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[ The PC Guide | Systems and Components Reference Guide | CD-ROM Drives | Recordable CD (CD-R) ]

CD-R Drives

Just as CD-R requires the use of special media, it also of course requires the use of a special CD-R drive. This drive is very different than a standard CD player because it must include a special laser. The laser is the key component from the drive's perspective, in that it is what burns the image into the CD-R media's dye layer.

CD-R drives are capable of reading disks as well as writing them, of course. Most support a large number of formats, to enable the reading and writing of a wide variety of CDs. Most drives are also faster than single speed; it is quite typical for the speed of the drive to be significantly lower when writing than when reading. For example, a drive might be specified to be 4X when reading, but only 2X when writing a disk.

While standard CD-ROM drives often use a variety of formats, SCSI is the interface of choice for the vast majority of CD-R drives. The main reason is that SCSI is a higher-performance interface that allows the flow of data to the drive to be maintained more easily, independently of what other activities are happening within the PC. This is critical for writing CD-R media because burning a disk requires an uninterrupted flow of data from wherever it is coming from (usually a hard disk) to the drive. CD-ROM drives are now also available for the ATAPI (IDE) interface.

Since the CD-ROM's laser is moving at a constant speed as it writes, it must have the data it needs available in a smooth flow; it cannot wait for the data if it is delayed because the disk cannot be stopped. The faster the drive spins the disk when writing, the more data flow is required. Due to how CDs are written, interrupting the writing of the disk will generally ruin it; burned disks that don't work are (semi-)affectionately referred to as "coasters", since about all they are good for is protecting your coffee table...

Many drives take specific steps to avoid this problem. One of the most common ones is the use of a substantial memory buffer that can supply data to the laser head in the event that the flow of data is interrupted. Another is the use of an image file: instead of copying the data to be recorded from its original locations, an exact image of the disk to be created is stored in a large file on disk first, and then transferred "whole" to the burned disk. This reduces the chances of an interruption while writing, but costs a good chunk of disk space.

Many users have a hard time when initially setting up their CD-R drives, getting them to write disks properly without destroying too much blank media in the process. Once the drives are set up, however, CD-R can be a fairly reliable format, allowing the user to create a large number of disks with minimal waste.

Next: CD-R Software

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