PC Hardware —

Audio Capability

Any CD-ROM drive that meets the Yellow Book standards (created by the audio industry for sound and adopted by the computer industry) has the ability to play back audio. Most CD-ROM drives contain the circuitry and chips to convert digital audio data into sound data. Most drives and sound cards also have a headphone jack, as well as audio jacks to connect to a stereo system. The only requirement is that the drive support the ISO 9660 standard for the file system. ISO 9660 is also known as the High Sierra Format. The ISO 9660 format is a standard for writing data to a CD-ROM for use in a cross-platform environment. This standard is compatible with MS-DOS, Windows, UNIX, Macintosh, and other operating systems.

Access Time

When purchasing or recommending a CD-ROM drive, you need to consider two values. The first is data transfer rate. The long-time standard for transfer rate has been 150 KB per second, and this is the basis for measuring CD-ROM drives today. A 2X CD-ROM drive operates at 300 KB per second, a 4X at 600 KB per second, and so on. A typical CD-ROM drive today will operate at 24X or 32X (4.8 MB per second) or faster. A hard disk drive typically operates between 800 KB and 1.8 MB per second.

The second value to look at is the drive's mean access time. This is the time it takes the head to move over half the tracks. Typical access time is 200 to 400 milliseconds (ms). Today's CD-ROM drives can have faster data transfer speeds than many hard drives, but their mean access time is 20 or so times slower. This means that while a CD-ROM drive will outperform the hard disk drive for copying or loading a large chunk of contiguous data, it will be beaten by the hard drive on random access tasks.

Although the transfer rate increases in multiples, the mean access time does not. The following table lists transfer rates and access speeds for some common CD-ROM drives.

CD-ROM Speed Transfer Rate Access Speed
4X 600 KB per second 220 ms
6X 900 KB per second 145 ms
8X 1200 KB per second 100 ms
12X 1800 KB per second 125 ms
16X 2.4 MB per second 100 ms
24X 3.6 MB per second 95 ms