. Therefore, the average access time is given in the drive passport as an average value for performing several readings of data 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 - 40 ... 250 ms.

    Cache capacity - is the capacity of the random access memory of the optical drive used to increase the speed of access to the data recorded on the medium (buffer memory). If special driver programs are used to control the drive, then the contents of the disk can be written to the cache memory in advance. Then access to a fragment of the requested data is much faster. The cache memory capacity of modern devices is from 64 to 2.048 MB.

    The drive buffer is a memory for short-term storage of data after reading it from the CD-ROM, but before transferring it to the controller board and then to the CPU. This buffering allows the disk device to transfer data to the processor in small chunks, rather than taking up its time with a slow transfer of a constant stream of data. An important feature of a drive is buffer fill rate, which affects the playback quality of animated images and movies. 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 moment they are issued to the system bus, to the total number of blocks that the buffer can accommodate. Too much major degree padding can lead to delays when issuing from the buffer to the bus; on the other hand, a buffer with too little filling will require more attention from the processor. Both of these situations result in jumps and tearing during playback.

    On CD-ROM burners, buffer memory is very important, as it ensures that information flows smoothly onto a CD-R or CD-RW. This allows for more reliable recording, as the recording of a track must not be allowed to stop, otherwise the entire disc may be damaged.

    Reading quality level is characterized by error rate(Eror Rate). This parameter displays the ability of the optical drive to correct write/read errors. Typically, the value of the error rate is 10-10...10-12. The error rate is an estimate of the probability that an information bit will be corrupted while it is being read. If the drive reads data from a dirty or scratched area of ​​the disk, it registers a group of error bits. If the error cannot be cleared by excessive error-correcting code (applied during write-read), then the drive slows down the data reading rate and repeats it many times. If the error correction mechanism fails to resolve the failure, then the message "Sector not found" (Sector not found) appears on the computer monitor. If the failure is corrected, 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 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 disc 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 channel of the IDE interface can support two embedded devices. The CD-ROM drive is connected to the I/O board via IDE interface along with a hard drive on a master/slave basis. However, in this case, the speed of data exchange with the hard disk is reduced. One way to solve this problem is to connect CD-ROM devices to various channels one 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 through the sound card controller. It should also not be forgotten that modern motherboards may contain built-in SCSI and IDE controllers, which generally eliminates the need for an additional I / O board to connect CD-ROM drives.

    Connecting audio channels. Almost everyone CD-ROM drive has a built-in digital-to-analog converter (DAC), as well as an output connector for outputting stereo signals. If there is audio information on the CD, the DAC converts it into an analog form and sends a signal to the headphone jack, as well as to the output audio jacks of the drive, from which, in turn, the signal goes to the amplifier and speaker system directly or through sound card. The advantage of the active output is that the audio signal from the CD-ROM is further 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 in various formats. To write 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 referred to as the CD-ROM and DVD-ROM formats, are defined in the ISO 9660 standard. Three format levels are given in this standard. The level 1 format (level 1) defines the names of the recorded files as MS DOS file names, that is, file names can contain up to eight characters given the three extension characters. Recorded files must occupy several following one after one sectors (non-fragmented recording). The level 2 format (level 2) allows the use of long file names, and the level 3 format (level 3) additionally allows files to be written to several sections of the disk (fragmented recording) in batch mode. In order to be able to write long names defined in the Windows operating system, the company Місrosofі supplemented 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 (at the beginning of the disc) is recorded on the disc (except for the content), which allows you to use the optical disc as a bootable disc.

    If the ISO 9660 standard is intended to ensure compatibility between CD-ROMs and DVD-ROMs used in different computer systems, then the UDF format (UniversalDiskFormatі - universal disc format) is designed to be compatible with read-only optical discs (ROM) and recordable optical discs (R or RW) in different operating systems. This format, like IS0 9660, allows long filenames and data to be written. Data is written to an optical disc in small chunks using the Package Writing (for CDs) or Incremental Writing (for DVDs) mode.
    Audio CD formats And Audio DVD used to record music. These are two different formats. Format Audio DVD provides better quality music recording.

    Video CD (VCD) and VideoDVD formats used to record movies. They 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 reproduction of the movie on CD-ROMs is provided by the Super Video format CD. To record movies in Video DVD format on CO-discs, the mini-DVD format is used.

    These formats are the most common, since they allow you to record music and movies, as well as text data, graphic data and programs on optical discs. These formats are supported by most disc drives and optical disc burning software. There are also other formats, designed mainly for such types of data that are used much less often, for example, Photo CD by Kodak, recording and playback of high quality digital photos. Both photos and music can be recorded in IS0 9660 or UDF formats as normal image 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 disc in the same way as it would to a floppy disk. Using this format, an optical disc can be accessed (read or written) from any applied task no call special programs reading and writing to optical discs.

    CD formats are sometimes named after 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, and the "green" book describes the CD-I(now almost never used), in "blue" - Enhanced CD and in "white" - Video CD.

    Information carriers - material that is intended for recording, storage and subsequent reproduction of information.

    Information carrier - a strictly defined part of a particular information system, which serves for intermediate storage or transmission of information.

    Information carrier - This physical environment in which it is fixed.

    Paper, photographic film, brain cells, punched cards, punched tapes, magnetic tapes and disks or computer memory cells can act as a carrier. Modern technology offers more and more new types of media. To encode information, they use the electrical, magnetic and optical properties of materials. Carriers are being developed in which information is recorded even at the level of individual molecules.

    All machine media are divided into:

    1. Perforated - have a paper basis, information is entered in the form of punches in the corresponding line and column. The amount of information is 800 bits or 100 Kb.

    2. Magnetic media - as they are used floppy magnetic disks and cassette magnetic tapes.

    3. (compact discs) is a metallized plastic compact disc with a diameter of 120 mm and a thickness of 1.2 mm. One of its sides has a reflective aluminum layer, which is covered with a protective varnish to prevent damage. Recording and reading of information is performed by a laser beam on a track going in a spiral from the center.

    Optical storage media– (compact discs) is a metallized plastic disc with a diameter of 120 mm. And 1.2mm thick. One of its sides has a reflective aluminum layer, which is covered with a protective varnish to prevent damage. Recording and reading of information is performed by a laser beam on a track going in a spiral from the center.

    Types of optical discs:

    1. CD-ROM (Compact Disk Read Only Memory) - CD, without the ability to record information.

    2. CD-R (Compact Disk Recordable) - a CD with the ability to record information once.

    3. CD-RW (Compact Disk Rewritable) - a CD with the ability to rewrite information.

    4. DVD (Digital Versatile Disk) - a digital multilayer disk for recording large amounts of information (up to 18 GB).

    ADVANTAGES: Reliability. Ability to record large amounts of information. Wear resistance.

    Compact Disk (CD) is a disc with a diameter of 120 mm (4.75 inches) or 80 mm (3.1 inches) and a thickness of 1.2 mm. The stroke depth is 0.12 µm, the width is 0.6 µm. The strokes are arranged in a spiral, from the center to the periphery. The stroke length is 0.9–3.3 µm, the distance between the tracks is 1.6 µm. CDs are made up of three to six layers. A standard five-inch disc can contain 650-700 MB of information, 74-80 minutes of high-quality stereo sound at 44.1 kHz sampling rate and 16-bit bit depth, or a huge amount of MP3 audio. About 180 MB of information is placed on three-inch disks. Sometimes there are discs called "business card" (business card). By appearance They look like a business card and are actually three-inch discs cut off on both sides. On such a CD is recorded from 10 to 80 MB.

    In the late 1970s, Sony and Philips began to jointly develop a single standard for optical storage media. Philips has created a laser player, and Sony has developed a technology for recording on optical media. At the suggestion of a corporation Sony size disk was equal to 12 cm, because. this volume made it possible to record the entire Ninth Symphony of Beethoven. In 1982, in a document called Red Book (Red Book), the standard for processing, recording and storing information on laser discs was published, as well as the physical parameters of the disc, i.e.: 1. The physical size of the disc. 2. Disk structure and data organization. 2. Recording data in a single stream from the center to the periphery. 3. Reading data at a constant linear velocity (Constant Linear Velocity, CLV).

    All data on the disk is divided into frames. Each frame consists of 192 bits for music, 388 bits for modulation and error correction data, and one control bit. 98 frames make up one sector (sector). Sectors are combined into a track (track). A maximum of 99 tracks can be recorded on a disc.

    During the recording and reading of information, when the laser beam moves from the center to the periphery, the disk rotation speed is ↓. This is necessary to ensure the ability to read and write the same amount of information in the same time. Therefore, without the use of CLV technology, when playing, for example, musical works, there would be a change in the speed of performance.

    Due to the relatively small size of laser discs compared to vinyl records, they have become known as compact discs, or CD (Compact Disk) for short. The first CDs were designed for recording and playing music and allowed up to 74 minutes of high-quality stereo sound to be stored. The standard for such discs was called CD-DA (Compact Disk Digital Audio).

    With the development of the computer industry, there was a need for a technology that allows you to store on CDs not only digital sound, but also various data. Computer programs couldn't fit on floppy disks, and user files got bigger and bigger.

    In 1984 a standard called the Yellow Book was published. Sony and Philips reorganized the structure of CDs and began to use new error correction codes - EDC (Error Detection and Correction) and ECC (Error Correction Code). The sector has become the main unit of data allocation. One sector contains: 12 bytes for synchronization, 4 bytes for headers, 2048 bytes for user data and 288 bytes for error correction. CAV (Constant Angular Velocity) technology was developed to read computer data. CAV technology allows you to read information from a disk faster than CLV technology, since when the laser beam moves from the center to the periphery, the data flow increases. Modern drives CDs support both technologies. Computer laser discs were called CD-ROM - Compact Disk ReadOnly Memory (literally - "read-only memory on CDs"). In the late 1990s, the CD-ROM drive became a standard part of every computer, and the vast majority of programs were distributed on CD-ROMs.

    The consumer market was expanding rapidly, production volumes were increasing, and the largest manufacturers were developing technology that allows the user to independently burn any information on a CD. In 1988, the world's first CD-R (Compact Disk Recordable) was released by Tajyo Yuden. The biggest challenge faced by CD burner designers is finding highly reflective materials. Tajyo Yuden successfully coped with the task. The alloy of gold and cyanine they used to make these drives was over 70% reflective. The same company developed a method for applying an active organic layer to the surface of a disc, as well as a technology for dividing a disc into tracks.

    DVD, DVD-R, DVD-RW, CD, CD-R, and CD-RW discs are produced by various companies: AMD, Amedia, Digitex, HP, Imation, MBI, Memorex, Philips, Smartbuy, Sony, TDK, Verbatim.

    DVD structure.

    In December 1995, 10 companies, united in the DVD Consortium, officially announced the creation of a single unified standard - DVD. The abbreviation DVD was originally deciphered as Digital Video Disc (Digital Video Disc), but later its meaning was changed to Digital Versatile Disc (Digital double-sided disc). The disc was fully compatible with the Red Book (Red Book) and Yellow Book (Yellow Book) standards. DVD is externally identical to CD, but allows you to record information that is 24 times larger in volume, that is, up to 17 GB. This was made possible by changing the physical characteristics of the disk and the use of new technologies. The distance between the tracks decreased to 0.74 µm, and the geometric dimensions of the pit were reduced to 0.4 µm for a single-layer disk and 0.44 µm for a two-layer disk. The data area has increased, the physical sizes of the sectors have decreased. A more efficient error correction code, RSPC (Reed Solomon Product Code), has been used, and more efficient bit modulation has become possible. DVD technology provides a huge number of formats and four types of designs in two sizes. A disc of this standard can be either single-sided or double-sided. Each side can have one or two working layers.

    Burning single-layer DVDs is similar to burning CDs, but burning dual-layer discs is quite different from the process described earlier.

    Dual-layer discs of the DVD-2 and DVD-9 types have two working layers for recording information. These layers are separated using a special translucent material. To perform its function, such a material must have mutually exclusive properties: it should reflect the laser beam well in the process of reading the outer layer and at the same time be as transparent as possible when reading the inner layer. Commissioned by Philips and Sony Corporation, 3M has created a material that meets these requirements: having a reflectance of 40% and the necessary transparency. DVDs are 0.6mm thick. For physical compatibility from CD to DVD, a polycarbonate substrate 0.6 mm thick was additionally glued.

    The CD specification does not provide for any copy protection mechanism - discs can be freely duplicated and played back. However, starting in 2002, various Western record labels began to attempt to create copy-protected CDs. The essence of almost all methods is to deliberately introduce errors into the data written to the disc, so that the disc is played on a household CD player or music center, but not on a computer. The result is a game of cat and mouse: such discs are far from being read on all household players, and on some computers they are read, software is being released that allows you to copy even protected discs, etc. The recording industry, however, leaves no hope and continues try new methods.

    There are also magneto-optical disks. : FLOPTICAL = FLOPPY (floppy) + OPTICAL.

    The surface of the magneto-optical disk is coated with a special material whose properties change under the influence of temperature and magnetic fields. All these discs differ from each other in diameter and the number of working surfaces. The amount of information is up to 10 GB.

    External memory

    Optical discs

    Optical (laser) disks 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 disc is recorded on one spiral track starting from the center of the disc and containing alternating sections of depressions and protrusions with different reflectivity.

    When reading information from optical discs, a laser beam installed in the drive falls on the surface of a rotating disc 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 by photocells into electrical impulses.

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

    There are two types of optical discs:

  • CD-disks (CD - Compact Disk, CD), which can be recorded up to 700 MB of information;
  • DVD-disks (DVD - Digital Versatile Disk, digital universal disk), which have a much larger information capacity (4.7 GB), since the optical tracks on them are thinner and more densely placed.
    DVDs can be double-layered (capacity 8.5 GB), while both layers have a reflective surface that carries information.
    In addition, the information capacity of DVD discs can be further doubled (up to 17 GB) since information can be recorded on both 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 DVD discs due to the use of a blue laser with a wavelength of 405 nanometers.

    Optical drives are divided into three types:

    • No write capability- CD-ROM and DVD-ROM
      (ROM - Read Only Memory, read-only memory).
      On CD-ROMs and DVD-ROMs store information that was written to them during the manufacturing process. Writing new information to them is not possible.
    • Write-once, read-once -
      CD-R and DVD±R (R - recordable, recordable).
      Information can be written to CD-R and DVD±R discs, but only once. Data is written to the disc by a high-powered laser beam, which destroys the organic dye of the recording layer and changes its reflective properties. By controlling the power of the laser, an alternation of dark and light spots is obtained on the recording layer, which, when read, are interpreted as logical 0 and 1.
    • With the ability to rewrite- 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 that can be brought to two different stable states of aggregation by heating, which are characterized by different degrees of transparency. When recording (erasing), the laser beam heats up a section of the track and puts it into one of these states.
      When reading, the laser beam has less power and does not change the state of the recording layer, and alternating sections with different transparency are interpreted as logical 0 and 1.

    The main characteristics of optical drives:

  • disk capacity (CD - up to 700 MB, DVD - up to 17 GB)
  • the speed of data transfer from the carrier to the RAM - measured in fractions, multiples of the speed
    150 KB/s for CD drives (The first CD drives had this reading speed) and
    1.3 MB / s for DVD drives (This was the speed of reading information in the first DVD drives)

    Currently, 52x-speed CD drives are widely used - up to 7.8 MB / s.
    CD-RW discs are recorded 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 the disk, measured in milliseconds (for CD 80-400ms).

    If the rules of storage (storage in cases in a vertical position) and operation (without scratches and dirt) are observed, optical media can retain information for decades.

    Additional information about disk structure

    The disk, created by an industrial method, consists of three layers. An informational pattern is applied to the basis of a disk made of transparent plastic by stamping. For stamping, there is a special matrix-prototype of the future disc, which extrudes tracks on the surface. Next, a reflective metal layer is sprayed onto the base, and then a protective layer of a thin film or special varnish is also sprayed 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 a recording layer is located between the base and the reflective layer, which can change under the influence of high temperature. When recording, the laser heats up the specified sections 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 below the first and has a transparency of about 40%. To read dual-layer discs, complex optical heads with a variable focal length are used. The laser beam, passing through the translucent layer, is first focused on the inner information layer, and upon completion of its reading, it is refocused on the outer layer.

    • Read more: