10.1 Compact Disc Fundamentals
Unlike magnetic storage devices, which store data on multiple concentric tracks, all CD formats store data on one physical track, which spirals continuously from the center to the outer edge of the recording area. All CD formats use 3,234-byte physical sectors, which allocate 882 bytes to control and error correction data, leaving 2,352 bytes available. Different CD formats use this space differently: audio CDs use the entire 2,352 bytes to store audio data; computer CDs use only 2,048 bytes to store user data, and allocate the remaining 304 bytes to store additional ECC and control data, including header data and synchronization data. (Audio CDs are addressable to within one second; computer CDs must be addressable by sector, or 1/75 second.) Sectors are grouped as logical numbered tracks, which are listed in the Table of Contents (TOC) for the disc, a special unnumbered track that is analogous to the File Allocation Table and root directory on a computer disk.
All current CD formats derive from the original CD-DA (Compact Disc - Digital Audio) format introduced in 1974 as a replacement for vinyl record albums. The following standards define the formats used for compact discs:
- Red Book
The original CD standard that defines Compact Disc - Digital Audio (CD-DA, the audio CD), a method that allows digital recording of 74 minutes of audio separated into tracks. Red Book also defines CD infrastructure, including disc dimensions, optical stylus, modulation and error correction standards, sub-code channels used for control and display, and the 16-bit Pulse Coded Modulation method used to store audio data. Red Book allows a CD to contain up to 99 tracks, each containing a single audio selection. Each sector contains 2,352 bytes of audio data, two 392-byte EDC/ECC layers, and 98 bytes of control data, which is divided into sub-codes (or sub-channels) identified as P through W. Control data allows jumping to the beginning of each track, and stores such information as track number, track time, and total time. All computer CD drives support the Red Book standard.
- Yellow Book
Contains extensions to Red Book that define the Compact Disc - Read Only Memory (CD-ROM) standard, which allows CDs to store digital computer data. Yellow Book defines two sector structures for user data and the error detection codes (EDC) and error correction codes (ECC) used to ensure data integrity. Mode 1 is the common CD-ROM format, and segments the 2,352 available bytes as 12 bytes sync, 4 bytes header, 2,048 bytes user data, 4 bytes EDC, 8 bytes blank, and 276 bytes ECC. Mode 2, which is never used, segments the 2,352 bytes as 12 bytes sync, 4 bytes header, and 2,336 bytes user data. All computer CD drives support the Yellow Book standard.
- CD-ROM XA
The original Yellow Book standard defined a means to store computer data, but made no provision for audio or video data. CD-ROM XA (Extended Architecture) extended Yellow Book with two new track types that allow a CD to store compressed audio and/or video data mixed with computer data. Mode 2, Form 1 is used for computer data, and segments the available 2,352 bytes as 12 bytes sync, 4 bytes header, 8 bytes subheader, 2,048 bytes user data, 4 bytes EDC, and 276 bytes ECC. Mode 2, Form 2 is used to store audio/video data, and uses 12 bytes sync, 4 bytes header, 2,324 bytes user data, and 4 bytes EDC. The subheader field describes sector contents, allowing Form 1 (data) sectors and Form 2 (audio/video) sectors to be interleaved within one track. CD-ROM XA compliant drives can separate Form 1 computer data from Form 2 audio/video on the fly, delivering each to the appropriate destination for processing. The only CDs you are likely to find using CD-ROM XA formats are Kodak PhotoCD and VideoCD (both CD-i Bridge formats), the Karaoke CD, and the Sony PlayStation CD.
Yellow Book defined the physical sector structure, but did not define logical file formats. This meant that early Yellow Book data CDs by necessity used proprietary file formats that were incompatible with each other. To address this problem, CD producers created the ad hoc High Sierra format, which was subsequently formalized almost without change by the ISO as ISO-9660. The strength of ISO-9660 was that it was universal ISO-9660 discs are readable by nearly any operating system. The other side of that coin was that ISO-9660 achieved this universality by restricting choices to the least common denominator e.g., filenames limited to 8.3, which was supported by all operating systems. The constraints imposed by ISO-9660 mean that it is seldom used any more except where universal compatibility is more important than filesystem features such things as huge tables of government data and other boring stuff. ISO-9660 is also still used occasionally to produce hybrid discs that are readable by both PCs and Macs. These discs use ISO-9660 formatting for the PC data and Mac HFS formatting for the Mac. But if the ISO-9660 format defined by CD-ROM XA is seldom used any more, it was at least a start.
- Green Book
An extension of Yellow Book that defines Compact Disc Interactive (CD-i). CD-i supports Mode 2, Form 2 audio, video, and picture data mixed with Mode 2, Form 1 computer data, which users can control interactively. CD-i CDs required a special CD-i player, which contained an embedded computer running a special operating system (OS/9, CD-RTOS), so CD-i tracks could not be played on normal CD-ROM drives. A hybrid format called CD Bridge defines a method for recording CD-i data on CD-ROM XA discs, allowing that data to be read by any CD-ROM XA drive. The only CD Bridge format still in common use is Kodak PhotoCD. The CD-i format achieved some popularity on dedicated CD-i players in the early to mid-90s for games, educational programs, encyclopdiae, and so on, but is now obsolete and has been replaced by various flavors of DVD.
- Orange Book
Defines standards for recordable CDs. Part I defines Compact Disc - Magneto-Optical (CD-MO); Part II, Compact Disc - Write Once (CD-WO, usually called CD-Recordable or CD-R); and Part III, Compact Disc - Erasable (CD-E, usually called CD-Rewritable or CD-RW).
Orange Book defines both single-session ( Disc-At-Once, DAO) recording and incremental multisession ( Track-At-Once, TAO) recording. Multisession allows recording an initial session that does not fill the disc, and subsequently adding one or more additional sessions until the capacity of the disc has been reached. Each new session contains a Table of Contents that lists both the old and new information on the disc, so any CD-ROM drive or CD player used to read multisession discs must be capable of locating and using the last-recorded Table of Contents. Any recent CD-ROM drive and most recent CD players can read multisession discs, but older drives and players usually cannot. Unless, that is, you use your CD burner to "finalize" the session, which closes the disc to further recording sessions and writes a final Table of Contents that can be read by any CD drive or player. Even then, very old players may not be able to read the disc, because their lasers and data pickups are incapable of dealing with the color and low reflectivity and contrast of recordable media.
- White Book
Defines the Video CD format, also known as Digital Video (DV), developed and promoted by Matsushita, JVC, Sony, and Philips. Video CDs are a type of CD-ROM XA bridge disc based on the Karaoke CD standard. They use MPEG-1 compression to store up to 70 minutes of full-screen, full-motion video with CD-quality audio, using CD-ROM XA Mode 2 Form 2. They can be played on a dedicated Video CD player, a CD-i player with a Digital Video cartridge, or a PC with a CD-ROM XA drive, an MPEG-1 decoder, and the necessary software. White Book is now obsolete, and has been replaced by DVD.
- Blue Book
Defines the Enhanced Music CD, also called CD-Extra or CD-Plus, which specifies a multisession format that stores mixed audio and data recorded as separate sessions to prevent standard CD players from attempting to "play" a data session. For compatibility with standard CD players, a Blue Book CD contains two sessions. The first session contains the audio tracks, and the second session a data track. A Blue Book CD stores a limited amount of data that is related to the audio, which comprises the major portion of the content on the CD. For example, a Blue Book CD of Johann Sebastian Bach's Brandenburg Concertos might include historical background and the score, while a rock CD might include album notes and lyrics. Blue Book CDs can be played on any standard audio CD player (which sees a Blue Book CD as a standard CD-DA disc), on PCs with compliant CD-ROM drives, and on dedicated players.