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Compact Disc

Retrieving Information From A Disc

The primary unit of data storage on a compact disc is a sector, which is 1/75-second long. Each sector on a CD contains 2352 bytes (processable units) of data, and each

Medium Capacity Cost per megabyte
Hard disk 100 megabytes ~$7.00
Paper 2 kilobytes per page ~$5.00
Magnetic tape 60 megabytes <$1.00
Floppy disk 1.44 megabytes <$0.50
CD-ROM 650 megabytes ~$0.01

Format name Description Notes
CD-ROM ISO 9660 Read-only memory Applies to MS-DOS and Macintosh files. This standard evolved from the Yellow Book specifications of Philips and Sony. Defined the Volume Table of Contents that tells the CD reader where and how the data are laid out on the disc.
CD-ROM High Sierra Read-only memory Based on a standard worked out in 1985 to resolve differences in leading manufacturers' implementations of ISO 9660.
CD-DA Digital audio Data drives that can read data and audio are called CD-DA.
PhotoCD Compressed images KODAK multisession XA system. Customers present an exposed 35 mm roll of color film for wet processing, and purchase a Photo CD for an additional charge. The negatives are then processed by a technician who scans each image at an imaging workstation. The images are written onto the Photo CD write-once media, color thumbnails of all of the images are printed, and the Photo CD is returned to the consumer in a CD jewel case with the index sheet inserted as a cover. The customer may return the same Photo CD to have more images written onto it, with the result that a multisession disc is produced. A Photo CD can hold 125 or more high resolution images. Photo CDS may be viewed using a Kodak Photo CD player connected to a television at the customer's home. Photo CD images can also be viewed using a CD-ROM/XA player attached to a computer. Photo CD images can be converted to other formats for incorporation into multimedia applications.

sector is followed by 882 bytes of data for detecting errors, correcting information, and controlling timing. Thus, a CD actually requires 3234 bytes to store 2352 bytes of data.

The disc spins at a constant linear velocity, which means that the rotational speed of the disc may vary from about 200 rpm when the data being read are near the outer of the disc to about 530 rpm when the data are located near the center of the disc. (This is because rotational speed = linear velocity × radius of sector.) The CD is read at a sustained rate of 150K (150,000) bytes per second, which is sufficient for good audio but very

Format name Description Notes
CD-ROM/XA Read-only memory Extended architecture. Data are read off a disc in alternating pieces, and synchronized at playback. The result is a simultaneous presentation of graphics and audio. CD-ROM/XA defined a new sector format to allow computer data, compressed audio data, and video/image information to be read and played back apparently simultaneously. Special enabling hardware is required on CD-ROM/XA players because the audio must be separated from the interleaved data, decompressed, and sent to speakers, at the same time the computer data are being sent to the computer.
CD-R (CD-WO or CD-WORM) Write-once May use multiple sessions to fill disc. Instead of burning pits into a substrate, the CD-R uses differences in reflectivity to fool the reader into believing that a pit actually exists. This format allows you to write your own CDS.
CD-ROM HFS Read-only memory The Macintosh Hierarchical File System (HFS) is Apple's method for managing files and folders on the Macintosh desktop. The HFS driver provides Macintosh users with the expected and familiar Apple desktop. This is the preferred format for delivery to Macintosh platforms, even though it does not conform to the ISO 9660 standard.
CD-I or CD-RTOS Interactive Philips Interactive motion video. CD-I discs are designed to work with Philips CD-I players, but the CD-I system also hooks up to the customer's TV or stereo. It can play audio CDS, and can also read Kodak PhotoCD discs. The CD-I is marketed for education, home, and business use.
CD-I Ready Interactive/Ready CD-Audio with features for CD-I player.
CD-Bridge Bridge Allows XA track to play on CD-I player.

Format name Description Notes
CD-MO Magneto-optical Premastered area readable on any CD player.
CD+G Mixed mode CD+G stands for CD audio plus graphics. This format allows the customer to play CD audio along with titles, still pictures, and song lyrics synchronized to the music. This CD may be best suited to karaoke-style discs, i.e., music playing along with on-screen lyrics.
CDTV ISO 9660 variant Commodore proprietary system.

slow for large image files, motion video, and other multimedia resources. Newer drives spin at twice or even three to six times this rate. Still, CD access speeds and transfer rates are much slower than those from a hard disc in a computer. This is expected to change as discs are made to spin faster and different types of lasers are perfected for use in computers.

The surface of the CD is essentially transparent. It must allow a finely focused beam of laser light to pass through it twice, first to the metallic layer beneath the plastic where the data reside, and then back to the receptors. Dirt, scratches, fingerprints, and other imperfections interfere with retrieval of the stored data.

Additional topics

Science EncyclopediaScience & Philosophy: Cluster compound to ConcupiscenceCompact Disc - Manufacture Of A Compact Disc, Retrieving Information From A Disc, Drive Specifications, Care Of Cd-roms - CD-ROM drives, Drive formats, Interfaces, Nonstandard SCSI interfaces