Compact disc

Compact disc (or CD) is an optical disc used to store digital data, originally developed for storing digital audio. It is the standard playback format for commercial audio recordings today.

Data structure

The smallest entity in the CD audio format is called a frame. A frame can accommodate six complete 16-bit stereo samples, i.e. 2×2×6 = 24 bytes. Data in a CD-ROM are organized in both frames and sectors.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

A CD-ROM sector contains 98 frames, and holds 98×24 = 2352 bytes.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

The CD-ROM is in essence a data disc, which cannot rely on error concealment, and it requires therefore 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.

Note that the CIRC error correction system used in the CD audio format has two interleaved layers.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

• A Mode-1 CD-ROM, which has the full third layer error correction capability, contains a net 2048 bytes of the available 2352 per sector.
• In a Mode-2 CD-ROM, which is mostly used for video files, there are 2336 user-available bytes per sector.

The net byte rate of a Mode-1 CD-ROM is 44.1k×2048/(6×98) = 153.6 kbyte/s. The playing time is 74 minutes, or 4440 seconds, so that the net capacity of a Mode-1 CD-ROM is 682 Mbyte.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Subcode

Besides digital audio, a CD contains digital data called "subcode", which is multiplexed with the digital audio.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

• The data in a CD are arranged in frames. A frame comprises 33 bytes, of which 24 are audio bytes (six full stereo samples), eight error correction, CIRC-generated, bytes plus one subcode byte.
• The eight bits of a subcode byte are available for control and display.
• The eight bits are used as eight different subcoding channels, and given letters designating their usage: P, Q, ?, W. Thus each channel has a bit rate of 7.35 (=44.1/6) kbit/s.

In each sector there are 2352 bytes (24×98) of audio content data and 96 bytes of subchannel data.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

The 96 bytes of subchannel information in each sector contain four packets of 24 bytes apiece:

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

1 byte for command,

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

1 byte for instruction,

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

2 bytes for parityQ,

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

16 bytes for data, and

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

4 bytes parityP.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Each of the 96 subchannel data bytes can be thought of as being divided into eight bits. Each of these bits corresponds to a separate stream of information. These streams are called "channels", and are labeled starting with the letter P, like so:

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Channel P is a simple pause/music flag, which can be used for low-cost search systems. Quite a few players ignore it in favor of the Q Channel.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Channel Q is used for control purposes of more sophisticated players. It contains positioning information, the Media Catalog Number (MCN), and International Standard Recording Code (ISRC). The ISRC is used by the media industry, and contains information about the country of origin, the year of publication, owner of the rights, as well as a serial number, and some additional tags:

Related Topics:
Media Catalog Number (MCN) - International Standard Recording Code (ISRC)

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

;Data: This track contains Data (rather than audio). Can be used for muting in audio CD players.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

;Copy Flag: Used by the Serial Copy Management System to indicate permission to digitally copy the track.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

;Four Channel Audio: The track uses four channel audio. This is very rarely used on Compact Discs.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

;Pre-Emphasis: The audio track was recorded with pre-emphasis. Used very rarely on Compact Discs.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Channels R?W are unused by Red-Book compliant CDs, and have been used for extensions to the standard.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

CD-Text

CD-Text is an extension of the Red Book standard for audio CDs. It allows for storage of additional information (e.g. album name, song name, and artist) on a standards-compliant audio CD. The information is stored in the lead-in area of the CD (there is roughly five kilobytes space there), or in the Subchannels R to W on the disc, which are not used on Red-Book compliant CDs. About 31 megabytes of information can be stored there. The text is stored in a format usable by the Interactive Text Transmission System (ITTS). ITTS is also used by Digital Audio Broadcasting or the MiniDisc.

Related Topics:
Red Book - Interactive Text Transmission System - MiniDisc

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Note that other extensions such as CD+G also use those subchannels to store graphics in.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~