CD-R (compact disc, recordable)

CD-R (for compact disc, recordable) is a type of write once, read many (WORM) compact disc (CD) format that allows one-time recording on a disc.

CD-R (for compact disc, recordable) is a type of write once, read many (WORM) compact disc (CD) format that allows one-time recording on a disc. The CD-R (as well as the CD-RW) format was introduced by Philips and Sony in their 1988 specification document, the Orange Book. Prior to the release of the Orange Book, CDs had been read-only audio (CD-Digital Audio, described in the Red Book), to be played in CD players, and multimedia (CD-ROM), to be played in computers' CD-ROM drives. After the Orange Book, any user with a CD recorder drive could create their own CDs from their desktop computers.

Like regular CDs (all the various formats are based on the original Red Book CD-DA), CD-Rs are composed of a polycarbonate plastic substrate, a thin reflective metal coating, and a protective outer coating. However, in a CD-R, a layer of organic polymer dye between the polycarbonate and metal layers serves as the recording medium. The composition of the dye is permanently transformed by exposure to a specific frequency of light. Some CD-Rs have an additional protective layer to make them less vulnerable to damage from scratches, since the data - unlike that on a regular CD - is closer to the label side of the disc. A pregrooved spiral track helps to guide the laser for recording data, which is encoded from the inside to the outside of the disk in a single continuous spiral. The laser creates marks in the dye layer that mimic the reflective properties of the pits and lands (lower and higher areas) of the traditional CD. The distinct differences in the way the areas reflect light register as digital data that is then unencoded for playback.

CD-R discs usually hold 74 minutes (650 MB) of data, although some can hold up to 80 minutes (700 MB). With packet writing software and a compatible CD-R or CD-RW drive, it is possible to save data to a CD-R in the same way that one can save it to a floppy disk, although - since each part of the disc can only be written once - it is not possible to delete files and then reuse the space. The rewriteable CDs, CD-RWs, use an alloy layer (instead of the dye layer) which can be transformed to and from a crystalline state repeatedly.

CD recorders (usually referred to as CD burners), were once much too expensive for the home user, but now are similar in price to CD-ROM drives. CD-Rs can be created in any CD-R or CD-RW drive.

This was first published in September 2005

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