What Is CD? The Definition of Compact Disc

[Latest Updated: November 6, 2018 by Donna Peng to Tips & Glossary]

CD

CD stands for Compact Disc. The CD is an optical disc format developed by Sony and Philips starting in 1979. It was originally developed as a format to store audio and once the biggest selling medium for recorded music from major and independent record companies. However, CD sales rates in the U.S. have decreased about 50% from their peak and CD is being replaced generally by other forms of digital storage and distribution.

The Features of CD

An audio CD can hold up to 80 minutes of audio data, stored using 16-bit PCM at a sampling rate of 44100 Hz. A standard CD is approximately 120mm in diameter, but 80mm discs also exist that can store about 20 minutes of audio.

The technology was eventually adapted and expanded to encompass data storage CD-ROM, write-once audio and data storage CD-R, rewritable media CD-RW, Video Compact Discs (VCD), Super Video Compact Discs (SVCD), PhotoCD, PictureCD, CD-i, and Enhanced CD.

What Is CD-R?

CD-R stands for Compact Disc - Recordable (originally named CD Write-Once). It is a write-once recordable CD media that is suitable for creating audio or data CD compilations (CD-ROM). Though a CD-RW disc can be erased and written again, many CD players and other equipment generally perform better with CD-R media (always check if a CD player supports CD-RW before you buy it). A standard CD-R is 120 mm in diameter and usually comes with a capacity of either 640MB or 700MB.

A CD-R will store 74 or 80 minutes of CD audio. CD-RW stands for Compact Disc - Rewritable. It is re-writable CD media that is suitable for data CD compilations (CD-ROM). Though a CD-RW disc can be erased and written again, many CD players and other equipment generally perform better with CD-R media (always check if a CD player supports CD-RW before you buy it). A standard CD-RW is 120 mm in diameter and usually comes with a capacity of either 640MB or 700MB. A CD-RW will store 74 or 80 minutes of CD audio, but as mentioned, is much less compatible with CD players than CD-R.

What Is CD-ROM?

CD-ROMs are popularly used to distribute computer software, including games and multimedia applications, though any data can be stored (up to the capacity limit of a disc). Some CDs hold both computer data and audio with the latter capable of being played on a CD player, while data (such as software or digital video)is only usable on a computer (such as ISO 9660 format PC CD-ROMs). These are called enhanced CDs.

CD-ROM discs are identical in appearance to audio CDs, and data are stored and retrieved in a very similar manner (only differing from audio CDs in the standards used to store the data). Discs are made from a 1.2 mm thick disc of polycarbonate plastic, with a thin layer of aluminum to make a reflective surface. The most common size of CD-ROM disc is 120 mm in diameter, though the smaller Mini CD standard with an 80 mm diameter, as well as numerous non-standard sizes and shapes (e.g., business card-sized media) are also available. Data is stored on the disc as a series of microscopic indentations.

A laser is shone onto the reflective surface of the disc to read the pattern of pits and lands ("pits", with the gaps between them referred to as "lands"). Because the depth of the pits is approximately one-quarter to one-sixth of the wavelength of the laser light used to read the disc, the reflected beam's phase is shifted in relation to the incoming beam, causing destructive interference and reducing the reflected beam's intensity. This pattern of changing intensity of the reflected beam is converted into binary data.

A CD-ROM sector contains 2,352 bytes, divided into 98 24-byte frames. Unlike a music CD, a CD-ROM cannot rely on error concealment by interpolation, and therefore requires a higher reliability of the retrieved data. In order to achieve improved error correction and detection, a CD-ROM has a third layer of Reed-Solomon error correction. A Mode-1 CD-ROM, which has the full three layers of error correction data, contains a net 2,048 bytes of the available 2,352 per sector. In a Mode-2 CD-ROM, which is mostly used for video files, there are 2,336 user-available bytes per sector. The net byte rate of a Mode-1 CD-ROM, based on comparison to CDDA audio standards, is 44100 Hz × 16 bits/sample×2 channels × 2,048 / 2,352/8=153.6 KBps=150 KBps. The playing time is 74 minutes, or 4,440 seconds, so that the net capacity of a Mode-1 CD-ROM is 682 MB or, equivalently, 650 MB.

In the 90s, recordable CD media showed up and has since become a huge success worldwide. Using recordable CD formats, which come mainly in 640MB and 700MB capacities, consumers can use easy software solutions like Nero to burn their very own audio CDs or create compilations of data CDs. They can also be used to create the inexpensive MPEG-1-based VideoCD and MPEG-2-based Super VideoCD formats, which are supported by a very large amount of stand-alone DVD players.

About the Author

Donna Peng    Donna Peng@DonnaPeng

Donna Peng has been working as a copywriter in Digiarty Software for many years. She has written loads of articles to help people handle their DVDs, videos and other stuff.

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Part 1The Features of CD

Part 2What Is CD-R?

Part 3What Is CD-ROM?

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