Page 68 - UNI 101 Computer Science Handout.

Faculty of Nursing
Adult care Nursing Department

Since the CD format was designed for maximum capacity, the decision was made to pack the bits on

the disk as tightly as possible by making each block the same length along the spiral track, regardless of

location on the disk. Thus, the disk is read at a constant linear velocity (i.e., CLV), using a variable speed
motor to keep the transfer rate constant. Since the angle of a block is smaller on the outer tracks, the

disk moves more slowly when outside tracks are being read. This is easily observable if you have access

to a portable CD player that allows you to observe the disk as it rotates. A CD-ROM typically stores
270,000 blocks of data. Each block is 2352 bytes long and holds 2048 bytes of data. In addition, there is

a 16-byte header, which provides 12 bytes to locate the start of a block and 4 bytes for block

identification. Due to the difficulty of the manufacturing process, errors can occur, so the CD-ROM

provides extensive means for correcting the errors. Therefore, each block also provides 288 bytes of an

advanced form of parity known as cross-interleaved Reed-Solomon error correcting code. This code
repairs not only isolated errors but also groups of errors that might result from a scratch or imperfection

on the disk. The resulting total data capacity of a single CD-ROM is approximately 550MB. The error

correction is occasionally omitted for applications where errors can be tolerated, such as audio, which
increases the capacity of a CD-ROM to about 630MB.

Blocks on a CD-ROM are identified by a 4-byte identification code that was inherited from the audio

origins of the medium. Three bytes, stored in binary-coded decimal (BCD) format, identify the block by

minute, second, and sector. There are 75 sectors per second and 60 seconds per minute. Normally, there
are 60 minutes, although this number can be increased to 70 minutes if necessary. This increases the

disk capacity to about 315,000 blocks. The fourth byte identifies a mode of operation. Mode 1, the

normal data mode, provides the data as we’ve described, with error correction. Mode 2 increases the

capacity by eliminating the error correction. Other modes are provided for special audio and video

features.

68 Academic Year 2025/2026