Compact Disc Read-Only Memory (CD-ROM)
|capacity||650 MiB to 879 MiB|
|lifespan||10 to 50 years|
|size||12 cm x 1.2 mm|
|developer||Philips , Sony|
Compact Disc Read-Only Memory ( CD-ROM ) is a permanent storage medium for digital data . It is the second application of the compact disc after the compact disc digital audio (audio CD) and is used, for example, to distribute software .
The storage capacity of a CD-ROM is approx. 700 MiB . When stored on the CD-ROM, this amount of data corresponds approximately to the amount of data of 80 minutes of music on an audio CD. Writable CDs even hold up to 879 MiB (100-minute CDs); however, they cannot be written to in all CD burners and cannot be read in all CD drives. Depending on the type and quality of the blank as well as the burner, it is also possible to burn outside the standardized range ("overburn"). However, this can lead to errors or loss of data in this area.
Different file systems can be used to format the data on a CD-ROM . CD-ROMs in accordance with the ISO 9660 standard can be read by various computer systems. Other common specifications offer extended possibilities (for example with regard to the length of file names ) and are based on this standard; Examples are the Rockridge (UNIX) and Joliet (Windows) file systems as well as the El Torito specification to make bootable CD-ROMs.
The CD-ROM is one of the CD formats specified in the “ Rainbow Books ” , in the case of CD-ROM in the “ Yellow Book ” (= data CDs) and “ Orange Book ” (= writable CD formats) with multisession capability). In compliance with the specifications described there, the CD-ROM may that of Philips award Compact Disc - Logo carry. There are also special formats such as CD-ROM XA (XA stands for eXtended Architecture), CD-Extra, CD-i, etc. When producing CD-ROMs and their derivatives, patents from Philips and Sony must be observed.
Structure of the data carrier
Only a plastic disk (made of polycarbonate ) with a diameter of 12 cm and a thickness of 1.2 mm is used here as the carrier medium . Analogous to the record , there is a spiral-shaped data track on this disc . Here, too, the information carriers are small recesses, the so-called "pits" and "lands", which have been reduced in scale many times over compared to the record, are of different lengths and have been designed in such a way that they can be read out with the aid of a laser beam . The change from "Pit / Land" or "Land / Pit" forms a 1, the structure "Land / Land" or "Pit / Pit" remains the same as a 0. Since the "Pits" and "Lands" are too short for consecutive ones this case must be ruled out. Recoding is therefore necessary, which is implemented with the 8-to-14 modulation . (With a further three merge channel bits separated, there are 17 channel bits for one data byte .) The "sound track" (data track) runs from the inside out. A table of contents is stored at the beginning of the data track. In the case of music CDs, this table of contents shows the number of music tracks, the playing time and the total playing time. With data CDs, the positions of the files and directories are not stored there, but only the positions of the tracks, which in turn normally contain a file system (with the positions of the files and directories). (See also file system or CDFS .)
The speed is regulated depending on the position of the scanning system on the spiral track in order to ensure a constant data scanning. For example, at 1-fold reading speed, the scanning system sets the speed on the inside to around 520 / min and on the outside to about 210 / min. User data is read out at speeds of around 153.6 kB / s (CD-ROM mode 1) to 176.4 kB / s (audio CD). At the maximum 72 times the reading speed, a data rate of up to 11.06 MB / s is possible. There is also additional information for error correction , coding and synchronization .
The question of how long the data can be effectively read is open. Estimates vary between 10 and 50 years, with aging being very dependent on temperature fluctuations; sunlight also makes the media age much faster. What is certain is that data on CD-ROM is significantly shorter than on paper or parchment . It is assumed that an optimal service life can be achieved by storage at a constant 20 ° C in absolute darkness. However, product quality also plays a major role, so poorly produced CDs can have reading problems after a few years.
A CD-ROM consists of a plastic carrier material with an aluminum coating. The digital information is applied on a spiral track. In places, indentations are pressed into the coating, so-called pits. These reflect a little earlier than the undamaged reflective spots called land because the CD-ROMs are pressed from the top and read from the bottom. Thus the pits are not visible from the reading side as depressions, but as hills. The transitions from land to pit and vice versa do not reflect the light. When reading, a weak laser beam scans the stored information.
The industrial production of a CD-ROM begins with premastering . The data to be saved on a CD-ROM is compiled and the associated error detection code (EDC / error correction) is calculated. The error detection code is used to eliminate errors when reading a CD-ROM using a special correction process ( Cross-interleaved Reed-Solomon Code , CIRC). With premastering, synchronization bytes and header information are added in front of the actual user data.
In the next production step , mastering , the data from the premastering is transferred to a photoresist layer with the help of a strong laser beam, washed out and silvered. The negative of a CD-ROM, a so-called glass master , is created. In most cases, the glass master is electroplated with nickel before the CD-ROM is manufactured , creating the so-called "father".
The actual CD-ROM production ( pressing ) takes place in an injection molding process (precisely: injection compression molding ). The starting material, liquid polycarbonate , is pressed into a mold with the help of the master, then coated with aluminum and sealed. Usually a CD label is also applied to the top of the CD-ROM using the screen printing process .
Norms and standards
- ISO / IEC 10149 , and identical standard ECMA-130 .
- A data volume of a maximum of 359849 extents (2048 bytes each) or 79: 59.74 minutes remains just within the maximum permissible tolerance values for the CD standard.
Special types and formats
Different types of media:
- CD-ROM (Compact Disc Read-Only Memory) - the industrially manufactured, ie "pressed" and countlessly reproduced, classic compact disc
- CD-R (Compact Disc Recordable) - write-once CD
- CD-RW (Compact Disc ReWritable) - multiple erasable and rewritable CD
- DVD (Digital Versatile Disc) - successor to the CD with multiple storage capacity
- HD-ROM (High Density ROM) is a further development of the CD-ROM from Norsam Technologies and IBM.
Different formats and file systems or possible content on the media mentioned:
- CD-DA (Compact Disc Digital Audio) - classic audio CD
- CD-i (Compact Disc Interactive) - now an obsolete multimedia format
- MiniDVD - Standard CD on which DVD-compliant data has been written
- GD-ROM - Giga Disc Read Only Memory This CD consists of two data areas and a separation area.
- CD-MRW (Compact Disc Mount Rainier Read / Write) - Use of standard CD-R (W) media for writing in MRW- enabled drives
- ISO 9660 - file system commonly used on CD-ROMs
- UDF ( Universal Disk Format ) - platform-independent file system for optical media, which supplements or replaces ISO 9660
Multimedia computer games
In 1991, the first published Multimedia - computer games on CD-ROM, previously several were floppy disks required for a game. The first such game for the PC was Sherlock Holmes: Consulting Detective . The first game consoles with a CD-ROM drive were FM Towns Marty from Fujitsu (1991, built-in drive) and the PC Engine (from 1988, but external device). Early games were often identical to the floppy disk versions or had additional cutscenes and music that played directly from the CD during the game.
Since the mid-1990s, with the spread of CD-ROM drives in PCs, an abundance of lexicons and other educational, entertainment and edutainment media under the heading of multimedia , often created with Macromedia's Software Director, came on the market less often also on the basis of HTML . Applications of the CD-ROM as an exhibition and museum medium were now also spreading : in many cases, so-called info terminals were now equipped with specially produced CD-ROMs in order to provide visitors with multimedia information. The interactive multimedia CD-ROM was still the early 2000s as a technically and conceptually The most recent medium until they finally one hand on the DVD-ROM and the USB flash drive multimedia with more capacity, and on the other by increasing and faster World Wide Web was replaced . It then appeared more in cheap segments and special niches in which it still exists today. For example, some professional databases, which are updated annually, continue to be sold exclusively on CD-ROM. Print media are often accompanied by a CD-ROM with additional data. The ability to copy digital data is a problem in marketing - digital images of CD-ROMs can be copied and distributed like any other data; virtual CD-ROM drives then make the computer believe that CD-ROMs are physically present.
Originally, according to the inventors' plans, the capacity should be 60 minutes of music and thus have a diameter of exactly 10 centimeters, so the CD-ROM could be put in the breast pocket of a shirt. But the seemingly arbitrary 74 minutes did not occur because the head of the development company like the 9th Symphony of Ludwig van Beethoven heard that 74 minutes is long, and therefore does not place would have found on an audio CD. Rather, the CD diameter, which is decisive for the playing time, was justified by the Philips management 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 0.5 cm larger.
The hole in the middle of the CD-ROM originates from the size of an old Dutch 10 cent coin. The developers at Philips always had these in their wallets. The size of the diameter of this coin appeared to be ideal for the application.
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