History

The CD format was developed as a joint cooperation between Sony (http://www.sony.com) and Philips (http://www.philips.com) in the late seventies, and was released by them as the Red Book standard in the eighties.

It was an instant success. It was introduced in the United States in 1983, and in the same year 30,000 players and 800,000 discs were sold. In 1986, the numbers raised to 3 million players and 53 million discs. In early nineties, players were selling at a rate of 9.2 million units each year, and at the same period of time an average of 288 million discs left store shelves.

Since the format was so amazingly successful, Sony and Philips got together again in 1987 to standardize Video CD (White Book). Later on, they would still cooperate in developing CD-ROM and CD-ROM XA (Yellow Book), Enhanced CD/CD Extra (Blue Book) and CD-R/CD-RW (Orange Book).

The Compact Disc logo was inspired by that of the previous Compact Cassette. It may only be used on discs that comply with the Red Book specifications.

Data structure

CD-R disc, bottom side, with interference colours

Under a microscope, all that is visible is a series of various-sized pits arranged in a long spiral, starting near the inner hole. As bit-times are counted off, a transition (pit-to-land, or land-to-pit) is interpreted as a "1" bit, while a constant region (all-land or all-pit) is interpreted as a "0" bit. Each 14 consecutive bits are grouped and decoded using Eight-to-Fourteen Modulation to get a byte. Each 14-bit EFM word alternates with a 3-bit merging word.

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 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. In total we have 33*(14+3) = 561 channel bits. A 27-bit unique synchronization word is added, so that the number of channel bit in a frame totals 588. The synchronization word cannot occur in the normal bit stream, and can thus be used to identify the beginning of a frame.

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 kB/s. The playing time is 74 minutes, or 4440 seconds, so that the net capacity of a Mode-1 CD-ROM is 682 MB.

A 1x speed CD drive reads 75 consecutive sectors per second.

A CD-ROM (data) sector contains 2352 bytes:

• 12 bytes: sync
• 4 bytes: sector ID
• 2 048 bytes: user data
• 4 bytes: error detection
• 8 bytes: null
• 276 bytes: error correction

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:

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 part of the CD+G extension to 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, where there is roughly five kilobytes of space available, or in the R through W Subchannels on the disc, which are not used by strict Red Book 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).

Note that the CD+G or “karaoke” extension also uses the R-W subchannels or subcodes to store low resolution graphics.

CD-ROM

For its first few years of existence, the compact disc was purely an audio format. However, in Yellow Book CD-ROM standard was established by Sony (http://www.sony.com) and Philips (http://www.philips.com), which defined a non-volatile optical data storage medium using the same physical format as audio compact discs, readable by a computer with a CD-ROM drive.

Recordability

Injection moulding is used to mass produce compact discs. A "stamper" is made from the original media (audio tape, data disc, etc.) by writing to a glass disc (referred to as a glass master) coated with a photosensitive dye with a laser. This dye is then etched, leaving the data track. It is then plated to make a positive version of the CD. Polycarbonate is liquified and injected into the mold cavity where the stamper transfers the pattern of pits and lands to the polycarbonate disc. The disc is then metallized with aluminum and lacquer coated.

Recordable compact discs are injection molded with a "blank" data spiral. A photosensitive dye is then applied, and then the discs are metallized and lacquer coated. The write laser of the CD recorder changes the characteristics of the dye to allow the read laser of a standard CD player to see the data as it would an injection molded compact disc. CD-R recordings are permanent. The resulting discs can be read by most CD-ROM drives and played in most audio CD players.

CD-RW is a re-recordable medium that uses a metallic alloy instead of a dye. The write laser in this case is used to heat and alter the chemical properties of the alloy and hence change its reflectivity. A CD-RW does not have as great a difference in the reflectivity of lands and bumps as a pressed CD or a CD-R, and so many CD audio players cannot read CD-RW discs, although the majority of standalone DVD players can.

See also

• DVD
• DVD-Audio
• SACD

External links

• Licensing the CD audio technology at Philips (http://www.licensing.philips.com/information/cd/audio/)
• The meaning of each CD book colour (http://www.disctronics.co.uk/technology/cdbasics/cd_books.htm)
• History of the CD (http://www.oneoffcd.com/info/historycd.cfm)
• CD-rw.org, a good resource on compact discs (http://www.cd-rw.org/)