CD-ROM is an abbreviation for compact disc read-only memory, a physical non-volatile storage of digital data. It is according to the audio CD, the second application of the compact disc
A CD- ROM stores between 650 MB ( amount to 74 minutes of music - the conventional audio CDs) and 879 MB ( 100 - minute CDs can not be read in all CD drives, although not described in all CD writers and ). Depending on the quality and type of the blank and after burner can be fired even outside the standard range ( overburn ). However, this can result in errors or loss of data in this area.
Almost all of today's computer systems have a CD -ROM drive, with which the data can be read. The CD-ROM is one of the few storage media that can be read by various computer systems, provided the data have been recorded according to the ISO 9660 standard. Other common file systems for CD -ROM include Rock Ridge (UNIX) and Joliet (Windows). Supports CD -ROM, the El Torito specification, it is a bootable medium. All of these specifications, however, put on the ISO to 9660 Norm.
The first CD was presented in 1979 at an exhibition in Tokyo. The CD- ROM is currently the most widely used (physical ) medium for distributing data and software.
The CD- ROM is one of the official CD- formats that are specified in the " colored books " ( Rainbow Books ), in the case of CD -ROM in the " Yellow Book " ( = data CDs ) and " Orange Book " ( = recordable CD formats with multi-session capability). Only if the specifications described therein are complied with, the CD -ROM must bear the official assigned by Philips Compact Disc logo. There are more specific formats, such as CD-ROM XA, CD-Extra, CD -I, etc. Since in the production of CD-ROM (and their derivatives), certain patents by Phillips and / or Sony be used is Check whether this be required by the patent rights holders royalties. The production and distribution of unlicensed CD -ROMs can be prosecuted or banned in Germany and other countries by the patentee or government agencies.
Construction of the data carrier
As a carrier medium is used here only one plastic sheet ( made of polycarbonate ) with a diameter of 12 cm and a thickness of 1.2 mm. On this disc is analogous to an LP, a spiraling data track. The information carrier are also small pits, called " pits " and "lands " which were, however, reduced by a multiple in scale compared to the record, vary in length and were designed so that they can be read using a laser beam. The change from "pit / land " or "land / pit" is a 1, consistent structure "land / country " or "Pit / Pit " a 0 Since the " pits " and "lands " in consecutive ones too short would, this case must be excluded. Therefore, a transcoding is needed, which is implemented with the 8-to- 14 modulation. (Separated with a further three Merge channel bit result 17 channel bits of a byte of data. ) The " soundtrack " ( data track ) here runs from the inside out. At the beginning of the data track an index is stored. In Music CDs this table of contents indicates the number of tracks, the box office duration and the total playing time. For data CDs there, the locations of the files and directories are not stored, but only the positions of the tracks, which in turn usually contain a file system (with locations of files and directories). ( See also file system or CDFS. )
The rotational speed is regulated depending on the position of the scanning on the spiral track, in order to ensure a uniform data sampling. Thus, in a 1-fold speed read by the scanning system, the speed is set to approximately 520/min and about the outside to the inside 210/min. This user data at a speed of about 153.6 kB / s ( CD - ROM Mode 1) to 176.4 KB / s ( audio CD) are read. At the maximum 72 times the reading speed, a data rate of up to 11.06 Mb / s is possible. In addition there are additional information for error correction coding and synchronization.
The question of how long the data can be read effectively, is open. Estimates vary between 10 and 50 years, with aging is highly dependent on temperature fluctuations; even sunlight makes the media age much faster. It is certain that keep data on CD- ROM significantly shorter than on paper or parchment. It is believed that an optimum service life can be achieved by storage at constant 20 ° C in absolute darkness. However, the product quality plays a major role, as bad as produced CDs may have problems reading quite a few years.
A CD-ROM is composed of a plastic substrate with an aluminum coating. The digital information is applied to a spiral track. There are some places wells pressed into the coating, called pits ( engl. pit = pit). This reflect little earlier than the undamaged reflective bodies country called, since the CD-ROM to be pressed from the top and can be read from the bottom. Thus, the pits are not visible on the reading side as well, but as a hill. The transition from land to pit and vice versa, does not reflect the light. Reading samples a weak laser beam from the stored information.
The industrial production of a CD- ROM starts with the pre-mastering. The data to be stored on a CD- ROM data are compiled, the associated error detection code (EDC / error correction ) is calculated. The error detection code is used for debugging when reading a CD -ROM by a special correction method (cross -interleaved Reed -Solomon code, CIRC ). When pre-mastering the actual user data is also preceded by synchronization bytes and header information.
The next production step, the mastering, the data is transferred from pre-mastering to a photoresist layer by a strong laser beam, washed and plated. The negative of a CD -ROM, called a glass master is created. In most cases, the glass master is galvanized before the CD -ROM production with nickel, the so-called " father " is created.
The actual CD -ROM production ( pressure) occurs ( exactly: injection-compression ) in an injection molding process. The starting material, polycarbonate, liquid is pressed by means of the master in a form, then coated with aluminum, and sealed. Usually a CD label will be applied by screen printing to the upper surface of the CD-ROM.
Norms and Standards
- ISO / IEC 10149, and identical standard ECMA- 130.
- A data set of more than 359849 extents ( each 2048 bytes ) or 79:59,74 minutes just remains within the maximum allowable tolerances for the CD standard.
Special types and formats
Different types of media:
- CD -ROM (Compact Disc Read - Only Memory) - the industrially produced, thus " squeezed " and countless mimeographed, classical compact disc
- CD -R ( Compact Disc Recordable) - once CD
- CD-RW ( Compact Disc ReWritable ), - repeatedly erasable and rewritable CD
- DVD (Digital Versatile Disc) - the successor to the CD with multiple storage
- ROM HD ( high-density ROM) is a further development of the CD -ROM Norsam Technologies and IBM.
Different formats and file systems respectively possible contents on the media mentioned:
- CD -DA ( Compact Disc Digital Audio ) - Classic Audio CD
- CD -i (Compact Disc Interactive ) - now outdated multimedia format
- MiniDVD - standard CD, which has been described with DVD - compliant data
- GD- ROM - Giga Disc Read Only Memory This CD consists of two data fields and a separation region.
- CD - MRW ( Compact Disc Mount Rainier Read / Write) - using standard CD -R ( W) media to describe in MRW compatible drives
- ISO 9660 - usual in the CD -ROM file system
- UDF (Universal Disk Format) - platform-independent file system for optical media, which replaces ISO 9660 supplemented or
Multimedia Computer Games
In 1991, the first multimedia computer games appeared on CD- ROM, previously several disks were needed for a game. The first such game for the PC was Sherlock Holmes: Consulting Detective. The first game console with CD -ROM drive were FM Towns Marty Fujitsu (1991, built-in drive) and the PC Engine (from 1988, but external device). Early games were often identical to the floppy versions or additionally had advanced cutscenes and music that was played directly from the CD while playing the game.
Multimedia CD- ROM
Since the mid- 1990s, a plethora of encyclopedias and other educational, entertainment and edutainment media came with the proliferation of CD -ROM drives in PCs under the heading of multimedia on the market, often created with the software Director of Macromedia, rare on the basis of HTML. It now also spread applications of CD -ROM as a museum and exhibition medium: in many cases so-called info-terminals have now been fitted with specially produced CD -ROMs to provide visitors with information multimedia. The interactive multimedia CD- ROM was still early 2000s as a technically and conceptually most recent medium until it was finally replaced on the one hand from the DVD- ROM and the USB flash drive with more capacity, and the other part of increasingly multimedia and faster World Wide Web. She stepped rather then to low- segments and specific niches on where it exists to this day: for example, about some annually updated, professional databases will continue to be sold exclusively on CD- ROM. Often print media are accompanied by a CD -ROM with additional data. The copyability digital data is a problem in the marketing - digital images from CD -ROMs can be copied and distributed as all other data, virtual CD -ROM drives pretend the computer then physically available CD -ROMs before.
Originally, the capacity should be on the planning of the inventor be 60 minutes of music and thus have a diameter of just 10 centimeters, the CD -ROM could thus be inserted into the breast pocket of a shirt. But the seemingly arbitrary 74 minutes was not due to the head of the development company enjoyed listening to the 9th Symphony by Ludwig van Beethoven, which is 74 minutes long, and would therefore not be found on an audio CD course. Rather, the key for playing time CD - diameter was established by the Philips leadership as follows: The Compact Cassette was a great success, the CD should not be much larger. The Compact Cassette had a diagonal of 11.5 cm, the engineers made the CD 0.5cm taller.
The hole in the center of the CD -ROM has its origin in the size of an old Dutch 10 cent coin. These were the developers of Philips constantly in her purse. The size of the diameter of the coin rail to be ideal for the application.