|What is it?||What are the Myths||How does it work?||Types||For the advanced user|
CD-ROM stands for Compact Disc Read Only Memory. It is a device that reads information stored on plastic compact disks. It is based on the same technology as audio CDs. It is called compact because it can store or "compact" large amounts of information. The information is read from pits and lands, like 1s and 0s. It is changed into binary so computers can read it. An optical reader reads the patterns of pits that stands for bytes. One CD can hold 650MB of data or 300,000 pages of text. Most CDs are read only, which means you cannot save data to the disk. This device is usually not used as a primary storage device for data. You will need other storage devices to save your work. Some CDs are writable, but you need a special CD-ROM to use them called a CD-R, Compact Disc Recordable. This device allows the computer to record data on a CD-R disk using a special recording device. These disks can be used on your computer, but the disk cannot be erased or changed.
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You can record (write to it) on a CD-ROM disk.
The CD-ROM disks that you buy in the store are for reading information only. You cannot save to a CD-ROM unless you have a special device and special software. Once a CD is written to, that's it. Some CDs let you put extra information on them called multi-session photo disks, but even saving to these disks requires special equipment. You can even listen to your audio compact disks on your computer if you have a multimedia computer with a sound card, but you cannot record or erase it. You cannot erase CDs like you can hard drives.
The CD-ROM drive is controlled by both the application software and the internal controller circuitry. The software sends instructions to the controller.
Data is recorded onto a CD in a clockwise spiral from the center. Its recorded and read in a series of lands and pits. Any land that the laser hits is reflected to the read head and is recorded as a 1 or a 0. They are counted in groups of 14 and then converted into standard 8-digit data.
The speed of the drive determines how fast the drive can transfer data to the computer (called the data transfer rate). The lower the access time, the faster the device works. Video, animation, and large graphics need faster transfer speeds because their files are larger. Video can be jerky at slow speeds. The recommended access time for a CD-ROM drive is 280 ms (milliseconds), though drives with access times through 400 ms are considered to be okay. Speeds of CD-ROM drives go from 1X to 20X. The 1X drive is the oldest and slowest and the 20X is currently (July 1997) the fastest.
CD-ROM drives can be both internal and external. The external drive is more expensive, but can also be used on other computers since it is portable.
Kinds of CD-ROM Drives
Read Only drives can retrieve data from the CD-ROM but cannot save new data to it. It cannot be used as the primary storage for the computer.
CD-R, Compact Disc-Recordable, allows the computer to record data on a CD-R disk using a special recording device. The disks can be used on a computer just like a regular CD-ROM disk, but they cannot be erased or changed.
DVD-ROM (Digital Video Disc- ROM just like the a CD-ROM drive, but has a larger storage capacity. It stores data starting at 4.7 GB. It can hold full length movies, and can also read CD-ROM disks. Of course it is the most expensive of the devices.
CDs can contain:
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