The CD ROM

CD ROMs are optical storage devices. Optical disk drives use lasers to write to and read from their media. When writing to an optical disk, a laser creates pits on its surface to represent data. Areas not burned into pits by the laser are called lands. The laser reads back the data on the optical disk by scanning for pits and lands. While they offer high-capacity storage, optical disk drives are not nearly as fast as hard disk drives.

There are three primary optical disk technologies available for storage:

• CD-ROM (Compact Disk - Read Only Memory)

• WORM (Write Once Read Many)

• re-writable

CD-ROMs are still the most popular form of optical disk storage, although the DVD will eventually take over. However, CD-ROMs are read-only. At the factory, lasers are used to create a master CD-ROM, and then a mold is made from the master. Plastic is injected into the mold, and the data pattern is pressed into the disk, much like a phonograph record.

Re-writable optical disks typically are used for data backup and archiving massive amounts of data, such as image databases. WORM drives are used almost exclusively for archival storage where it is important that the data cannot be changed or erased after it is written, for example, for financial records storage.

If you've ever watched an audio CD player that had a clear case, you may have noticed that the head starts reading at the centre while the CD spins fast, and continues to the outer edge where the CD spins much slower. Audio CD players read data at a constant angular velocity (CAV) of 150 KB/sec. 

A CD-ROM drive reads disks differently from a music disk, in that it spins at a constant linear velocity (CLV), instead of a constant angular velocity (CAV). This means that there is not a constant data transfer rate across the CD (more data per rotation can be stored in the outer regions). By utilising CAV techniques, the data transfer rate can be significantly increased for the majority of your data. One way of doing that is by increasing the rotation speed of the drive. Each multiplier of the standard 150 KB/sec. makes the data transfer rate faster, so a 2X drive reads 300 KB/sec., a 4x 600 KB/sec and so forth. However, top reading speed can only be achieved at the outer regions of the CD. Therefore the transfer rate may start at 3600 KB/sec (24X) at the inner regions and increase to 7200 KB/sec (48X) at the end of the disc, giving an overall performance of 36X.

As with hard disks, SCSI and IDE ATAPI drives are available. Parallel port drives are still in use for some laptops as external devices, but are much slower than SCSI or ATAPI (The interface used by the IBM PC systems for accessing CD-ROM devices).

You have to setup your CD ROM just like you setup you hard drive, as both are IDE devices. Depending if your CD ROM will be connected as a MASTER or a SLAVE, you have to set the jumpers accordingly in order to tell the CD ROM how it ranks in the IDE hierarchy.

Most sound files are by default decoded by the CPU itself and the decoded information is then simply forwarded to the sound card, but conventional audio CDs do not need to be decoded and can bypass the CPU. Every CD ROM drive has a small audio output socket featuring four pins. The cable that leads from this output socket to the sound card is usually provided by the CD ROM drive manufacturer. Make sure you connect it, otherwise you won't be able to hear your audio CDs playing.