Compact disc

Compact disc (or CD) is an optical disc used to store digital data, originally developed for storing digital audio. It is the standard playback format for commercial audio recordings today.

Recordability

Injection moulding is used to mass produce compact discs. A "stamper" is made from the original media (audio tape, data disc, etc.) by writing to a glass disc (referred to as a glass master) coated with a photosensitive dye with a laser. This dye is then etched, leaving the data track. It is then plated to make a positive version of the CD. Polycarbonate is liquified and injected into the mold cavity where the stamper transfers the pattern of pits and lands to the polycarbonate disc. The disc is then metallized with aluminum and lacquer coated.

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Recordable compact discs are injection molded with a "blank" data spiral. A photosensitive dye is then applied, and then the discs are metallized and lacquer coated. The write laser of the CD recorder changes the characteristics of the dye to allow the read laser of a standard CD player to see the data as it would an injection molded compact disc. CD-R recordings are permanent. The resulting discs can be read by most CD-ROM drives and played in most audio CD players.

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CD-RW is a re-recordable medium that uses a metallic alloy instead of a dye. The write laser in this case is used to heat and alter the chemical properties of the alloy and hence change its reflectivity. A CD-RW does not have as great a difference in the reflectivity of lands and bumps as a pressed CD or a CD-R, and so many CD audio players cannot read CD-RW discs, although the majority of standalone DVD players can.

Related Topics:
CD-RW - DVD

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