Speed is important when viewing animation or video such as those found in multimedia encyclopedias and games. CD-ROM drive’s speed measured by its data transfer rate. Here we use an X to denote the original transfer rate of 150 KB per second. The speed is donated in terms of how much faster it is than a standard audio CD player. Therefore a 2X drive is twice the speed of your stereo’s CD player, and so on. Technology as advanced quite a bit. Today, 24X or 32X is standard. Many of us still have 6X or 8X drives, though and these are fine.

            When reading a CD-ROM, a low power laser beam is focused on the rotating CD-ROM and the read head views its reflection. When the beam reflect back from the CD-ROM, it’s intensity changes as it moves from “lands” to “pits”. These variations in the laser beam are decoded as data by the CD-ROM drive. It should be noted, that unlike hard disc which rotate at the constant angular velocity (CAV). Figure 1.2a show the layout of a disc using CAV. The disc is divided into a number of pie-shaped sectors and into a series of concentric tracks. CD-ROMs rotate at constant linear velocity (CLV) of about one meter per second. (See Figure 1.2b for the layout of CDs and CD-ROMs using CLV) The linear dimensions of the track are the same at the start of play as they are the ends of the data, so the drive must rotate the disc faster when the head is reading data near the center and more slowly as the outer tracks are read..

 A compact disc begins play at the end of the spiral nearest the center, with the initial rate of rotation ranging between 468 and 568 rpm. The CLV for any single disc is fixed; it does not fluctuate. The main advantage of the CLV is the constantly dense recording of information that this make possible, regardless of distance from the center.

            The CLV is natural determinant of the audio CD’s length of play. If we pick a mathematically convenient sped midway within the CLV range of  1.2 to 1.4 m/s –namely, 1.28 m/s which is equivalent to slightly under 3 mph. Seventy-four minutes is a standard length of play for CDs, and our conveniently chosen 1.28 m/s speed an calculate to this as follows:

Length of play (in minutes)

=  length  =       5681.0155m          =73.79

    speed       1.28m/s x 60s/minute