CD

CD
Name	Compact Disc
Caption	A standard 120 mm optical disc
Type	Optical disc
Encoding	Various
Capacity	Typically 700 MiB (up to 80 minutes audio)
Read	780 nm semiconductor laser
Standard	Rainbow Books
Developed by	Philips and Sony
Usage	Audio, data storage
Released	1982

CD. The Compact Disc is a digital optical disc data storage format that was co-developed by Philips and Sony and released in 1982. It revolutionized the music industry by replacing analog formats like the vinyl record and compact cassette with a durable, high-fidelity digital medium. The format's success was built upon the technical standards outlined in the Red Book and its subsequent adaptation for computer data storage.

History

The development of the Compact Disc was a joint effort between the Dutch electronics firm Philips and the Japanese corporation Sony, following earlier laser disc research by companies like MCA Inc. The format was publicly demonstrated in 1979 and commercially launched in Japan with the release of Billy Joel's album *52nd Street*. A key figure in its standardization was the conductor Herbert von Karajan, whose advocacy at the 1981 Salzburg Festival provided significant prestige. The collaboration between the two companies ensured a unified technical standard, preventing a format war similar to the earlier VHS versus Betamax conflict. The success of the CD-DA format for audio quickly led to its adaptation for data, formalized in the Yellow Book, which enabled the rise of the CD-ROM for software distribution and multimedia applications like Microsoft Encarta.

Physical characteristics

A standard CD is a disc 120 millimeters in diameter and 1.2 millimeters thick, composed of a polycarbonate plastic substrate, a reflective metallic layer, and a protective lacquer coating. Data is stored as a series of microscopic pits and lands arranged in a continuous spiral track from the center to the outer edge. This spiral is read by a 780 nm wavelength semiconductor laser beam, which reflects differently off pits and lands. The disc rotates at a variable speed, a system known as Constant linear velocity, to maintain a constant data rate regardless of the laser's position. The reflective layer is typically made of aluminium, though some early discs used gold for longevity, and the entire structure is protected by the disc's label.

Data encoding and storage

Data on a CD is encoded using Eight-to-fourteen modulation, a scheme that converts 8-bit data bytes into 14-bit channel bits for more reliable reading. This is combined with Cross-interleaved Reed–Solomon coding, a powerful error-correction system that allows the player to reconstruct data even if the disc is scratched or dusty. For audio CDs, the digital audio is stored in a Pulse-code modulation format at a 44.1 kHz sampling rate with 16-bit resolution. The data stream is organized into sectors, with each sector containing 2,352 bytes of user data for audio. The maximum storage capacity of a standard 74-minute disc is approximately 650 MiB, though 80-minute discs holding 700 MiB later became common.

Types and formats

Several standard formats evolved from the original audio CD. The CD-ROM is a read-only format for computer software and data, while the CD-R allows users to record data once using an organic dye layer. The CD-RW is an erasable format that uses a phase-change alloy. Specialized audio formats include the CD Single for shorter releases and the CD Video which combined digital audio with analog NTSC video. Enhanced formats like CD+G added simple graphics for use with karaoke machines. The technical specifications for these formats are detailed in a series of standards known as the Rainbow Books, including the Orange Book for recordable discs.

Manufacturing

Commercial CD production is a high-precision replication process. It begins with the creation of a glass master disc using a laser beam recorder that etches the data pattern into a photoresist layer. This master is then electroplated to create a metal stamper. In injection molding, polycarbonate is pressed against this stamper to form clear disc substrates containing the pit-and-land pattern. The substrate is then coated with a thin reflective layer of aluminium via sputter deposition, followed by a spin-coated protective lacquer. Finally, the label is printed on top, often using screen printing or offset printing. This entire process is distinct from the recording process used for CD-R discs, which are "burned" by users with a laser that darkens an organic dye.

Use and impact

The CD's introduction had a profound impact on the music industry, leading to a massive resurgence in album sales and profits for labels like Warner Music Group and EMI as consumers replaced their analog collections. It became the dominant software distribution medium for personal computers, enabling the spread of complex applications and games from companies like Microsoft and Adobe Inc.. The technology also facilitated the creation of multimedia experiences and was central to early digital archives like the Gutenberg Project. Although its prominence has waned with the rise of Internet-based distribution, streaming media, and solid-state drive storage, the CD established the foundational optical storage technology that led to the DVD and Blu-ray formats.

Category:Computer storage media Category:Audio storage Category:1982 introductions Category:Optical disc authoring