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Compact Disc

Compact Disc

Background

Ever since the invention of the phonograph in 1876, music has been a popular source of home entertainment. In recent years, the compact disc has become the playback medium of choice for recorded music.

A compact disc, or CD, is an optical storage medium with digital data recorded on it. The digital data can be in the form of audio, video, or computer information. When the CD is played, the information is read or detected by a tightly focused light source called a laser (thus the name optical medium). This article will focus on audio compact discs, which are used to play back recorded music.

The history of the compact disc can be traced back to the development of electronic technology and particularly digital electronic technology in the 1960s. Although the first applications of this technology were not in the recording area, it found increasing use in audio components as the technology evolved.

During the same period, many companies started experimenting with optical information storage and laser technology. Among these companies, electronic giants Sony and Philips made notable progress in this area.

By the 1970s, digital and optical technologies had reached a level where they could be combined to develop a single audio system. These technologies provided solutions to the three main challenges faced by the developers of digital audio.

The first challenge was to find a suitable method for recording audio signals in digital format, a process known as audio encoding. A practical method of audio encoding was developed from the theories published by C. Shannon in 1948. This method, known as pulse code modulation (PCM), is a technique that samples a sound during a short time interval and converts the sample to a numerical value that is then modulated or stored for later retrieval.

The storing of audio signals in digital form requires a large amount of data. For instance, to store one second of music requires one million bits of data. The next challenge, therefore, was to find a suitable storage medium to accommodate any significant amount of sound. The solution to this problem came in the form of optical discs. An optical disc can store large amounts of data tightly compressed together. For example, one million bits of data on a CD can occupy an area smaller than a pinhead. This information is read by means of a laser beam that is capable of focusing on a very narrow area as small as 1/2500th of an inch.

The final challenge of digital audio was to process the densely packed information on compact discs quickly enough to produce continuous music. The solution was provided by the development of integrated circuit technology, which allow the processing of millions of computations in just micro-seconds.

By the late 1970s, a common set of standards for the optical storage discs had been developed by the joint efforts of Sony and Philips. A consortium of 35 hardware manufacturers agreed to adopt this standard in 1981 and the first compact discs and compact disc players were introduced in the market in 1982.

Raw Materials

A compact disc is a deceptively simple looking device considering the technology required to make it. CDs consist of three layers of materials:

  • A base layer made of a polycarbonate plastic.
  • A thin layer of aluminum coating over the polycarbonate plastic.
  • A clear protective acrylic coating over the aluminum layer.

Some manufacturers use a silver or even gold layer instead of the aluminum layer in the manufacture of their compact discs.

Design

The compact disc is designed strictly according to the standards established by Sony and Philips in order to maintain universal compatibility. A CD is 4.72 inches (120 millimeters) in diameter and .047 inches (1.2 millimeters) thick. The positioning hole in the middle is .59 of an inch (15 millimeters) in diameter. A CD usually weighs around .53 of an ounce (15 grams).

A standard CD can store up to 74 minutes of data. However, most CDs contain only about 50 minutes of music, all of which is recorded on only one side of the CD (the underside). The recorded data on the CD takes the form of a continuous spiral starting from the inside and moving outward. This spiral or track consists of a series of indentations called pits, separated by sections called lands. A tiny laser beam moving along the track reflects light back to a photo sensor. The sensor sees more light when it is on a land than when it is on a pit, and these variations in light intensity are converted into electrical signals that represent the music originally recorded.

The Manufacturing
Process

Compact discs must be manufactured under very clean and dust free conditions in a "clean room," which is kept free from virtually all dust particles. The air in the room is specially filtered to keep out dirt, and occupants of the room must wear special clothing. Because an average dust particle is 100 times larger than the average pit and land on a CD, even the smallest dust particle can render a disc useless.

Preparing the disc master

  • 1 The original music is first recorded onto a digital audio tape. Next, the audio program is transferred to a 3/4-inch (1.9 centimeters) video tape, and then data (called subcodes) used for indexing and tracking the music is added to the audio data on the tape. At this point, the tape is called a pre-master.
  • 2 The pre-master tape will be used to create the disc master (also called the glass master), which is a disc made from specially prepared glass. The glass is polished to a smooth finish and coated with a layer of adhesive and a layer of photoresist material. The disc is approximately 9.45 inches (240 millimeters) in diameter and .24 of an inch (six millimeters) thick. After the adhesive and photoresist are applied, the disc is cured in an oven.
  • 3 Next, both the pre-master tape and the disc master are put into a complex laser cutting machine. The machine plays back the audio program on the pre-master tape. As it does so, the program is transferred to a device called a CD encoder, which in turn generates an electrical signal. This signal powers a laser beam, which exposes or "cuts" grooves into the photoresist coating on the glass disc (the disc master).
  • 4 The grooves that have been exposed are then etched away by chemicals; these etched grooves will form the pits of the CD's surface. A metal coating, usually silver, is then applied to the disc. The disc master now contains the exact pit-and-land track that the finished CD will have.

Electroforming

  • 5 After etching, the disc master undergoes a process called electroforming, in which another metal layer such as nickel is deposited onto the disc's surface. The phrase "electro" is used because the metal is deposited using an electric current. The disc is bathed in an electrolytic solution, such as nickel solphamate, and as the electric current is applied, a layer of metal forms on the disc master. The thickness of this metal layer is strictly controlled.
  • 6 Next, the newly applied metal layer is pulled apart from the disc master, which is put aside. The metal layer, or father, contains a negative impression of the disc master track; in other words, the track on the metal layer is an exact replica, but in reverse, of the track on the disc master.
  • 7 The metal father then undergoes further electroforming to produce one or more mothers, which are simply metal layers that again have positive impressions of the original disc master track. Using the same electroforming process, each mother then produces a son (also called a stamper) with a negative impression of the track. It is the son that is then used to create the actual CD.
  • 8 After being separated from the mother, the metal son is rinsed, dried, polished and put in a punching machine that cuts out the center hole and forms the desired outside diameter.

Replication

  • 9 The metal son is then put into a hollow cavitya dieof the proper disc shape in an injection molding machine. Molten polycarbonate plastic is then poured into this die to form around the metal son. Once cooled, the plastic is shaped like the son, with the pits and groovesonce again in a positive impression of the original disc master trackformed into one side.
  • 10 The center hole is then punched out of plastic disc, which is transparent at this stage. Next, the disc is scanned for flaws such as water bubbles, dust particles, and warps. If a flaw is found, the disc must be discarded.
  • 11 If the disc meets the quality standards, I it is then coated with an extremely thin, reflective layer of aluminum. The coating is applied using vacuum deposition. In this process, aluminum is put into a vacuum chamber and heated to the point of evaporation, which allows it to be applied evenly to the plastic disc.
  • 12 Finally, a clear acrylic plastic is applied to the disc to help protect the underlying layers from physical damage such as scratches. After the label is printed, generally using a silk-screening process, the compact disc is complete and ready for packaging and shipment.

Quality Control

A compact disc is a very precise and accurate device. The microscopic size of the data does not allow for any errors in the manufacturing process. The smallest of dust particles can render a disc unreadable.

The first quality control concern is to ensure that the clean room environment is properly monitored, with controlled temperature, humidity, and filtering systems. Beyond that, quality control checkpoints are built into the manufacturing process. The disc master, for instance, is inspected for smoothness and its photoresist surface for proper thickness by means of laser equipment. At later stages in the process, such as before and after the aluminum coating is deposited and after the protective acrylic coating is applied, the disc is checked automatically for warps, bubbles, dust particles, and encoding errors on the spiral track. This mechanical checking is combined with human inspection using polarized light, which allows the human eye to spot defective pits in the track.

In addition to checking the discs, the equipment used to manufacture them must be carefully maintained. The laser cutting machine, for instance, must be very stable, because any vibration would make proper cutting impossible. If strict quality control is not maintained, the rejection rate of CDs can be very high.

The Future

The massive storage capabilities, accuracy of data, and relative immunity from wear and tear will continue to make compact discs a popular medium for music and video applications. The hottest new product stirring public interest is CD-Interactive or CD-I, a multimedia system that allows users to interact with computers and television.

Manufacturing techniques will continue to be streamlined and improved, requiring smaller facilities and less human intervention in the process and resulting in lower CD rejection rates. Already in the first decade of CD manufacture, the manufacturing and quality control processes have become almost completely automated.

Where To Learn More

Books

Brewer, Bryan. The Compact Disc Book: A Complete Guide to the Digital Sound of the Future. Harcourt Brace, 1987.

Nakajima, H. Compact Disc Technology. IOS Press, 1991.

Pohlmann, Ken C. Principles of Digital Audio. 1985.

Pohlmann, Ken C. The Compact Disk Handbook, 2nd ed., A-R Editions, 1992.

Periodicals

Bernard, Josef. "Compact Discs-Bit by Bit," Radio-Electronics. August, 1986, p. 62.

Birchall, Steve. "The Magic of CD Manufacturing," Stereo Review. October, 1986, p. 67.

Rashid Riaz

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Compact Disc

Compact disc

A compact disc (CD), or optical disc, is a thin, circular wafer of clear plastic and metal measuring 4.75 inches (120 centimeters) in diameter with a small hole in its center. CDs store different kinds of data or information: sound, text, or pictures (both still and moving). Computer data is stored on CDs in a format called CD-ROM (Compact Disc-Read Only Memory).

All CDs and CD-ROMs are produced the same way. Digital data (the binary language of ones and zeroes common to all computers) is encoded onto a master disc, which is then used to create copies of itself. A laser burns small holes, or pits, into a microscopic layer of metal, usually aluminum. These pits correspond to the binary ones. Smooth areas of the disc untouched by the laser, called land, correspond to the binary zeros. After the laser has completed burning all the pits, the metal is coated with a protective layer.

Audio CDs

Audio or music CDs were introduced in 1982. They offered many advantages over phonograph records and audio tapes, including smaller size and better sound quality. By 1991, CDs had come to dominate the record industry. In an audio CD player, a small infrared laser shines upon the pits and land on the metal layer of the disc as the disc spins. Land reflects the laser light while pits do not. A mirror or prism between the laser and the disc picks up the reflected light and bounces it onto a photosensitive diode (an electronic device that is sensitive to light). The diode converts the light into a coded string of electrical impulses. The impulses are then transformed into waves for playback through stereo speakers.

CD-ROMs

CD-ROMs (and audio CDs) contain information that cannot be erased or added to once the discs have been created. While audio CDs contain only sound information, CD-ROMs store incredible amounts of

text, graphic (video), or sound information. Discs that contain information in more than one of these media are referred to as multimedia. Since video and sound require large amounts of disc storage space, most multimedia CD-ROMs are text-based with some video or sound features added. Information on a CD-ROM is retrieved the same way it is on an audio CD: a laser beam scans tracks of microscopic holes on a rotating disc, eventually converting the information into the proper medium. Because of their high information storage capacity, CD-ROMs have become the standard format for such large published works as software documentation and encyclopedias.

WORMs

WORM (Write Once, Read Many) systems are a little more complicated than CD-ROM systems. Writable WORM discs are made of different material than consumer CD-ROMs. When a WORM disc is created, a laser does not burn pits into a microscopic layer of metal as with a CD-ROM. Instead, in a heat-sensitive film a laser creates distortions that reflect light. These distortions represent bits of data. To read the disc, the laser is scanned over the surface at lower power. A detector then reads and decodes the distortions to obtain the original signal.

WORM discs allow the user to write new information onto the optical disc. Multiple writing sessions may be needed to fill the disc. Once recorded, however, the data is permanent. It cannot be rewritten or erased. WORM discs are especially suited to huge databases (like those used by banks, insurance companies, and government offices) where information might expand but not change.

MODs

Magneto-optical discs (MODs) are rewritable, and operate differently than either ROM or WORM disc. Data is not recorded as distortions of a heat-sensitive layer within the disc. Rather, it is written using combined magnetic and optical techniques. Digital data (binary ones and zeros) is encoded in the optical signal from the laser in the usual manner. Unlike the ROM or WORM discs, however, the MOD write layer is magnetically sensitive. An external magnet located on the write/read head aligns the binary ones and zeros in different directions. The MOD is read by scanning a laser over the spinning disc and evaluating the different directions of the digital data. The MOD is erased by orienting the external magnet so that digital zeros are recorded over the whole disc.

[See also Computer, digital; DVD technology; Laser ]

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Compact Discs

COMPACT DISCS

COMPACT DISCS (CDs) are small, thin, plastic discs twelve centimeters in diameter that contain a metallized surface that holds optically recorded digital information, such as sound, images (still and motion), and computer programs. Data is recorded by creating microscopic pits along a single track on the metallized surface; playback incorporates a red laser beam reflected onto the surface that measures the pits and translates them into binary information. A standard CD can hold between 74 and 82 minutes of audio, or approximately 780 million bytes of data. CDs are nearly unaffected by the number of times they are played. The disc's durable surface tolerates fingerprints and small scratches, making it an ideal solution for optically storing and preserving digital information.

Development of the CD to replace vinyl records began in the 1970s with Royal Philips Electronics of the Netherlands and Sony Corporation of Japan. Philips produced the optical storage technologies, while Sony pioneered error correction circuitry. The result was a set of industry standards established in the late 1970s for the CD's physical and logical characteristics, which among other things, ensured compatibility among discs and players from diverse manufacturers. This standard, known as the Compact Disc Digital Audio system, was in place in the early 1980s, and in 1983, the compact disc and the first CD players were introduced to consumers.

As standards evolved, so did the uses for CDs. Changes in recording techniques allowed for specialized uses such as the CD-Read-Only Memory (CD-ROM) for use in computers, CD-Interactive (CD-I), a stand-alone audio and video hardware system designed for audio and visual data, and the Video-CD (VCD) for high-quality video playback. The rewriteable CD (CD-RW) standard, created in 1996, enabled nearly anyone with a home computer and a CD-RW drive to record music, data, and video on a compact disc.

The future remains bright for the compact disc. The Digital Versatile Disc (DVD) standard has increased storage capacities to nearly five gigabytes (4.7GB) of information, or twenty-eight times that of its CD-ROM cousin, and DVD-Video has pushed VHS videotapes from store shelves as the preferred format for popular movies. Upcoming innovations in manufacturing processes, such as the improved pinpoint light-focusing ability of the blue-violet laser beam and higher transfer rates of players and recorders, will see DVD storage capacities climb to nearly thirty gigabytes of data on one shiny disc.

BIBLIOGRAPHY

Armstrong, Elizabeth. "DVD Lasers: Why Blue Beats Red." Wired (June 2002):50.

Pohlmann, Ken C. Principles of Digital Audio. 4th ed. New York: McGraw-Hill, 2000.

MichaelRegoli

See alsoDigital Technology .

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compact disc

compact disc (CD), a small plastic disc used for the storage of digital data. As originally developed for audio systems, the sound signal is sampled at a rate of 44,100 times a second, then each sample is measured and digitally encoded on the 43/4 in (12 cm) disc as a series of microscopic pits on an otherwise polished surface. The disc is covered with a transparent coating so that it can be read by a laser beam. Since nothing touches the encoded portion, the CD is not worn out by the playing process. Introduced in 1982, the CD offered other advantages over the phonograph record and recording tape—smaller size, greater dynamic range, extremely low distortion—and met with rapid consumer acceptance; the CD became the music carrier of choice by 1991, when sales exceeded those of audiocassettes.

Other CD formats include CD-ROM [Compact Disc–Read Only Memory], a form of CD that is read (but not written to) by computer using a CD-ROM drive and that can contain computer programs and digitized text, sound, photographs, and video; CD-R [Compact Disc–Recordable] and CD-RW [Compact Disc–ReWritable], which can be written to one time and multiple times, respectively. Interactive CDs (CD-I, CDTV, and other formats) can store video, audio, and data. Photo CD is a format that holds digitized photographs and sound. There are also CD-ROMs that require special players with built-in microcomputers.

Other optical disk formats include digital versatile (or video) discs and videodiscs. A digital versatile disk (DVD) holds far more information than a CD. DVD players are backward compatible to existing technologies, so they can also play a CD (or CD-ROM), but a CD player cannot be used with a DVD (or DVD-ROM). The videodisc, or laser disk system, uses 12-in. (30-cm) disks for video recording. Its technology, unlike that of the CD, is an analog system that uses a laser to read a variable-width track, much like a conventional phonograph record.

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compact disc

compact disc (CD) Disc used for high-quality digital sound reproduction. It is a plastic disc with a shiny metal layer and a transparent protective plastic coating. The sound signal consists of millions of minute pits, pressed into one side of the metal. On replay, a narrow laser beam is reflected from the rotating disc's surface. A sensor detects changes in the beam, and forms an electrical signal of pulses. This is processed and decoded to form a sound signal that can be amplified for reproduction on loudspeakers. See also CD-ROM

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compact disc

com·pact disc (also com·pact disk) (abbr.: CD) • n. a small plastic disc on which music or other digital information is stored, and from which the information can be read using reflected laser light.See also CD-ROM.

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CD

CD • abbr. ∎  certificate of deposit. ∎  civil defense. ∎  compact disc. ∎  corps diplomatique.

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compact discs

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CD (abbreviation for compact disc)

CD: see compact disc.

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Compact Disc

Compact Disc

Manufacture of a compact disc

Retrieving information from a disc

CD-ROM drives

Drive specifications

Drive formats

Interfaces

Nonstandard SCSI interfaces

Care of CD-ROMs

Multimedia

Compact discs of the near future

Resources

In 1978, Philips and Sony together launched an effort to produce an audio compact disc(CD) as a method of delivering digital sound and music to consumers. The two companies continued to cooperate through the 1980s and eventually worked out standards for using the CD technology to store computer data. These recommendations evolved into the CDROM technology of today.

The CD-ROM (compact discread only memory) is a read-only optical storage medium capable of holding 700 megabytes of data (on the order of 500, 000 pages of text), 80 minutes of high-fidelity audio, or some combination of the two. Legend has it that the

Table 1. Costs of Information Storage . (Thomson Gale.)
Costs of information storage
Medium Capacity Cost per megabyte
Hard disk100 megabytes$7.00
Paper2 kilobytes per page$5.00
Magnetic tape60 megabytes<$1.00
Floppy disk1.44 megabytes<$0.50
CD-ROM650 megabytes$0.01

audio duration of the compact disc was chosen so that it could contain a slow-tempo rendition of Beethovens Ninth Symphony. As can be seen from Table 1, compact discs offer a high volume of data storage at a lower cost than other media.

The first users of CD-ROMs were owners of large databases: library catalogs, referencesystems, and parts lists. Typical applications of CD-ROMs as storage media now include storage of such information as the following:

  • every listing from all of the Yellow Pages in the United States
  • maps of every street in the country
  • facsimile numbers for all publicly held businesses and government institutions
  • a 21-volume encyclopedia

CD-ROMs are expected to achieve significant impact in storage of the following kinds of documents:

  • business reference materials
  • interactive educational materials for schools
  • scholarly publications
  • government archives
  • home-based reference materials

Manufacture of a compact disc

A compact disc is a thin wafer of clear polycarbonate plastic and metal measuring 4.75in (120 mm) in diameter with a small hole in its center. The metal layer is usually pure aluminum that is spread on the polycarbonate surface in a layer that is only a few molecules thick. The metal reflects light from a tiny infrared laser as the disc spins in the CD player. The reflections are transformed into electrical signals and then further converted to meaningful data for use in digital equipment.

Information (either audio on a music CD or data of many kinds on a CD-ROM) is stored in pits on the CD that are 13 microns long, about 0.5 micron wide, and 0.1 micron deep. There may be more than 3 mi(4.8 km) of these pits wound around the center hole on the disc. The CD is coated with a layer of lacquer that protects the surface. By convention, a label is usually silkscreened on the backside.

Compact discs are made in a multistep process. First a glass master is made using photolithographic techniques. An optically ground glass disc is coated with a layer of photoresist material 0.1 micron thick. A pattern is produced on the disc using a laser; then the exposed areas on the disc are washed away, and the disc is silvered to produce the actual pits. The master disc is next coated with single-molecule-thick layers of nickel, one layer at a time, until the desired thickness has been achieved. The nickel layer is next separated from the glass disc and used as a metal negative.

For low-production runs, the metal negative is used to make the actual discs. Most projects require that several positives be produced by plating the surface of the metal negative. Molds or stampers are then made from the positives and used in injection molding machines.

Plastic pellets are heated and injected into the molds, where they form the disc with pits in it. The plastic disc is coated with a thin aluminum layer for reflectance and with a protective lacquer layer. The disc is then given its silkscreened label and packaged for delivery. Most of these operations take place in a clean-room because a single particle of dust larger than a pit can destroy data. Mastering alone takes about 12 hours of work.

Retrieving information from a disc

The primary unit of data storage on a compact disc is a sector, which is 1/75 of a second long. Each sector on a CD contains 2, 352 bytes (processable units) of data, and eac sector is followed by 882 bytes of data for detecting errors, correcting information, andcontrolling timing. Thus, a CD actually requires 3, 234 bytes to store 2, 352 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 150 kB (150,000 bytes) per second, which is sufficient for good audio but very 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

Table 2. Compact Disk and Drive Formats. (Thomson Gale.)
Compact disc and drive formats
Format nameDescriptionNotes
CD-ROM ISO 9660Read-only memoryApplies 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 SierraRead-only memoryBased on a standard worked out in 1985 to resolve differences in leading manufacturers implementations of ISO 9660.
CD-DADigital audioData drives that can read data and audio are called CD-DA.
PhotoCDCompressed imagesKODAK multisession XA system. Customerspresent an exposed 35 mm roll of color film for wet processing, and purchase a Photo CD for an additional charge. The negatives are thenprocessed 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 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 multises-sion 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 atelevision at the customers home. Photo CD images can also be viewed using CD-ROM/XA player attached to a computer. Photo CD images ca be converted to other formats for incorporation into multimedia applications.
CD-ROM/XARead-only memoryExtended 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/X 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, decompessed, and sent to speakers, at the same time the computer data are being sent to the computer.
CD-R (CD-WOor 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-R (CD-WO or CD-WORM)Write-onceMay 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-ROMHFS read-only memoryThe Macintosh Hierarchical File System (HFS) is Apples 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-RTOSInteractivePhilips Interactive motion video. CD-I disc are designed to work with Philips CD-I players, but the CD-I system also hooks up to the customerss 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 ReadyInteractive/ReadyCD-Audio with features for CD-I player
CD-BridgeBridgeAllows XA track to play on CD-I player
CD-MOMagneto-opticalPremastered area readable on my CD player.
CD+GMixed modeCD+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 karaokestyle discs, i.e., music playing along with on-screen lyrics.
CDTVISO 9660 variantCommodore proprietary system.

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 afinely 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 othe imperfections interfere with retrieval of the stored data.

CD-ROM drives

Each CD-ROM drive for a personal computer (PC) may be characterized according to the following:

  • drive specifications
  • formats the drive can read
  • interface the drive requires to connect the computer

Drive specifications

The drive specifications tell the drives performance capabilities. These specifications commonly include the following:

  • The data transfer rate, which specifies how much data the drive can read from a data CD and transfer to the host computer when reading one large, sequential chunk of data.
Table 3. Multimedia Storage Requirements (Thomson Gale.)
Multimedia storage requirements
One minute of...Storage space required
audio, mono700 kilobytes
700 kilobytesmore than 1.5 megabytes
animation2.5 to 5.5 megabytes
video20 to 30 megabytes, compressed
  • The access time, which is the delay between the drive receiving the command to read and its actual first reading of the data.
  • The buffer size, which is the size of the cache of memory stored in the drive. Not all drives are equipped with memory buffers.

Drive formats

The data on compact discs need to be organized if the CD-ROM drive and computer are to make sense of the data. The data are therefore encoded to conform to certain standards. Although advanced CD-ROM standards are still evolving, most drives today comply with earlier CD-ROM formats.

Interfaces

The CD-ROM interface is the physical connection of the drive to the PCs expansion bus. The three typical interfaces are SCSI Standard, SCSI-2 and ASPI, and nonstandardSCSI.

SCSI standard interfaces

Small Computer System Interface (SCSI) refers to a group of adapter cards that conformto a set of common commands. These adapter cards allow a chain of devices to be strung from a single adapter. Consequently, SCSI interfaces are preferred for connecting a CD-ROM drive to a personal computer.

SCSI-2 and ASPI interfaces

SCSI-2 and Advanced SCSI Programming Interfaces (ASPI) take into account rapid enhancements of computer interface technology. SCSI-2 incorporates several enhancements, including greater data throughput and improved read and write technologies. ASPI provides a standard software interface to the host adapter hardware.

Nonstandard SCSI interfaces

Nonstandard SCSI interfaces may not accept installation of multiple SCSI devices; in cases where this is not a problem, they may prove acceptable.

Care of CD-ROMs

Audio CDs tend to be more forgiving than CDs that will be read by computers. The audioCD player can fill in any missing data on the disc because audio data are easily interpolated, with the result that scratches do not have much effect on the quality of the sound produced when the CD is played.

Computer data are less predictable than audio data. Consequently, it is not as easy tointerpolate when data are missing. Because computer data are digital (either 0s or 1s), the computer cannot represent missing data by an average value lying somewhere between 0 and 1. Because small scratches are inevitable, the CD-RO incorporates a scheme that makes it possible to reconstruct any bit from the surrounding data. This scheme is called Error Correction Code (ECC). ECC permits the CD-ROM to undergosome surface damage and still remain usable, but it does not replace the need to exercise care when handling a disc.

Multimedia

Multimedia is a computer application that employs more than one medium to convey information. Examples of multimedia include:

  • text with graphics
  • text with photos
  • text with sound
  • text with animation
  • text with video
  • graphics with sound
  • photos with sound
  • animation with sound
  • video with sound

As indicated in Table 3, some multimedia combinations require very large amounts of disc storage space. To date, most multimedia applications have been text-based with multimedia features added. Many multimedia applications, however, make heavy use of memory-intensive features such as video and sound. Although the CD-ROM is not a requirement for multimedia on the personal computer, its impressive storage capacity makes it a logical choicefor delivering multimedia documentation.

Compact discs of the near future

Recordable and erasable-rewritable CDs (CD-RWs) have given the compact disc greater versatility. Erasable CDs became common by 2001. CD-RWs are versatile data storage of moderate amounts of data because they can be overwritten at well, with the potential of cycling up to a thousand times. DVDRWS capable of holding about 4.5 GB rewriteably are now commonplace (as of 2006). High-density and multilayer CD-Rs, CD-RWs, and DVDs are also being developed but have not yet become market standards for desktop and laptop computer users.

Improvements are also coming to audio CDs as manufacturers seek new features that wil improve sales. Enhanced audio CDs now include music videos, lyrics, scores that the home musician can play, and interviews with the musicians. Enhanced audio CDs can be played o a CD-ROM drive and viewed on a monitor or connected television set. High Definition Compatible Digitals, or HDCDs, are also being marketed. They produce more realistic sound but require a CD player with a built-in decoder.

KEY TERMS

Bit and byte A bit is the smallest element representing data in a computers memory. A byte consists of eight bits that the computer processes as a unit.

Kilobyte and megabyte One thousand bytes is a kilobyte. One million bytes is a megabyte.

Developments in technical and scientific uses of CDs are also being explored. One of the most promising is a portable medical laboratory called the LabCD, marketed starting i 2004. A drop of blood is placed on a special CD combining software and microfluidic channels. As the CD spins in a special player/analysis unit, it acts like a centrifuge and separates the cells in the blood. They slip into receptacles in the CD that contain testin chemicals, and sensors in the special player read the results of a range of blood tests including DNA tests. This technology opens the possibility for ambulances to carry LabCDs and the CD-sensing machine and to perform on-the-spot analyses for drug and alcohol use or DNA tests at crime scenes.

See also Computer, digital.

Resources

Books

Meinders, Erwin R., et al. Optical Data Storage: Phase-Change Media and Recording. New York: Springer, 2006.

Tominaga, J. and T. Nakano. Optical Near-Field Recording: Science and Technology. New York: Springer, 2005.

Other

Audio Engineering Society. The Compact Disc Story. 1998. <http://www.exp-math.uni-essen.de/ïmmink/pdf/cdstory.pdf> (accessed October 23, 2006).

Randall S. Frost

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Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

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The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Compact Disc

Compact disc

In 1978, Philips and Sony together launched an effort to produce an audio compact disc (CD) as a method of delivering digital sound and music to consumers. The two companies continued to cooperate through the 1980s and eventually worked out standards for using the CD technology to store computer data. These recommendations evolved into the CD-ROM technology of today.

The CD-ROM (compact disc-read only memory) is a read-only optical storage medium capable of holding 600 megabytes of data (approximately 500,000 pages of text), 70 minutes of high fidelity audio, or some combination of the two. Legend has it that the size of the compact disc was chosen so that it could contain a slow-tempo rendition of Beethoven's Ninth Symphony. As can be seen from Table 1, compact discs offer a high volume of data storage at a lower cost than other media.

The first users of CD-ROMs were owners of large databases: library catalogs, reference systems, and parts lists. Typical applications of CD-ROMs as storage media now include storage of such information as the following:

  • every listing from all of the Yellow Pages in the United States
  • maps of every street in the country
  • facsimile numbers for all publicly held businesses and government institutions
  • a 21-volume encyclopedia

CD-ROMs are expected to achieve significant impact in storage of the following kinds of documents:

  • business reference materials
  • interactive educational materials for schools
  • scholarly publications
  • government archives
  • home-based reference materials

Manufacture of a compact disc

A compact disc is a thin wafer of clear polycarbonate plastic and metal measuring 4.75 in (120 mm) in diameter with a small hole in its center. The metal layer is usually pure aluminum that is spread on the polycarbonate surface in a layer that is only a few molecules thick.

The metal reflects light from a tiny infrared laser as the disc spins in the CD player. The reflections are transformed into electrical signals and then further converted to meaningful data for use in digital equipment.

Information (either audio on a music CD or data of many kinds on a CD-ROM) is stored in pits on the CD that are 1-3 microns long, about 0.5-micron wide, and 0.1-micron deep. There may be more than 3 mi (4.8 km) of these pits wound around the center hole on the disc. The CD is coated with a layer of lacquer that protects the surface. By convention, a label is usually silkscreened on the backside.

Compact discs are made in a multistep process. First a glass master is made using photolithographic techniques. An optically ground glass disc is coated with a layer of photoresist material 0.1-micron thick. A pattern is produced on the disc using a laser; then the exposed areas on the disc are washed away, and the disc is silvered to produce the actual pits. The master disc is next coated with single-molecule-thick layers of nickel, one layer at a time, until the desired thickness has been achieved. The nickel layer is next separated from the glass disc and used as a metal negative .

For low production runs, the metal negative is used to make the actual discs. Most projects require that several positives be produced by plating the surface of the metal negative. Molds or stampers are then made from the positives and used in injection molding machines.

Plastic pellets are heated and injected into the molds, where they form the disc with pits in it. The plastic disc is coated with a thin aluminum layer for reflectance and with a protective lacquer layer. The disc is then given its silkscreened label and packaged for delivery. Most of these operations take place in a cleanroom because a single particle of dust larger than a pit can destroy data. Mastering alone takes about 12 hours of work.


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

TABLE 1. COSTS OF INFORMATION STORAGE
Medium Capacity Cost per megabyte
Hard disk100 megabytes~$7.00
Paper2 kilobytes per page~$5.00
Magnetic tape60 megabytes<$1.00
Floppy disk1.44 megabytes<$0.50
CD-ROM650 megabytes~$0.01
TABLE 2. COMPACT DISC AND DRIVE FORMATS
Format name Description Notes
CD-ROM ISO 9660Read-only memoryApplies 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 SierraRead-only memoryBased on a standard worked out in 1985 to resolve differences in leading manufacturers' implementations of ISO 9660.
CD-DADigital audioData drives that can read data and audio are called CD-DA.
PhotoCDCompressed imagesKODAK 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

TABLE 2. COMPACT DISC AND DRIVE FORMATS (cnt'd)
Format name Description Notes
CD-ROM/XARead-only memoryExtended 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-onceMay 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 HFSRead-only memoryThe 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-RTOSInteractivePhilips 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 ReadyInteractive/ReadyCD-Audio with features for CD-I player.
CD-BridgeBridgeAllows XA track to play on CD-I player.
TABLE 2. COMPACT DISC AND DRIVE FORMATS (cnt'd)
Format name Description Notes
CD-MOMagneto-opticalPremastered area readable on any CD player.
CD+GMixed modeCD+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.
CDTVISO 9660 variantCommodore 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.


CD-ROM drives

Each CD-ROM drive for a personal computer (PC) may be characterized according to the following:

  • drive specifications
  • formats the drive can read
  • interface the drive requires to connect the computer

Drive specifications

The drive specifications tell the drive's performance capabilities. These specifications commonly include the following:

  • The data transfer rate, which specifies how much data the drive can read from a data CD and transfer to the host computer when reading one large, sequential chunk of data.
  • The access time, which is the delay between the drive receiving the command to read and its actual first reading of the data.
  • The buffer size, which is the size of the cache of memory stored in the drive. Not all drives are equipped with memory buffers.

Drive formats

The data on compact discs need to be organized if the CD-ROM drive and computer are to make sense of the data. The data are therefore encoded to conform to certain standards. Although advanced CD-ROM standards are still evolving, most drives today comply with earlier CD-ROM formats.


Interfaces

The CD-ROM interface is the physical connection of the drive to the PC's expansion bus. The three typical interfaces are SCSI Standard, SCSI-2, and ASPI, and nonstandard SCSI.


SCSI standard interfaces

Small Computer System Interface (SCSI) refers to a group of adapter cards that conform to a set of common commands. These adapter cards allow a chain of devices to be strung from a single adapter. Consequently, SCSI interfaces are preferred for connecting a CD-ROM drive to a personal computer.


SCSI-2 and ASPI interfaces

SCSI-2 and Advanced SCSI Programming Interfaces (ASPI) take into account rapid enhancements of computer interface technology. SCSI-2 incorporates several enhancements, including greater data throughput and improved read and write technologies. ASPI provides a standard software interface to the host adapter hardware.

TABLE 3. MULTIMEDIA STORAGE REQUIREMENTS
One Minute Of... Storage Space Required
audio, mono700 kilobytes
audio, stereomore than 1.5 megabytes
animation2.5 to 5.5 megabytes
video20 to 30 megabytes, compressed


Nonstandard SCSI interfaces

Nonstandard SCSI interfaces may not accept installation of multiple SCSI devices; in cases where this is not a problem, they may prove acceptable.


Care of CD-ROMs

Audio CDs tend to be more forgiving than CDs that will be read by computers. The audio CD player can fill in any missing data on the disc because audio data are easily interpolated, with the result that scratches do not have much effect on the quality of the sound produced when the CD is played.

Computer data are less predictable than audio data. Consequently, it is not as easy to interpolate when data are missing. Because computer data are digital (either 0s or 1s), the computer cannot represent missing data by an "average" value lying somewhere between 0 and 1. Because small scratches are inevitable, the CD-ROM incorporates a scheme that makes it possible to reconstruct any bit from the surrounding data. This scheme is called Error Correction Code (ECC). ECC permits the CDROM to undergo some surface damage and still remain usable, but it does not replace the need to exercise care when handling a disc.


Multimedia

Multimedia is a computer application that employs more than one medium to convey information. Examples of multimedia include:

  • text with graphics
  • text with photos
  • text with sound
  • text with animation
  • text with video
  • graphics with sound
  • photos with sound
  • animation with sound
  • video with sound

As indicated in Table 3, some multimedia combinations require very large amounts of disc storage space.

To date, most multimedia applications have been text-based with multimedia features added. Many multimedia applications, however, make heavy use of memory-intensive features such as video and sound. Although the CD-ROM is not a requirement for multimedia on the personal computer, its impressive storage capacity makes it a logical choice for delivering multimedia documentation.



Compact discs of the near future

Recordable and erasable CDs will give the compact disc greater versatility. Compact disc recorders went on the market in 1997 and allow the user to record audio from various sources on CDs. The recorders require some finesse in use because the recording procedure used depends on the type, quality, and input device of the source material. If the source is a CD that can be played on a machine with digital optical output, it can be connected directly to the CD recorder as input and be dubbed much like audio tapes. The recorder evaluates the sonic range of the original and digitally synchronizes it; if tracks are recorded from several CDs, the recorder must resynchronize with each track. Also, the CD recorder does not erase, so care is needed during recording to copy the desired tracks only.

Erasable CDs became common by 2001. Erasable CDs or CD-RWs allow flexible data storage because they can be overwritten when the data on them becomes obsolete. CD-RWs are important as a publishing medium because they can be used to display multi-media presentations. Consequently, today's desktop-published newsletter may become text with audio and video displays. High-density CD-Rs and CD-RWs are also being developed.

Improvements are also coming to audio CDs as manufacturers seek new features that will improve sales. Enhanced audio CDs now include music videos, lyrics, scores that the home musician can play, and interviews with the musicians. Enhanced audio CDs can be played on a CD-ROM drive and viewed on a monitor or connected television set. High Definition Compatible Digitals, or HDCDs, are also being marketed. They produce more realistic sound but require a CD player with a builtin decoder.

Developments in technical and scientific uses of CDs are also being explored. One of the most promising is a portable medical laboratory called the LabCD. A drop of blood is placed on the CD near the center hole. As the CD spins, it acts like a centrifuge and separates the cells in the blood. They slip into receptacles in the CD that contain testing chemicals, and sensors read the results of a range of blood tests including DNA tests. This technology opens the possibility for ambulances to carry LabCDs and the CD-sensing machine and to perform on-the-spot analyses for drug and alcohol use or DNA tests at crime scenes.

See also Computer, digital.


Resources

books

Bosak, S,. J. Sloman, and D. Gibbons, The CD-ROM Book. Indianapolis, IN: Que Corporation, 1994.

Vaughan, T., Multimedia: Making It Happen. Berkeley, CA: Osborne-McGraw Hill, 1994.


Randall S. Frost

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bit and byte

—A bit is the smallest element representing data in a computer's memory. A byte consists of eight bits that the computer processes as a unit.

Kilobyte and megabyte

—One thousand bytes is a kilobyte. One million bytes is a megabyte.

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Compact Disc

Compact Disc



Throughout the history of recorded sound, technological changes have periodically reshaped the music industry. One such change was the introduction of the 331⁄3-rpm long-playing record (LP; see entry under 1940s—Music in volume 3) and the 45-rpm "single" in 1948. Recorded at speeds of thirty-three and one-third and forty-five revolutions per minute, vinyl records improved the way people listened to music and they offered new opportunities for musical innovation. The introduction of the compact disc (CD) in 1983 was another momentous change. By the late 1980s, the compact disc had almost completely replaced vinyl records, which could become scratched or worn out. Compact discs used digital technology to take sound and convert it to samples that could be read by a laser beam of light. The digital coding could then be placed on a 41⁄2 inch disc.

A number of reasons are responsible for the success of CDs: they are portable, they never wear out, and each disc can hold much more music than previous media. Digital technology allows people to take a lot of music with them on these discs, which are light and easy to carry. Many discs can hold eighty minutes of music or more. Electronics manufacturers introduced portable CD listening devices such as Sony's Walkman (see entry under 1970s—Music in volume 4), first used for cassettes but later adapted for CDs. They can also be put into car stereos. Now the original recording a person buys can move easily from the home stereo to the portable stereo to the car stereo. CDs are also more durable than vinyl records. Only the


laser beam touches the disc, so the disc can be played indefinitely without ever wearing out like vinyl records. The discs also never develop the pops and hiss of vinyl records.

Compact discs added a much-needed jolt to the music industry. Many people, impressed by the sound quality, durability, and portability of CDs, repurchased many of their old records on the discs. The popularity of the CD also led record companies to mine their collections and release box sets of older music, much of which had disappeared from record shelves. These box sets reinvigorated people's interest in older music. Often the box sets came with elaborate packaging that contained detailed histories of the artists and information about the music. By the late 1980s, the CD had become the dominant form of music technology and an important part of American culture.


—Timothy Berg


For More Information

Millard, Andre. America on Record: A History of Recorded Sound. New York: Cambridge University Press, 1995.

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Compact Discs

Compact Discs

With the 1983 mass market introduction of CDs (compact discs), the face of the music recording and retail industry changed dramatically. As the price of compact disc players tumbled from $1,500 to $500 and below, CDs were quickly adopted by music consumers and pushed the long-playing vinyl record virtually off the market.

CDs offered extremely high sound quality, free from the scratches or needle dust "noise" found on vinyl records. Compact discs were the first introduction of digital technology to the general public. Records and tapes had been recorded using analog technology. Digital recording samples sounds and represents them as a series of numbers encoded in binary form and stored on the disc's data surface. The CD player's laser light reads this data and when it is converted back into an electric signal, it is then amplified and played through headphones or loudspeakers. On a CD nothing except light touches the disc with no wear to the recording. In addition to the superior sound of CDs, the new technology also allowed any song, or any part of a song, to be accessed quickly. Most CD players could be programmed to play specific songs, omit songs, or reorder them, providing a "customization" previously unavailable with cassette tapes or records.

While the portability of cassette tapes—with hand-held cassette players like the Sony Walkman and the ubiquity of cassette players in automobiles—slowed the domination of CDs in the market, manufacturers quickly produced products to offer the superior sound quality, flexibility, and longevity of CDs in automobiles and for personal use. By the end of the century, CD players that could hold several CDs at a time were installed in automobiles and people could carry personal CD players to listen to their favorite music with headphones while exercising. Although manufacturers and record companies had for the most part stopped manufacturing both cassette players and prerecorded tapes, cassette tapes continued to be used in home recording and in automobiles. By the late 1990s, cassettes were no longer a viable format for prerecorded popular music in the United States although they remained the most used format for sound recording worldwide.

The physical size of CDs altered the nature of liner notes and album covers. The large size of LP covers had long offered a setting for contemporary graphic design and artwork, but the smaller size of the CD package, or "jewel box," made CD "cover art" an oxymoron. Liner notes and song lyrics became minuscule as producers tried to fit their material into 5-by-5 inch booklets. The "boxed set," a collection of two or more CDs in a longer cardboard box, became popular as retrospectives for musicians and groups, collecting all of a musician or groups' output including rare and unreleased material with a booklet of extensive notes and photographs. Ironically, the average playing time of a CD was more than 70 minutes but most albums continued to hold about 40 minutes of music, the amount available on LPs.

Though the long-playing records were technologically obsolete and no longer stocked on the shelves of major music retailers, they were still found in stores specializing in older recordings and used by rap and hip-hop performers who scratched and mixed records to make their music. LPs also experienced a minor resurgence in 1998 from sales to young people interested in the "original" sound of vinyl. Nevertheless, the CD had become the dominant medium for new music by the end of the century.

—Jeff Ritter

Further Reading:

Millard, Andre. America on Record: A History of Recorded Sound. Cambridge University Press, 1995.

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