For multimedia applications, theCD- ROM(Compact Disc Read Only Memory) has crystallized out of the multitude of CD variants. This shiny silver disc, 12 cm in diameter, can only be read but not written to. The information layer is covered with a plastic layer that protects it from damage and contamination.
The most important features of the CD-ROM are the storage capacity of up to 650 MB, the data transfer rate of 150 kbit/s and the access time of about 100 ms to a few hundred milliseconds. The CD-ROM format is specified in the Yellow Book and standardized according to ISO 9660. The compact disc (CD) is formatted in sectors that run in a spiral from the inside to the outside. A CD-ROM can have up to 333,000 sectors, with a 12- byte synchronization field in front of each sector. The sector structure and synchronization are identical for all compact discs. What differs are the formats.
On the CD-ROM, the synchronization field is followed by a header (4 bytes) and the 2 KB long data field, which is terminated by a 4 byte error detection code( EDC). This is followed by 8 bytes of unused data and the 276 byte error correction( ECC). This format is called the Mode 1 format and provides three levels of error detection and error correction of data. Mode 2 provides two levels for error detection of audio and video. Error correction is done with the Reed-Solomon code.
The tracks for storing the data are spiral-shaped, with the sectors always having the same length. The data is stored in the form of pits and lands. The data is read out with an infrared laser with a wavelength of 780 nm.