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Subject: CD-R, CD-RW, DVD Glossary of Terms
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Tech Article Number: WBTA00000031

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CD: Compact Disk
This term is loosely applied when describing a variety of formats, from the production (mass produced) audio and data disks, to the rewritable versions, write once (CD-R) or write many "re-writable" CDs (CD-RW). The standard CD disk can hold about 650MB of data on a single disk.

CD-R: Compact Disk - Recordable
This CD-ROM technology allows you to use write-once CD-ROM media to record music, data, or "hybrid" on a special CD-ROM disk that can be read in most CD-ROM drives as long as the disk format is compatible.

CD-RW: Compact Disk - ReWritable" or "Read/Write
This CD-ROM technology allows you to record multiple times to the same CD-ROM disk media. It allows you to erase previous information that was recorded to the CD-ROM media, and record new music, data, or a combination of both to the CD-ROM disk. A limitation of this media is that it can only be read in other CD-RW drives or CD-ROM drives that are "multi-read" compatible. Many older CD-ROM drives will not be able to read this media.

DVD: Digital Video Disk
This is a high-capacity version of the compact disk (CD), using the same 5 inch disk form factor. DVD storage capacities are about four times as high as compact disks. A single-sided DVD disk can hold 2.6GB of data, compared to the 650MB of CDs. The DVD specification also supports double-sided versions, although currently, no double-sided DVD drives appear to be in production, meaning you would need to remove and "flip" the disk over to play side two. DVD drives record data using the Universal Data Format (UDF).

DVD-ROM: Read-Only version of Digital Video Disk
Production DVD disks containing movies, audio, and or data cannot be modified or erasedThink of this as the "high density" version of a CD-ROM disk.

Just starting to appear commercially, the DVD-RAM disk is a rewritable version of the DVD disk. Just as with the CD-RW disks, information, video, audio, and or data can be recorded to the disk, but it can later be modified, erased, or updated. All this, but with the storage capacity found with the DVD specification.

ISO: International Standards Organization
ISO 9660: A common CD-ROM disk format standard, defining how data will be structured on the disk media, cataloged, and accessed. This is similar to a floppy disk format in that any CD-ROM player or drive that is compatible with this format will be able to read and access the information (data or music) stored on the disk. Most CD-ROM drives on the Windows (PC), Macintosh, and UNIX platforms are capable of reading this CD-ROM disk format. (Keep in mind that just because a system can view the CD and your data files, programs are still platform specific, and can only run on those systems they were developed for.)

Data is written to a CD as a session. The session includes writing the data track (and audio track(s) if creating a multi-mode or "hybrid" CD), followed by writing the disk catalog. It is the catalog that is read by the CD-ROM drive to determine the contents of the CD. With a multi-session CD, several sessions, complete with catalogs, are written to the CD media. Only the last catalog is read by multi-session compatible CD-ROM drives. To access data from any or all of the sessions on a CD-ROM, the previous catalog data is combined with any new files written to the disk. You can "erase" a file by recording a new catalog that does not include a reference to previously recorded file data. (The file data is still there, but multi-session CD drives would ignore it.) Single session CD-ROM drives can only read the first session on the CD. Audio CDs cannot be recorded in multiple sessions, they are strictly single-session CDs.

UDF - Universal Data Format
A CD-ROM disk format standard introduced with CD-RW media requires Direct CD or compatible software to access the data. The Universal Data Format is also used when creating DVD media. UDF was designed to overcome technical limitation of the original CD standard, the ISO 9660 file format.

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