Page 69 - UNI 101 Computer Science Handout.

Faculty of Nursing
Adult care Nursing Department

Figure 8 Layout of a CD-ROM versus a Standard Disk.

It is possible to mix data, audio, and video on the same disk. Data blocks on CD-ROMs are sometimes
called large frames. Data is stored on the disk in the form of pits and lands. These are burned into the

surface of the master disk with a high-powered laser. The disk is reproduced mechanically, using a

stamping process that is less expensive than the bit-by-bit transfer process required of magnetic media.
The disk is protected with a clear coating. Figure below shows a basic diagram of the read process.

A laser beam is reflected off the pitted surface of the disk as a motor rotates the disk. There flection is

used to distinguish between the pit sand lands, and these are translated into bits. On the disk itself, each

2352-byte data block, or large frame, is broken up into 9824-byte small frames. Bytes are stored using a
special 17-bit code for each byte, and each small frame also provides additional error correcting facilities.

Translation of the small frames into more recognizable data blocks is performed within the CD-ROM

hardware and is invisible to the computer system. The bit-encoding method and additional error

correction built into the small frames increases the reliability of the disk still further. DVD technology is
essentially similar to CD-ROM technology.

The disk is the same size, and is formatted similarly. However, the use of a laser with a shorter light

wavelength (visible red, instead of infrared) allows tighter packing of the disk In addition, the laser can

be focused in such a way that two layers of data can be placed on the same side of the disk, one

69 Academic Year 2025/2026