Since the introduction of the audio compact disc (CD) in 1982, this digital information carrier has been the nucleus of many developments. Today, the CD is not only used for music, but also for computer data and video. This article discusses the various CD standards that have evolved over the years. These are world standards which have been defined in the Red Book, Yellow Book, Green Book, Orange Book and White Book.
The Compact Disc was born in Eindhoven, The Netherlands, at the Philips Research Laboratories. There, the concept of optical registration of digital data was developed. In 1980, when Philips N.V. and Sony agreed to promote the CD as a carrier of digital audio, it was difficult to foresee the overwhelming effect the silver disc would have on the music industry. In five years’ time, some 30 million CD players were operational, and 450 million CDs sold. The gramophone, Edison’s invention, and the most important sound medium for 80-odd years, was dead and buried at ‘record’ speed.
Since then, there’s no stopping the CD. The shining silver disc is used for an increasing number of applications. The White Book, for instance, published in 1994 by Philips and JVC, gives details of the video CD, a CD variant which looks poised to open the attack on the pre-recorded video tape. In the course of this year, both the computer industry and manufacturers of consumer electronics will introduce various systems which support digital video based on the video CD. This puts the video CD in a position to become accepted just as rapidly as the CD-DA (audio CD).
Today there are so many CD standards that it is difficult to see the wood for the trees.
Red Book, CD-DA
The standard for digital audio, CD-DA, may be found in the Red Book, which contains all technical specifications concerning registration and error correction pertaining to the CD. The Red Book has become the basis for later CD standards and developments, which were once beyond the specs laid down in the Red Book because of high technical demands on the CD.
The information on a CD consists of a spiral-shaped string of holes pressed into a transparent polycarbonate layer. Next, a reflecting aluminium layer is applied which is covered with a transparent protection layer. The space between two holes is called an island. A laser beam scans the spiral from centre of the disc. Because the recording device in the laser drive unit is capable of detecting whether the light is reflected by a hole or an island, it is possible to recover the information pressed on the CD. Variations in the intensity of the reflected light are used to focus the laser via a complex servo system.
The spiral on the CD is divided into three sections: the lead-in, the program (data) and the lead-out. To make the best possible use of the space on the CD, the spiral is scanned at a constant speed. The CD’s rotation speed is therefore dependent on the position of the laser. In the program section, the rotation speed can vary between 197 and 539 revolutions per minute. Because a mechanical speed control alone does not enable a constant data flow, the player has a buffer memory which has to be half filled all the time. If less than 50% of the available space is used, the drive speed is increased. If more than 50% is used, the speed is lowered accordingly. The data read by the laser is clocked out of the buffer with the aid of a quartz-controlled oscillator. The buffer/clock system guarantees a perfectly stable datastream into the DAC which follows the CD drive unit.
All data on an audio CD represents sound signals which have been sampled at a frequency of 44.1 KHz. Each sound channel is linearly digitized at a resolution of 16 bits. The result is a datastream with a speed of 1.4112 Mbit per second.
Yellow Book, CD-ROM
The next standard to appear on the market was defined in the Yellow Book, again produced jointly by Philips and Sony. Once the enormous storage capacity of CDs was fully recognized, the idea of a CD-ROM was born. The CD-ROM is marketed as the replacement for magnetic media such as floppy disks and hard disks. It has a number of advantages and disadvantages. The advantages include low cost, large storage capacity, non-volatile character, and reliability. On the down side, a CD-ROM is a read- only medium.
To maintain compatibility with the CD-DA, the CD-ROM also contains one long spiral. This is in stark contrast with the long-established system of parallel tracks on magnetic media. Because the CD-ROM is also read at constant speed, it has a fairly large random read access time. Any time a file has to be read, the motor speed has to be adjusted until the right reading speed is achieved, and that takes time. Furthermore, the data transfer speed offered by the CD-ROM is relatively low at about 150 KByte per second. Fortunately, double, triple and quadruple speed CD-ROM drives have arrived, and we may see further improvements in the near future.
The information in the Yellow Book does not go much further than a description of the idea to store computer information in the program section. Further, it indicates the types of error correction methods which can be added to the system. The Yellow Book contains neither descriptions of the structure of files and directories as used with computers, nor information on how audio-visual (multi-media) information is to be stored. Consequently, the ISO-9660 and CD-ROM-XA standards were introduced later. The ISO-9660 standard was initially developed by the High Sierra Group, and taken over by the ISO in 1988. It describes the file structure used with IBM-compatible PCs, and has become the basis for later standards. The UNIX platform uses the RRIP (Rock Ridge Interchange Protocol), while Apple Macintosh users have the HFS (Hierarchical File System) for their CD-ROMs.
Green Book, CD-i
In 1986, less than a year after the introduction of the CD-ROM, the CD Interac-tive (CD-i) was announced as a multi-media system for the consumer market. The complete system description may be found in the Green Book. Multi-media systems which process audio-visual information are an interesting application area of the CD-ROM. As already mentioned in the description of the CD-DA, audio information is recorded at a speed of 1.4112 MBit/s. Because video information is far more complex than audio information, the amount of data required for digital video will be even greater. Both with audio and video, there is an interaction between the necessary datastream and the quality of the encoded signal. The CD-i specification describes lots of possibilities for the recording of multi-media information such as sound, still-video, animations and video. Moreover, the information is stored in an interleaved pattern, which means that, for instance, audio and video are interwoven in blocks. That allows the CD-i player to read several types of information simultaneously.
The essential difference between CD-i and CD-ROM is that the specifications of the former include a complete description of the hardware which is necessary to be able to use the CD-i software. The Green Book also provides information on the structure of the data pressed on a CD-i disc. A marked difference with the Yellow Book, which has nothing on data storage formats.
Chapter 9 of the Green Book describes how MPEG1 full-screen video encoded according to ISO 11172 may be added to CD-i information. This system is called ‘digital video on CD-i’ and has been added to the CD-i system as an option.
CD-ROM XA (eXtended Architecture) denotes an extension of the CD-ROM standard drawn up to make the medium better suited to multi-media applications. Broadly speaking, the XA standard enables a number of CD-i compatible audio and video formats to be used with CD-ROM also. Furthermore, it introduces the possibility of different start directories, which enables you to run a CD under different operating systems. Finally, the CD-ROM-XA standard, like the CD-i, is structurally compatible with the ISO-9660 standard. That makes the CD-ROM-XA suitable for many different computer systems. This type of disc is, however, unsuitable for CD-i players because they often lack the appropriate programs. Despite the fact that the XA standard has been with us for some time, little software has become available so far which utilizes this standard.
According to the definitions in the Yellow Book, sectors on a CD have a length of 2,352 bytes, and are subdivided into a number of fields to make them accessible to computers. The start of a sector is indicated by a synchronization pattern of 12 bytes. This is followed by a 4-byte header which contains its absolute address in minutes, seconds and sectors, as well as a mode byte. That leaves 2,336 bytes available for data storage in each sector.
All sectors in one track use the same mode. Sectors exist in three different modes. In Mode-0 sectors, all remaining 2,336 bytes are empty, i.e., zero. Mode-1 sectors contain 2,048 bytes of user data, protected by an EDC (error-detecting code) and an ECC (error- correcting code). This error protection is additional to the protection already used for CDs. Although the standard error correction for CDs is application independent, it is not powerful enough for computer applications. This is because small data errors are unacceptable in computer systems, although they may go by unnoticed in audio systems.
In Mode-2, which is the format used for normal CD-ROMs that do not require extra error correction (for instance, digitized audio/video), the remaining 2,336 bytes are available to hold data. The structure of Mode-2 sectors is further detailed in the Green Book to ensure compatibility with the ISO-9660 standard. The extension with Mode-2 is used with CD-i and CD-ROM-XA. It involves the storage of data in sectors by assigning a sub- header to each sector. This sub-header contains a file and channel number coding information for details on the type of data contained in the sector, as well as a sub-mode byte. The file number is used to distinguish between sectors of different, interleaved, files. The channel number is used to support the different channels a file may be composed of. The sub-mode byte contains the end-of-file (EOF) and end-of-record (EOR) bits, an interrupt trigger bit to enable synchronization, and a real-time bit which indicates that the file is used in real time mode.
Finally, three bits follow which indicate the type of data (video, audio, computer, etc.) in the sector. This extension has two options: mode-2/form-1 and mode-2/form-2, where the form bit indicates whether or not the additional error correction used in Mode-1 sectors is employed. Mode-2/form-1 sectors do have this extra error correction (EDC and ECC), while mode-2/form-2 sectors have an EDC block only. The absence of extra error correction is justified if the CD contains audio or video information. With these file types, the absence of the real-time bits (even occasionally) is much more important than small data errors. Because the extra error correction is not used, 280 bytes are available for data storage. With a real-time file stored in a mode-2/form-1 sector, the error detection and correction operations have to be performed in real time also, which makes high demands on the relevant hardware.
The eight space bytes in a Mode-1 sector are usually zero, and used for the header in Mode-2. Consequently, the locations for the data and the EDC fields are moved by eight bytes. A mode-2/form-2 sector is closed off by a 4-byte EDC field only, which may contain parity bits which serve as quality indicators in the CD production process. These bytes may also be made zero. If they are used, it is recommended to employ the same EDC as with mode-2/form-1 sectors.
Orange Book, CD-MO and CD-WO
The Compact Disc Magneto-Optical (CD-MO) and Compact Disc Write Once (CD-WO) are specified in the Orange Book. This extension has given the CD an even wider application area, because it specifies how CDs may be produced in small quantities. The CD-MO enables the information on a CD to be re-written several times. By contrast, the CD-WO system allows data to be written once only. The CD-MO has a magneto- optical layer in which the information is stored in a completely different manner than with a normal CD. The read section of the laser recognizes a CD-MO disc from a change in the polarization direction of the laser light. A special drive unit is required for the reading and writing of this type of CD.
A CD-WO, a.k.a. CD Recordable (CD-R) has three status levels: it is either blank, partly written on, or completely written on. Like the CD-MO, the CD-WO contains a special pre- recorded track with information required for tracking and timing. The CD-WO contains a light absorbing layer of which the reflection characteristics are modified with a special laser during recording. A CD-WO enables CD’s to be ‘burnt’ which are compatible with the Red, Yellow and Green Book. These discs can be played on any conventional CD player or CD-ROM drive.
The CD-WO standard also supports the use of multiple partitions on a single CD (‘multi- session’). Each of these partitions has its own lead-in, program and lead-out sections, and is compatible with the standards in the Red, Green or Yellow Book. Such a multi- session CD can only be read by a special multi-session drive. An ordinary CD-ROM drive can only read the data in the first session.
The CD-Bridge standard was developed to bridge the CD-i and CD-ROM standards. It is a very open standard with plenty of room for the implementation of the various system specifications. A CD-Bridge disc is a CD-ROM-XA disc which also contains a CD-i program. Conse-quently, this type of disc may be used in a computer as well as in a CD- ROM-XA drive. The popular photo-CD and Video-CD are examples of CD-Bridge discs.
The Photo-CD is a CD-Bridge disc of which the standard was designed by Philips and Kodak. This type of CD is intended for the storage of photographs which have been digitized at a very high solution, and so allows you to view photographs on your TV or computer screen. The relevant file may be processed further on the PC, or printed on paper.
A Photo-CD may be an ordinary CD, produced with conventional means, or a CD-WO. The latter format allows photos to be added to the CD during further sessions. Obviously, pre-recorded Photo-CDs have one session only, and can be read by all types of CD- ROM.
The standard version of the Photo-CD is based on 35-mm photographic negatives which are digitized at a resolution of 3782 x 2048 pixels. The file recorded on the CD consists of five sub-files: Base/16, Base/4, Base, 4xBase and 16xBase. Thanks to data compression techniques, an ordinary CD can hold more than 100 digitized 35-mm photographs. These days it is also possible to put larger film formats onto a Photo-CD. This is particularly interesting for professional users of photographic material.
White Book, Video-CD
The last CD-Bridge variant discussed in this article is the Video-CD. This was developed for the storage of full-motion, full-screen, video, as already described in Chapter 9 of the Green Book (CD-i). The pictures on the CD are encoded to the MPEG-1 standard. A normal 12-cm diameter CD has storage capacity for 74 minutes of digital video. The technical specifications of the Video-CD are described in the White Book. Initially, this version was also known as the Karaoke-CD standard.
To further the acceptance of this medium, the standard explicitly describes the possibility of adding full-motion video decoder to a conventional audio CD player. As a matter of course, that is only possible if the player is modified accordingly.
CD-Video, not to be confused with Video-CD, is a CD format which has been in use for some time to record five to six minutes of analogue video on a CD. This format is described as an extension in the Red Book. In addition to the video information, the CD has spare capacity for about 20 minutes of digital audio. After a few false starts, this format soon went into oblivion. Today, it is hardly produced any more.
The CD Background Music (CD-BGM) format was developed for background music (‘Muzak’) systems, and is based on technologies derived from the CD-i standard. By virtue of the ADPCM encoding used, a single CD can hold up to 20 hours of background music at reasonable quality.
CD+MIDI and CD+G were also developed on the basis of the audio CD. These formats enable graphics and MIDI data to be put on a CD, together with digital audio.
The last format to be mentioned is called CD-i Ready. With these audio CDs, an amount of CD-i software is stored in the background. When the CD is played on an audio CD player, this extra functionality is not noticed because the disc behaves just like any other music CD. The CD-i software comes to life once the disc is inserted in a CD-I player. Additional functions which are then available include song texts, photographs, discographies, etc. To make sure that the CD-i information does not interfere with the music reproduction, the CD-i player first reads its own information, and stores it in a buffer. Next, the associated audio track is played.
A plethora of CD standards is currently in use, and the overall situation is complex enough to look pretty bewildering at first glance. As a general tip, formulate your requirements before you go out and purchase a CD player or a CD-ROM drive. Use the information in Table 2 to check for yourself which standards are actually required for your particular applications, and then select a player which supports at least those standards.