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Glossary
 

The CD was launched in 1982 for high quality digital audio and has become one of the most successful examples of consumer electronics technology. The CD Technology pages provide an introduction to the following formats, which are fully described in the coloured books available from Philips.
  • CD audio, for high quality, noise free digital audio and one of the most successful examples of consumer electronics technology.
  • CD-ROM, an extension of the CD format for computer software, games and multimedia applications.

For printable documents on CD and DVD technology please visit CD and DVD documents.

Essential Technologies

The compact disc makes use of a number of essential technologies that were invented well before 1982. In particular the following three technologies have helped to make the CD possible: .

  • Pulse Code Modulation (PCM), invented in 1937 by Alec Reeves who was working in for STL in London at the time. He received a CBE in 1969 for his work on PCM. See CD digital audio for more information.
  • Reed-Solomon error correction codes, which were invented by Reed and Solomon in 1960, but were based on earlier work by Hamming in 1950. See CD Data Coding for more information.
  • The Laser which was invented at the Bell Labs in 1958, (see Laser Technology).

Laser Technology

CDs rely on laser technology to read (and write) the data on discs. The word LASER stands for Light Amplification by Stimulated Emission of Radiation. Lasers generate coherent light, which allows the light beam to be focused to a very small spot size, the spot diameter being equal to the wavelength divided by the numerical aperture.

The advent of lasers and in particular low cost laser emitting diodes has allowed the compact disc technology to become one of the most successful consumer electronic technologies of all time.

Diagram of PitsIn the late 60s, Philips developed the laser video disc, the first such application of the laser for a consumer electronics product. The 30cm disc was capable of storing up to 60mins of analogue video per side.

A low power laser was used to read the video information stored in pits in the disc surface. The video and audio signals are represented in analogue form by these pits which were arranged in a spiral pattern, like vinyl records.

CD Laser Diodes

CD players use light emitting diode lasers, which are compact and low cost, to read the data contained in pits in the surface of the disc.

Diagram showing laser diode and opticsThe laser diode is mounted on a swivel arm which can be moved in a radial direction across the disc surface while the disc is rotated. This allows the laser beam to follow the pits accurately.

A semi-reflective mirror allows the reflected light to pass back to a photo detector. When the laser beam falls on a pit very little is reflected. The changing light pattern detected is then converted into a series of zeros and ones which are then decoded into the original audio or computer data signal.

Unlike laserdiscs, CDs use a digital technique where the pits indicate whether a data bit is '0' or '1'. Also laserdiscs can be either CAV (Constant Angular Velocity) or CLV (Constant Linear Velocity), but all CDs use CLV. This means that the pit sizes do not vary from inside to outside of the disc but the angular velocity does vary.

CD Books

The specifications for all CD formats are contained in the 'coloured books', which are summarised below.
  • Red Book, which defines the physical properties of the compact disc, digital audio encoding, modulation system, error correction and subcode channels. CD TEXT was added in 1996.
  • Yellow Book for CD-ROM and, in a separate yellow book, CD-ROM XA specifications. They define the physical parameters, sector structure, additional error correction and, for CD-ROM XA, the audio and image coding which are now obsolete.
  • White Book for Video CD and Super Video CD, defining the video tracks, still images, playlists and scan information for fast forward/reverse.
  • Blue Book for CD Extra or Enhanced Music CD, including the use of multisession, the directory structure and mandatory files and directories.
  • Orange Book for CD-R and CD-RW media formats.

As there are not enough colours there are also the following books:

  • Photo CD, for the storage of photographic files, including image data coding, and playlists.
  • Multisession CD for pre-recorded multisession CDs, including data/session format and data retrieval structure.

These Books are obtainable from Philips Intellectual Property & Standards,


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