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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING CDs: technologies and standards. Reference data
Encyclopedia of radio electronics and electrical engineering / Reference materials Regularly reading the computer press, communicating with people related to computers, being interested in advertising the latter in print and on television, it is easy to see what gigantic steps this branch of technology is developing. Imagination is shaken by ever more advanced processors, high-quality monitors, printers, and often completely new products. Many of the capabilities of computing technology are due to its peripheral equipment. This article describes one of the types of peripherals - devices for long-term storage of information on optical CDs. Over the relatively short history of computing technology, many types of media have changed, on which information can be stored indefinitely: paper punched cards and punched tapes, magnetic tapes, drums, floppy and hard disks of various sizes and capacities, and, finally, magneto-optical and optical disks. Manufacturers of high-tech accessories for computers today probably have a lot of promising ideas in this area, but so far magneto-optical and optical disks are gaining more and more popularity. In this article, we will focus only on optical discs, which appeared quite a long time ago, but are constantly being improved and are steadily gaining more and more popularity. The more common name for optical discs is "compact disc" or CD-ROM (CD for short). A CD is capable of storing a huge amount of information in a small physical volume. The possibility of repeated reading of the recorded data without wear of the media is also important, due to the absence of any mechanical contact of the reading device with the surface carrying the information. To this should be added the relatively low cost of the disks themselves and the devices needed to work with them. These advantages cannot fail to attract everyone who has to store huge amounts of data with minimal risk of losing it. And there are more and more of them. Wherever there are computers, there are bound to be powerful programs, archives and databases, images and sounds digitized. All this is conveniently stored on a CD. PRINCIPLES OF DESIGN AND OPERATION A modern CD is a plastic disc about 120 in diameter and about 1 mm thick, with a 15 mm hole in the center. Around the hole there is an area about 10 mm wide for clamping in the spindle that rotates the disc. One side of the CD is usually beautifully designed and provided with brief information about the contents of the records. The other - glitters and shimmers with all the colors of the rainbow. It has another visually distinguishable ring around the clamping area, which is stamped with a serial number in a barcode or other code, often understandable only to the disc manufacturer. Next is the data area, which gives the rainbow effect when viewed in reflected light. From the outer edge of the CD has a transparent protective ring of small width [1]. The most common CDs have the structure shown in Fig. 1. The thinnest reflective layer 1 of aluminum is applied to the base 2 of acrylic plastic. The metal is covered with a transparent protective polycarbonate film 3. The data is read by a laser beam 4. The usual process of making a CD consists of several stages: preparing data for recording, making a master disc (original) and matrices (negatives of the master disc), replicating a CD.
Information is applied to the smooth surface of an aluminum master disk by a laser beam, which, by changing the structure of the metal (in other words, by burning it), creates microscopic cavities on it. The alternation of differently reflective pits and flat areas represents the data in the usual binary form for computers. Note that the dimensions of the cavities formed by the laser beam are very small - on a segment whose length does not exceed the thickness of a human hair, several tens of them can be accommodated [2]. What follows is reminiscent of the production of conventional gramophone records. The negative copies of the master disc serve as matrices for pressing the information-bearing depressions on the surface of the CD itself, which remains to be covered with aluminum, applied with a protective layer and provided with the necessary inscriptions. It is worth noting that there are other technologies for the production of CDs, including rewritable and rewritable, some of which will be discussed below. READING INFORMATION Under the CD, inserted into the drive with the shiny side down and fixed in a rotating spindle, a reader moves along the radius with the help of a servomotor (Fig. 2). It consists of a semiconductor laser 1, a beam-splitting prism 2 with a lens 3 that focuses the beam on the disk surface 4, and a photodetector 5. The lens is equipped with drives for fine-tuning the beam position on the information track. It is clear that a laser of much lower power is used for reading than the one used to burn the depressions on the surface of the master disk [3].
The beam reflected by the aluminum surface is directed by the prism to the photodetector. If it is reflected from a shiny island between the depressions, an electric current appears in the photodetector circuit, the presence of which is interpreted as logical 1. The beam that falls into the depression is mostly scattered, as a result, the illumination of the photodetector and the current generated by it decrease - logical 0 is fixed. The sensitive surface of the photodetector is divided into four sectors. This allows the microprocessor controlling the drive to determine if the beam is correctly positioned. If the beam deviated from the desired position (and this, as a rule, happens due to errors in the manufacture of the CD and the drive), the spot created by it on the surface of the photodetector will also shift, as a result of which its sectors will be illuminated unequally. Comparing the currents generated by each of the elements of the receiver, the microprocessor generates commands that correct the position of the lens, and hence the beam on the surface of the reflective layer. DATA STRUCTURES As already mentioned, data is recorded on a CD as a sequence of pits and intervals between them, forming one physical information track. Just one, in contrast to the usual way of recording on magnetic disks. This single track is a spiral that starts at the center of the disk and unwinds towards its edge. This CD is a bit like a traditional record, differing from it in the direction of the spiral and the non-contact method of reading data. The track begins with the service area necessary for drive synchronization: the reader must "know" when to expect the arrival of each of the recorded bits of information. A physical track can be divided into multiple logical tracks. The continuous stream of bits read from the CD is divided into eight-bit bytes, logically grouped into sectors. Each sector consists of 12 bytes of synchronization, four bytes of a header containing the sector number and information about the type of record in it, 2048 bytes of the main data area and 288 bytes of additional information. Several types of sectors are used. The first one is for digital audio only. The second one is the main one for all CDs. Its header is extended to 12 bytes due to the area of additional information. The rest of this area is occupied by a data reading error detection code (four bytes) and two codes that allow them to be corrected: P-parity (172 bytes) and Q-parity (104 bytes). In sectors of the third type, the additional information area is made available to the user. So each of them can contain up to 2336 bytes of data, but without the ability to control the correct reading and error correction. Each logical track consists of sectors of only one type [4]. The first sectors of the CD contain its contents (Volume Table of Contents, VTOC) - something like a file allocation table (FAT) on magnetic disks. In general, the basic CD format according to the HSG standard (see below) is in many ways reminiscent of the format of a floppy disk, on the zero track of which not only its main parameters (number of tracks, sectors, etc.) are indicated, but also information about the placement of data is stored (directories and files). The system area contains directories with pointers or addresses of areas where data is stored. The essential difference from a floppy disk is that the direct addresses of files located in subdirectories are indicated in the root directory of a CD, which greatly facilitates their search. The classic "single" data reading speed, which only audio CD players work with today, is 175 KB/s, or about 75 sectors per second. Each logical track containing 300 sectors is played back at this rate in 4 seconds. The entire CD, if it consists only of sectors of the second type, contains 663,5 MB of data. Computers use CD drives, which provide much faster data reading by increasing the spindle speed and correspondingly changing a number of other technical characteristics. Drives with 12x and 16x speed increase are common today. But there are also those in which it is 24 and even XNUMX times more than "single". CD STANDARDS Musical optical CDs replaced mechanically recorded vinyl (phonograph records) in 1982, almost simultaneously with the advent of the first personal computers from IBM. This was the result of a collaboration between two giants of the electronics industry, the Japanese firm Sony and the Dutch Philips. The history of the choice of CD capacity is curious. Sony CEO Akio Morita decided that the new products should meet the requirements of classical music lovers. After conducting a survey, it turned out that the most popular classical work in Japan - Beethoven's ninth symphony - sounds about 73 minutes. Apparently, if the Japanese were more fond of Haydn's short symphonies or Wagner's operas performed in their entirety in two evenings, the development of the CD could have taken a different path. But the fact remains. It was decided that the CD should be 74 minutes and 33 seconds long. Thus was born the standard known as the "Red Book" (Red Book). Not all music lovers were satisfied with the chosen duration of the sound, but compared to 45 minutes of short-lived vinyl records, this was a significant step forward. When 74 minutes of music were counted into information capacity, it turned out to be about 640 MB [2]. The two firms mentioned above also played a leading role in the development of the first digital CD standard, the so-called "Yellow Book". The disks created on its basis, capable of storing, in addition to audio data, also text and graphic data, were called CD-DA (CD-Digital Audio). The CD-DA header contains information that allows you to determine the type of data recorded. The standard, however, did not regulate the logical and file recording formats. Their choice was completely entrusted to manufacturing firms. As a result, a Yellow Book-compliant CD could often only be read by a device of the model for which it was designed. Such a situation, especially in connection with the great commercial success of the CD, of course, could not satisfy anyone. In the common interest, it was urgent to find a compromise. The second de facto standard for digital CDs was HSG or simply High Sierra. We note an interesting detail: it is named after a hotel and casino in one of the towns of California, where the main CD producers gathered to discuss their problems. This document was advisory in nature and was proposed to provide at least some compatibility. It defined both the logical and file formats of the CD. Unfortunately, there was no suitable color for the book with the HSG standard. Nevertheless, it turned out to be so attractive that the main provisions of the international standard ISO 9660 adopted somewhat later coincided with HSG. ISO 9660 describes the CD-ROM file system. According to the first level standard, it resembles the similar MS DOS system: file names can be up to eight characters long and have a three-character extension separated by a dot. Special characters are prohibited in names (for example, "~", "-", "=", "+"), only uppercase (capital) Latin letters, numbers and underscores are used. Each file is supplied with a version number, which is separated from the extension by a ";" character. Directory names cannot have extensions. You can nest up to eight directories. The ISO 9660 Level 32 standard allows files to be named up to XNUMX characters long, subject to the restrictions described above. CDs created according to this standard are unsuitable for use in a number of operating systems, including MS DOS. Before continuing with CD standards, let's look at the notion of a recording session. Most CDs are single-session (Single Session), since all data is recorded on them in one technological cycle or recording session. However, after the appropriate technologies and special disks were developed, it became possible to perform additional recording sessions, adding new portions of data to the existing ones. Multisession CDs include PhotoCD and CD-ROM XA (extended architecture) CD formats. PhotoCD technology was proposed by Eastman Kodak as a means of creating and viewing digital photographs. On a special disk, you can digitally burn images from any 35 mm slides and negatives one by one. However, a PhotoCD-compatible drive is required to fully read the information. A normal HSG or ISO 9660 standard will only be able to read the entry made in the first session, since the VTOC, located at the beginning of the information track, has information about it only. The CD-ROM XA standard is top compatible with High Sierra and ISO 9660. However, it has many more features. First, it allows multi-session recording. Secondly, you can store graphics, text and sound data on the same disk, and graphics can include both still pictures and animation, and full-motion movies. The main feature of CD-ROM XA is the so-called interleaving of blocks of heterogeneous information. For example, the first video frame can be followed by its soundtrack, after which the next frame is located, etc. This contributes to the synchronism of sound and image playback, significantly reduces the required amount of the intermediate buffer, compared to that required for the usual arrangement of data on disk. Another feature of the XA standard is the compression of audio data, which makes it possible to record several hours of audio information on one disc (instead of the usual 74 minutes). Although compression algorithms for a wide variety of data are actively used in many branches of computer technology, this advantage of CD-ROM XA is not yet widely used. Another attempt by Sony and Philips to exhaustively regulate not only the logical and file formats, but also the contents of the files themselves on digital CDs resulted in a standard known as the Green Book. Actually, this is an extended version of the XA CD-ROM standard. Green Book drives can read CD-DA, CD-ROM, CD-ROM XA, CD-I and Kodak PhotoCD [2] formats. The CD-I (Interactive) format mentioned here for the first time deserves a description. Real-time audio and video devices with advanced word processing and graphics capabilities are considered sources of interactive information for CD-I. Computer programs are expected to be widely used to process all kinds of data. With regard to information and system tasks in the CD-I format, possible data types and ways of encoding them are determined, as well as the organization of the necessary means of supporting disk systems. From a technical point of view, the CD-I format is based on CD-ROM technology, but for the consumer it is close to CD-DA. On one disc, you can combine tracks of CD-DA and CD-I records, use CD-DA decoding equipment in CD-I systems. CD-I format discs are most often used in the fields of education (distance learning and self-study using reference books, albums, "talking" books), entertainment (music with text, notes, pictures, games), leisure activities (drawing and drawing, film making , real-time animation, poetry writing), tourism (maps, navigation devices, information about places of interest), disease diagnostics, and many others. The latest CD standard in use today is the Orange Book. In its first part, we are talking about magneto-optical storage devices (CD-MO), which allow erasing and overwriting information. The second part is devoted to WORM (Write Once Read Many) and CD-R (Recordable) drives. Data can only be added to these devices. It is not possible to delete an existing entry. Almost all CD drives currently sold meet the requirements of the second part of the "Orange Book" - they can read CDs of all the described formats, including rewritable ones. The standards discussed refer to CDs suitable for use on IBM-compatible personal computers. Of course, there are formats designed for other systems, such as Macintosh HFS for Apple Macintosh computers, but we will not touch on them. In the first part of the article, almost all popular data storage formats on CD-ROM were considered. One of their features is the difference between the structure of the CD file system and that adopted in MS DOS. Thus, in order to access the recorded data, it is necessary to convert their format. To solve this problem, Microsoft has released a special software driver called Microsoft CD Extentions (MSCDEX.EXE). It is very common, included with MS DOS and almost all CD-ROM drives. When using MSCDEX.EXE, the operating system treats the CD as if it were a regular magnetic disk (except that the data can only be read). To load the driver, the AUTOEXEC.BAT file must contain a command (written in one line) MSCDEX /D: name [/D: name2...] [/E] [/K] [/S] [/V] [/L:letter] [/M:number] Its parameters (optional - in square brackets) specify the following: /D:name [/D:name2...] - names of CD-ROM drives installed in the computer. They must match those specified in similar parameters of commands in the CONFIG.SYS file that these drives are started with. The default name is MSCD001. /E - Allows placing disk sector buffers in extended memory, if available. /K - MS DOS can read CDs using the Japanese Kanji character set. /S - Allows access to the CD-ROM from the local computer network. /V - During startup, MSCDEX will display statistics on the screen. /L:letter - this letter will indicate the logical drive corresponding to the CD-ROM drive. If it is not set, the driver uses the first free one. For example, on a system that already has drives A, B, and C, the CD-ROM will default to drive D, and if the /L:H option is present, it will be drive H. If there is more than one CD drive, the rest will get the next unused letters. /M:number - how many CD sector buffers the driver will create. There can be from two to 30 (default - 10) and each will take about 2 KB in memory. The more buffers, the better the system performance. MSCDEX.EXE must be used in conjunction with CD - ROM drive drivers, described as devices (DEVICE) in the CONFIG.SYS file. These drivers are specific to each drive model, shipped with them, and also have several options. Unfortunately, it is not possible to list all options [2]. INTERFACES The interface links the CD-ROM drive and the computer. It is its characteristics that determine the speed of interaction of these devices. Each new type of disk and drive to it that appears on the market must have an interface that allows you to transfer large amounts of data without delay and with the least CPU load. Quite often, manufacturers supply a CD-ROM drive along with a controller that implements the so-called Proprietary interface. It is often located on the sound card, which is connected to a CD - ROM purchased as part of a multimedia kit. This is usually a simplified implementation of one of the standards discussed below. Very rarely (due to the low data transfer rate) is communication through a parallel port intended for a printer. Usually, some models of external drives are connected to it, since this does not require opening the computer. The port is most often configured to work in one of the advanced modes: EPP (Enchanced Parallel Port) or ECP (Extended Capabilities Port). To connect CD - ROM to portable computers, converters of their interface to parallel are often used. Many CD - ROM drives are equipped with an IDE interface (also known as AT - Bus, ATA), which is common for hard magnetic disks (hard drives). Its peculiarity is the implementation of controller functions in the drive itself, which makes connecting to a computer quite simple. A few years ago, Western Digital developed the EIDE standard - an improved (Enhanced) IDE, which was supported by five other leading companies. It allows you to install up to four hard drives, CD-ROM drives or streamers into your computer. The SCSI interface is popular (read "tell"). With it, connect many peripheral devices that require high data transfer rates. The usual speed for this interface is 2...4 MB/s. Physically, the SCSI bus is a flat cable with 50-pin connectors. Up to eight peripheral devices can be connected to it. The standard provides for two ways to transmit signals over the bus: common mode and differential. The latter is characterized by increased noise immunity and allows you to increase its length. To ensure undistorted signal transmission, terminations must be connected to the bus lines on both sides (the set of resistors provided for this is often referred to as a terminator). In the SCSI - 2 version, throughput is increased by increasing the clock frequency and reducing the critical time parameters of the bus through the use of the latest integrated circuits and high-quality cables. There are improved versions of this interface: "fast" (Fast) and "wide" (Wide). In the latter, 24 additional communication lines are provided and the devices are connected by another cable (68-wire). For CD - ROM drives, "wide" SCSI - 2 is practically not used [5]. The programming interface of the main (host) SCSI adapter installed in the computer is defined by the ASPI (Advanced SCSI Programming Interface) standard, developed by Adaptec, a leading manufacturer of such devices. The software modules of this standard fit together quite easily. The main one is the host manager. Device drivers are associated with it. If an ASPI-compatible driver is supplied with the SCSI CD-ROM drive, it will work with all host adapters (interface boards) from Adaptec and most other manufacturers. RECORDABLE CD We have already said more than once that technology is developing very rapidly and what was new yesterday is a familiar thing today, and tomorrow it is hopelessly archaic. Let's consider some perspective directions of CD development. Recordable CDs, already popular today, continue to be widely distributed. They are intended not for mass replication of programs and other information, but for single recordings or the production of a small number of copies. CD-R (Recordable - rewritable) fully comply with the requirements of the second part of the "Orange Book". Most recording devices support multi-session. The structure of CD - R is shown in fig. 3. It consists of several layers: carrier polycarbonate 1, organic 2, in which the laser beam "burns out" information, reflective (gold) 3 and protective 4 made of varnish resistant to external influences, on which the label is printed [6].
Several fundamentally different types of organic layer are used. It is made from materials of very complex chemical composition. During CD-R recording, small areas 5 of the organic layer are heated by a powerful focused laser beam and change optical properties (begin to scatter light). In unheated places, the layer remains transparent and transmits laser light 6 during data reading. The latter reaches the gold reflective layer and, returning back, hits the beam-splitting prism, and then onto the light-sensitive sensor. The higher light reflectance of gold than aluminum compensates for the energy loss of the readout beam in the organic layer. Although the way information is written to regular and rewritable CDs is different, the result is the same - a sequence of reflective and non-reflective patches that any CD-ROM drive can read. It should be noted that CD-R has some advantages over similar WORM disks that have a large capacity (double-sided - up to 1,2 GB), but due to their very high cost, they are not widely used [4]. NEW STANDARD: DVD TECHNOLOGY The last type of optical disc we'll cover in this article is the DVD. Today it is the newest and most promising standard. Just as CDs slowly replaced vinyl LPs, DVDs will gradually replace CD-ROMs in the future [6]. Initially, the abbreviation DVD was deciphered as Digital Video Disk (digital video disc), then - Digital Versatile Disk (digital universal disk), and today it is not deciphered at all. This technology has been under development for a very long time, but has finally reached the milestone, followed by widespread adoption. In particular, at the largest Russian computer exhibition Comtek'98, several video disks made using DVD technology were demonstrated [7].
Externally, a DVD resembles a regular CD, but the information on it can be recorded seven times more (4,7 GB). This value is typical for a single-layer, single-sided disc (SLSS). The information capacity of a double-layer single-sided (DLSS) is 8,5 GB, a single-layer double-sided (SLDS) is 9,4 GB, and a double-layer double-sided (DLDS) is about 17 GB, i.e. 26 times more than a modern CD - ROM. A DLDS disk (Fig. 4) consists of two glued substrates 1 0,6 mm thick each with information and protective layers several micrometers thick deposited on them. To read data from each of the information layers, the laser beam 4 is focused on one of them: translucent near-surface 2 or reflective depth 3. The laser diode of the reader does not operate in the infrared range characteristic of CD (wavelength 780 nm), but emits red light with a length waves of 650 and 635 nm, which made it possible to reduce the size of the pit (the area occupied by a unit of information on the working surface of the disk) by almost half and, accordingly, reduce the distance between the recording tracks. Increased requirements for focusing accuracy forced the use of an enlarged aperture lens. The high recording density required fault-tolerant data coding (EFM Plus 8/16) and the use of a reliable read error correction system (Reed-Solomon codes) [8]. There are five DVD substandards (books): A - DVD - ROM, B - DVD - Video, C - DVD - Audio, D - DVD - WO, E - DVD - RAM. It is not difficult to guess the content of each of the books. DVD-ROMs will replace the digital computer CDs we use today. DVD - Video with recording of moving images in full screen format will inevitably replace household video cassettes with magnetic tape. DVD - Audio is a replacement for the current audio CDs. DVD - WO (Write Once - write once) are similar to rewritable CD - R. Great hopes are placed on the most technologically sophisticated DVD - RAM. In the distant (or not so) future, they will replace CD - RW (Rewritable - rewritable) or CD - E (Erasable - erasable), prototypes of which are just beginning to appear on the market. Each of the listed sub-standards is predicted with the brightest prospects. But it will be possible to truly appreciate their merits only by seeing them in action. So far, consumers should be impressed by the amount of content that DVDs are supposed to offer. Literature
Authors: A. Denisenko, A. Balabanov, Nizhny Novgorod
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