Compact Disc

The CD (Compact Disc, 激光唱片) is a digital optical storage medium jointly developed by Philips and Sony.

From the late 1970s to the early 1980s, the two companies standardized audio encoding, disc dimensions, and playback parameters, resulting in the CD-DA specification, later known as the “Red Book.” The first commercial CDs and the Sony CDP-101 player were released in Japan in October 1982 and entered the European and North American markets in 1983.

A standard CD has a diameter of 120 millimeters and a thickness of 1.2 millimeters; there is also an 80-millimeter Mini CD. Factory-pressed discs are primarily made of polycarbonate, with information arranged along a spiral track extending from the center outward. The pits and flat areas on the disc do not directly correspond to the digits 0 and 1; instead, the reading system detects changes in reflected light caused by the transition between the two, and the data is then reconstructed through 8-to-14 modulation, synchronization, and error correction. Players use a near-infrared laser with a wavelength of approximately 780 nanometers; during normal playback, the optical head does not come into contact with the disc surface.

“CD” is a generic term for a family of physical media and formats and is not synonymous with an audio CD.

CD-DA (Compact Disc Digital Audio), used for pre-recorded music, stores stereo linear PCM at a sampling rate of 44.1 kHz and a quantization depth of 16 bits, with a raw audio data rate of 1,411.2 kbit/s. The audio is written continuously to sectors and tracks, rather than being stored as WAV or other standard file formats. When ripping a CD, a computer must read the CD-DA audio sectors and then encapsulate the PCM data into files such as WAVE, AIFF, or FLAC. A single CD-DA sector contains 2,352 bytes of audio data, with 75 sectors played per second. The directory area records track start positions, control flags, and the total duration of the disc; tracks may also use index points, pre-gap, and pre-emphasis flags. The standard directory only stores track positions and does not include general text fields such as track titles and artist names; the CD-Text extension uses subcode areas to add this type of information. Early 120-millimeter audio CDs had a nominal playback time of approximately 74 minutes. The later, more widespread 80-minute discs pushed track spacing and recordable range to the limits allowed by the specification, enabling the storage of approximately 80 minutes of audio. When converted to CD-ROM user data, these are typically referred to as 650 MB and 700 MB, respectively; different sources may use decimal MB, binary MiB, or the total number of sectors, so the capacity figures may not be entirely consistent.

CD-ROM and CD-DA use similar physical signals and basic error correction, but computer data includes sector addresses, synchronization fields, and additional error detection and correction information. A typical Mode 1 sector provides 2,048 bytes of user data from 2,352 raw bytes. ECMA-130 and ISO/IEC 10149 specify the mechanical, optical, and recording characteristics of 120-millimeter read-only CD-ROMs, while standards such as ISO 9660 define the file and directory structures. Data CDs, MP3 CDs, and audio CDs can therefore have the same physical form but employ different logical organizations and playback methods. CD-Rs use an organic dye recording layer; the recording laser alters its optical properties to simulate the signal variations of a pressed disc, and once written, the data cannot be erased. CD-RWs use reversible phase-change material that can be switched multiple times between crystalline and amorphous states. The reflectivity and recording characteristics of recordable discs differ from those of factory-pressed discs; some early audio players could not reliably read CD-Rs or did not support CD-RWs at all. Multi-track configurations and unsealed discs can also affect recognition by standard players and different operating systems.

Applications developed around CD-DA include CD+G, Enhanced CD, and HDCD. CD+G incorporates low-resolution graphics into the subcode and is commonly used for karaoke; Enhanced CD uses multisession or hybrid mode to provide both standard audio and computer data simultaneously; HDCD encodes specific control information into a CD-DA-compatible PCM signal, requiring corresponding decoding to utilize its extended features. These terms describe different logical formats or signal processing methods; the discs themselves remain part of the CD family.

CD-DA uses an interleaved Reed–Solomon code to handle continuous and burst read errors; players can also interpolate or mute audio samples that cannot be fully corrected. CD-ROM builds on this by adding stricter error-checking mechanisms to prevent computer files from being altered without an error being reported. Scratches and smudges can interfere with focusing and tracking; since the reflective layer of a CD is close to the label side, deep scratches on the label or peeling of the protective coating can sometimes directly damage the data layer. Material oxidation, dye fading, and bonding defects can also affect long-term readability, and the actual lifespan depends on manufacturing quality, light exposure, temperature, and humidity.