After Philips and Sony created the Red Book CD-DA format, they began work on other format standards that would allow CDs to store computer files, data, and even video and photos. These standards control how the data is formatted so that the drive can read it, and additional file format standards can then control how the software and drivers on your PC can be designed to understand and interpret the data properly.

Note that the physical format and storage of data on the disc as defined in the Red Book was adopted by all subsequent CD standards. This refers to the encoding and basic levels of error correction provided by CD-DA discs. The other "books" specify primarily how the 2,352 bytes in each sector are to be handled, what type of data can be stored, how it should be formatted, and more.

Red Book—CD-DA

The Red Book introduced by Philips and Sony in 1980 is the father of all compact-disc specifications because all other "books" or formats are based on the original CD-DA Red Book format. The Red Book specification includes the main parameters, audio specification, disc specification, optical stylus, modulation system, error correction system, and control and display system. The latest revision of the Red Book is dated May 1999.

Yellow Book—CD-ROM

The Yellow Book was first published by Philips, Sony, and Microsoft in 1983 and has been revised and amended several times since. The Yellow Book standard took the physical format of the original CD-DA, or Red Book, standard and added another layer of error detection and correction to enable data to be stored reliably.

It also added additional synchronization and header information to enable sectors to be more accurately located. Yellow Book specifies two types of sectoring—called Mode 1 (with error correction) and Mode 2—which offer different levels of error detection and correction schemes.

Some data (computer files, for example) can't tolerate errors. However, other data, such as a video image or sound, can tolerate minor errors. By using a mode with less error correction information, more data can be stored, but with the possibility of uncorrected errors.

In 1989, the Yellow Book was issued as an international standard by the ISO as ISO/IEC 10149, Data Inter change on Read-Only 120mm Optical Discs (CD-ROM). The latest version of the Yellow Book is dated May 1999.

Green Book—CD-i

The Green Book was published by Philips and Sony in 1986. CD-i is much more than just a disc format; instead it is a complete specification for an entire interactive system consisting of custom hardware (players) designed to be connected to a television, software designed to deliver video and audio together with user interactivity in real time, and the media and format.

A CD-i player is actually a dedicated computer usually running a variant on the Motorola 68000 processor line, as well as a customized version of the Microware OS/9 Real Time Operating System. CD-i enables both audio and video to share a disc and enables the information to be interleaved so as to maintain synchronization between the pictures and sounds.

To fit both audio and video in the same space originally designed for just audio, compression was performed. The video was compressed using the Moving Picture Experts Group-1 (MPEG-1) compression standard, whereas the audio was compressed with adaptive differential pulse code modulation (ADPCM).

ADPCM is an audio encoding algorithm that takes about half the space for the same quality of standard PCM, and even less if quality is reduced by lowering the sampling rate or bits per sample. Using ADPCM, up to 8 hours of stereo or 16 hours of mono sound can fit on one CD.

The "differential" part of ADPCM refers to the fact that it records the differences between one signal and the next (using only 4-bit numbers), which reduces the total amount of data involved. ADPCM audio can be interleaved with video in CD-i (and CD-ROM XA) applications. The Yellow Book defines two CD-ROM sector structures, called Mode 1 and Mode 2.

The Green Book (CD-i) refines the Mode 2 sector definition by adding two forms, called Mode 2, Form 1 and Mode 2, Form 2. The Mode 2, Form 1 sector definition uses ECC and allows for 2,048 bytes of data storage like the Yellow Book Mode 1 sectors, but it rearranged things slightly to use the 8 formerly unused (blank or 0) bytes as a subheader containing additional information about the sector.

The Mode 2, Form 2 definition drops the ECC and allows 2,324 bytes for data. Without the ECC, only video or audio information should be stored in Form 2 sectors because that type of information can tolerate minor errors.

All types of media were produced for CD-i, but because the files use the OS/9 file format, they can't be viewed by a PC without special drivers. One of the best resources for CD-i technical information, utility software, drivers, and emulators is The New International CD-I Association Web site.

Today, the CD-i format is largely obsolete. The last revision of the standard was produced in May 1994. Philips sold off its entire consumer CD-i catalog to Infogrames Multimedia in 1998, which now owns the rights for virtually all consumer CD-i titles ever produced.

Philips made a final run of CD-i players in 1999, and it is doubtful any new ones will ever be produced. The legacy of CD-i lives on in the other formats that use specifications originally devised for CD-i, such as the Mode 2, Form 1 and Form 2 sector structures found in CD-XA and the MPEG-1 video format later used in the White Book (CD-Video).

CD-ROM XA

CD-ROM XA originally was defined in 1989 by Philips, Sony, and Microsoft as a supplement to the Yellow Book. CD-ROM XA brings some of the features originally defined in the Green Book (CD-i) to the Yellow Book (CD-ROM) standard, especially for multimedia use. CD-ROM XA adds three main features to the Yellow Book standard.

The first consists of the CD-i-enhanced sector definitions (called forms) for the Mode 2 sectors; the second is a capability called interleaving (mixing audio and video information); and the third is ADPCM for compressed audio. The latest version of the CD-ROM XA standard was released in May 1991.

Orange Book

The Orange Book defines the standards for recordable CDs and originally was announced in 1989 by Philips and Sony. The Orange Book comes in three parts: Part I describes a format called CD-MO (magneto-optical), which was to be a rewritable format but was withdrawn before any products really came to market; Part II (1989) describes CD-R; and Part III (1996) describes CD-RW. Note that originally CD-R was referred to as CD-WO (write-once), and CD-RW originally was called CD-E (erasable).

The Orange Book Part II CD-R design is known as a WORM (write once read mostly) format. After a portion of a CD-R disc is recorded, it can't be overwritten or reused. Recorded CD-R discs are Red Book and Yellow Book compatible, which means they are readable on conventional CD-DA or CD-ROM drives.

The CD-R definition in the Orange Book Part II is divided into two volumes. Volume 1 defines recording speeds of 1x, 2x, and 4x the standard CD speed; the last revision, dated December 1998, is 3.1. Volume 2 defines recording speeds up to 48x the standard CD speed. The latest version released, 1.2, is dated April 2002.

The Orange Book Part III describes CD-RW. As the name implies, CD-RW enables you to erase and overwrite information in addition to reading and writing. The Orange Book Part III CD-RW definition is broken into three volumes.

Volume 1 defines recording speeds of 1x, 2x, and 4x times the standard CD speed; the latest version, 2.0, is dated August 1998. Volume 2 (high-speed) defines recording speeds from 4x to 10x standard CD speed; the latest version, 1.1, is dated June 2001.

Volume 3 (ultra-speed) defines recording speeds from 8x to 32x; the latest version, 1.0, is dated September 2002. Besides the capability to record on CDs, the most important feature instituted in the Orange Book specification is the capability to perform multisession recording.

Photo CD

First announced back in 1990 but not available until 1992, Photo CD is a standard for CD-R discs and drives to store photos. The current version 1.0 of the Photo CD standard was published in December 1994.

You simply drop off a roll of film at a participating Kodak developer, and they digitize and store the photos on a specially formatted CD-R disc called a Photo CD, which you can then read on virtually any CD-ROM drive connected to a PC running the appropriate software.

Originally, Kodak sold Photo CD "players"designed to display the photos to a connected TV, but these have since been dropped in favor of simply using a PC with software to decode and display the photos.

White Book—Video CD

The White Book was introduced in 1993 by Philips, JVC, Matsushita, and Sony. It is based on the Green Book (CD-i) and CD-ROM XA standards and allows for storing up to 74 minutes of MPEG-1 video and ADPCM digital audio data on a single disc. The latest version (2.0) was released in April 1995.

Video CD 2.0 supports MPEG-1 compression with a 1.15Mbps bit rate. The screen resolution is 352x240 for NTSC format and 352x288 for European PAL format. In addition, it supports Dolby Pro Logic-compatible stereo sound. You can think of video CDs as a sort of poor man's DVD format, although the picture and sound quality can actually be quite good—certainly better than VHS or most other videotape formats.

You can play video CDs on virtually any PC with a CD-ROM drive using the free Windows Media Player (other media player applications can be used as well). They also can be played on most DVD players and even some game consoles, such as the Playstation (with the correct options). Video CDs are an especially big hit with people who travel with laptop computers, and the prerecorded discs are much cheaper than DVD—many cost as little as $5.

Super Video CD

The Super Video CD specification 1.0, published in May 1999, is an enhanced version of the the White Book Video CD specification. It uses MPEG-2 compression, an NTSC screen resolution of 480x480, and a PAL screen resolution of 480x576; it also supports MPEG-2 5.1 surround sound and multiple languages. Most home DVD-creation programs can create Video CDs or Super Video CDs.

Blue Book—CD EXTRA

Manufacturers of CD-DA media were looking for a standard method to combine both music and data on a single CD. The intention was for a user to be able to play only the audio tracks in a standard audio CD player while remaining unaware of the data track.

However, a user with a PC or dedicated combination audio/data player could access both the audio and data tracks on the same disc. The fundamental problem with nonstandard mixed-mode CDs is that if or when an audio player tries to play the data track, the result is static that could conceivably damage speakers and possibly hearing if the volume level has been turned up.

Various manufacturers originally addressed this problem in different ways, resulting in a number of confusing methods for creating these types of discs, some of which still allowed the data tracks to be accidentally "played" on an audio player. In 1995, Philips and Sony developed the CD EXTRA specification, as defined in the Blue Book standard.

CDs conforming to this specification usually are referred to as CD EXTRA (formerly called CD Plus or CD Enhanced Music) discs and use the multisession technology defined in the CD-ROM XA standard to separate the audio and data tracks. These are a form of stamped multisession disc.

The audio portion of the disc can consist of up to 98 standard Red Book audio tracks, whereas the data track typically is composed of XA Mode 2 sectors and can contain video, song lyrics, still images, or other multimedia content. Such discs can be identified by the CD EXTRA logo, which is the standard CD-DA logo with a plus sign to the right.

Often the logo or markings on the disc package are overlooked or somewhat obscure, and you might not know that an audio CD contains this extra data until you play it in a CD-ROM drive. A CD EXTRA disc normally contains two sessions.

Because audio CD players are only single-session capable, they play only the audio session and ignore the additional session containing the data. A CD-ROM drive in a PC, however, can see both sessions on the disc and access both the audio and data tracks.

Purple Book

The Purple Book defines the standards for double-density CD-ROM (DDCD), CD-R (DDCD-R), and CD-RW (DDCD-RW) media and drives. It was announced by Sony and Philips in July 2000, and the current 1.0 standard was released in July 2001.

Purple Book–compliant rewritable drives can read and write standard CD, CD-R, and CD-RW media and achieve their higher 1.3GB (versus 650MB for standard drives) by modifying the following features of existing CD-ROM, CD-R, and CD-RW standards:

• The track pitch has been reduced from 1.6 micrometers to 1.1 micrometers, and the minimum pit length has been reduced from 0.833 micrometers to 0.623 micrometers to enable double-density recording.

• CIRC7, instead of regular CIRC, error correction is used.

• An expanded ATIP address format is used.

DD drives support digital rights management. They are designed to prevent the creation of DD music CDs. Although DDCD drives have twice the capacity of traditional drives, very few of them were sold. Sony offered several models in 2001 but has since discontinued them, although DDCD media is still available.