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The first CD-ROM drives transferred data at 150KB per second. Speeds doubled to 300KB and continued upward to more than 40 times the original. Access times range from 80 to 120ms.
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Earlier CD-ROM drives used a caddy. The disc must be inserted into the caddy, and the caddy inserted into the drive. Today, most CD-ROM drives are caddy less. The disc is placed into a tray.
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Digital data is carved into the CD-ROM as pits (low spots) and lands (high spots). As the laser shines into the moving pits and lands, a sensor detects a change in reflection when it encounters a transition from pit to land or land to pit. Each transition is a 1. The lack of transitions is 0s. There is only one laser in a drive.
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The Re-Write CD-ROM and DVD drives are valuable for data storage and allows you to make copies of software music CD's and DVD's. CD-RW drives can also be used to write CD-R discs, and they can read CD-ROMs. CD-RW disks have a lower reflectivity than CD-ROMs and CD-R's, and older CD-ROM drives that do not have Multi-Read capability cannot read them. Initially known as CD-E for CD-Erasable, a CD-RW disk can be rewritten a thousand times.
CD-RW disks can be used to master CD-ROMs, and the same software used for CD-R creation supports this application. However, unlike CD-R's, in which the entire disc or an entire track is recorded at once, CD-RW's support the Universal Disk Format (UDF). Like the file system on a hard disk, files can be added and deleted. This "fixed packet writing" capability requires pre-formatting like a floppy disk. CD-RW's use phase change technology to alter the reflectivity of the disk's surface.
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DVD (digital versatile disk) drives will soon be able to hold up to 17 GB of data on both sides of the disk. Single sided DVD's can hold 3.7 GB. The only drawbacks to these drives are their transfer rate which is increasing with time. DVD-ROM is like a large CD-ROM, allowing for data and interactive material and audio and video.
