CD-ROM and DVD-ROM Drives
Both the compact disc read-only memory (CD-ROM) and the digital versatile disc read-only memory (DVD-ROM) drive are based on technology from the audio-visual industry, and both have become standard equipment on computers. A compact disc can hold a maximum of approximately 737 MB of data while a DVD disc can hold up to 4.7GB (single layer) or 8.5GB (dual layer) of data on a single side of the disc and twice that amount on double-sided DVD disks. Both CD-ROM and DVD-ROM drives make use of highcapacity optic media in the form of a silvery platter that holds digital data that is read by means of a laser beam. Many new computers come with a DVD-ROM drive that are compatible with compact discs. DVDROM drives require additional decoding hardware, either on the drive itself or on a companion card, to play mpeg videos effectively. Without this additional hardware, the playback of mpeg files will be erratic with lost frames and erratic audio playback.
A CD platter is composed of a reflective layer of aluminum applied to a synthetic base that is composed of polymers. A layer of transparent polycarbonate covers the aluminum. A protective coating of lacquer is applied to the surface to protect it from dust, dirt, and scratches.
CD-recordable (CD-R) discs use materials other than aluminum. They often have a yellow or green cast on the data side. Some CD-ROM drives, such as the older IDE CD-ROM drives are not able to read CD-R discs.
Data is written to a CD at a press by creating pits and lands on the CD's surface. A pit is a depression on the surface, and a land is the height of the original surface. The transition from a land to a pit or a pit to a land represents a binary character of 1 while continuous lands and continuous pits represent binary 0. The reading of data is based on the speed at which the CD rotates and the reflection of light. The data transfer speed of a CD-ROM drive is expressed in relation to the rotation speed of an audio CD player. The latter can transfer 150 KB of data per second.
A 2X CD-ROM drive operates at 300 KB per second, a 4X at 600 KB per second, etc. Table below lists transfer rates for some common CD-ROM drives.
| The Transfer Rate of the CD-ROM | |
| CD-ROM Speed | Transfer Rate |
| 4X | 600 KB per second |
| 6X | 900 KB per second |
| 8X | 1.2 MB per second |
| 12X | 1.8 MB per second |
| 16X | 2.4 MB per second |
| 24X | 3.6 MB per second |
| 32X | 4.8 MB per second |
| 40X | 6.0 MB per second |
| 48X | 7.2 MB per second |
| 52X | 7.8 MB per second |
A single-speed DVD-ROM drive provides a data transfer rate of 1.385MBps, which is roughly equivalent to a 9X CD-ROM. This, however, does not mean that a 1X DVD-ROM drive can read CDs at 9X rates.
DVD-ROM drives spin a disk 2.7 times faster than a CD-ROM drive of the same speed. Therefore, many DVDROM drives list two speeds, one for reading DVD disks and another for reading CD disks. Thus, a DVDROM drive listed as a 16X/40X would indicate the performance when reading DVD disks and CD disks, respectively.