Definition

X (compact disc access time)

Part of the Multimedia and graphics glossary:

In compact disc ( CD ) and digital versatile disc ( DVD ) technology, X is a base multiplier that expresses the speed with which data could be read (the read access time) from the compact disc in its original version, which was 150 kilobytes (KB) per second. As successively faster CDs arrived, manufacturers adopted the convention of indicating the read time in terms of the original speed. Thus, a 2X CD had a read access time of 300 KB per second and so forth. Our table relates each common drive speed to its read access time. It also shows the range of revolutions per minute (RPM) used to make the read access time possible.

CD/DVD Drive Speed Maximum Data Transfer Rate RPMs (revolutions per minute)
1X CD-ROM 150 KB/sec 200 - 530
2X CD-ROM 300 KB/sec 400 - 1060
4X CD-ROM 600 KB/sec 800 - 2120
8X - 12X CD-ROM 1.2 MB/sec 1600 - 4240
24X - 50X 1.8 - 6 MB/sec 2400 - 6360 approximately
1X DVD-ROM 1.25 MB/sec No exact data, but much slower than 1X CD-ROM

There are only minor increases in read access time as one moves from the 24X, 32X, and 40X drives. Higher rotation speeds are (somewhat) prone to noise and vibrations and may cause performance to vary from drive to drive.

It seems unlikely, because of these vibrations and performance variations, that read access times will increase much above present levels. Even though a hard drive can reach much faster speeds of rotation, its enclosure stabilizes the entire mechanism and therefore avoids much of the noise and vibration inherent in the open CD-ROM drive.

This was last updated in September 2005
Contributor(s): Steven Raimi
Posted by: Margaret Rouse

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