The CD-ROM (Compact Disc Read Only Memory) wasn't developed specifically for the PC. It was designed initially for use as an audio storage device to replace the cassette tape. It hasn't been totally successful in that mission cassettes are still around but it has gained acceptance and proven to be very popular. However, the CD-ROM did discover a ready and willing market of personal computer users. The CD-ROM and its comparatively huge storage capacity (over floppy disks) was very attractive to software and multimedia producers and soon virtually all software, including databases, books, encyclopedias, and other materials not available to the PC in the past suddenly became very available and accessible for the PC.
Because the majority of software titles are available only on CD-ROM, today's PCs have a CD-ROM drive. A CD-ROM drive is as common on PCs today as floppy drives were only a few years ago. In fact, some manufacturers now replace the floppy disk drive with a single CD-ROM on their latest PC models. The CD-ROM is by far the most common method of software distribution and data storage due to their combination of high capacity and easy, inexpensive manufacturing.
When CD-ROMs were first introduced to the market, most software distributors included floppy disks along with the CD-ROM version of the program or provided a coupon that could be mailed in for a CD-ROM version of the software. In the past few years, this was reversed, and the coupon became the means of getting a diskette version of a software package. Today, the coupon has disappeared altogether and the CD-ROM is now the only option available. A PC without a CD-ROM drive simply is not able to install the vast majority of PC software available on the market today. Some CD-ROM software even requires that your PC's drive meet a specific minimum requirement. For example, if you have an older CD-ROM drive, such as a 4X, in your system, it may need to be replaced before it can run some newer CD-ROM titles that require at least a 12X drive.
THE TECHNOLOGY OF THE CD AND CD-ROM
The CD-ROM uses compact disc (CD) technology, the same technology used to record the music on your favorite audio CDs. The physical media used for recording data, programs, music, and multimedia on a CD-ROM is the same as that used to record music. In fact, the physical disc (see Figure 10 1) is the same for both.
CD-ROM Formats
There are a number of different formats and applications of the CD technology, not all of which are for the computer. The two that most people are familiar with are the formats used for music CDs and data CD-ROMs, but there are a few others. The format of the CD is the pattern and method used to record its contents. The CD is often compared to the old vinyl record because they are produced in a similar fashion and their contents is recorded in a spiraling pattern, as opposed to data arranged in tracks, as on a music cassette or a disk drive.
CD Digital Audio (CD DA)
The first standard CD format was the one used to produce audio CDs that could play in all regular CD players: CD Digital Audio, or CD DA. The CD DA standard was defined in what is called the Red Book, a specification developed by the two originators of the CD technology, the Royal Philips Electronics Company and the Sony Corporation. The Red Book standard, issued in 1980, defines the technical specification for CD DA (audio CD), including sampling and transfer rates, the data format for the digital audio, and the physical specifications for compact discs, including the media's size and the spacing of tracks. The Red Book defined the standard for the structure of the media and how a CD is read that is still used today.
The technical details of the Red Book standard include:
16 bit sample.
Sampling is at 44.1kHz (kilohertz), which is about twice the highest frequency that humans can hear.
Sampling is done in stereo.
Each one second of sound stored on the CD requires 176,400 bytes.
Compact Disc Read Only Memory (CD-ROM)
The large capacity of the CD was attractive to nonmusic producers as well, including software publishers, database producers, and multimedia developers. The CD-ROM holds about 640 million bytes of data. The CD-ROM technology had approximately the same speed as the CD DA, which was designated as IX (one times) the relative speed of a music CD at about 150KB per second.
In order to store data, the CD DA standard had to be modified. In 1984, Philips and Sony issued the Yellow Book standard that defined the CD-ROM for storing computer data. The Yellow Book defined two new kinds of content sectors: Mode 1 and Mode 2. Mode 1 sectors store computer data and Mode 2 sectors are used to store compressed audio or video and graphic data. This new standard recognized the need for the CD-ROM to store data more precisely than the audio CD.
Audio CD (CD DA) has 99 accessible tracks on which music is stored. The Yellow Book defined the CD-ROM with what amounted to a file system. Both Mode 1 and Mode 2 sector formats have a few bytes at the front of each sector. Table 10 1 lists the contents of a Mode 1 sector, showing the space used for the header and error detection and correction.
The size of CD DA and CD-ROM Mode 1 and Mode 2 sectors are the same, but the amount of user data varies because of sync bytes, header bytes, and error correction and detection. The CD DA format uses all 2,352 bytes of a sector for user data (music). CD-ROM Mode 1 blocks have 2,048 of user data and Mode 2 blocks provide 2,336 user bytes. Because of the amount of data they transfer, the two modes have different transfer speeds (about 1.22Mbps for Mode 1 and 1.4Mbps for Mode 2).
In the Mode 1 sector, the first 12 bytes of the header are sync bytes that are used for sector separation. The sync bytes at the beginning of a sector are intended to identify the sector mode, but since the value of the sync bytes could coincidentally appear in the user bytes, the length of the sector is also used to identify the mode type. The next four bytes are the header bytes, three of which are used for addressing. The fourth byte indicates the mode used to record the contents of the sector. The address stored in the header bytes contains the length of any blocks in the sector in minutes and seconds, plus other identifying information.
The header byte mode indicator contains the CIRC (Cross Interleaved Reed Solomon Code), which is the standard error detection and correction method used by CD DA and
CD-ROM formats. On CD-ROM Mode 1 discs, the CIRC method used is called Layered EDC/ECC (error detection code/error correcting code), which determines if an error has occurred in a data block and corrects it. This error detection and correction method requires the use of additional bytes at the end of the sector. EDC uses 4 bytes, ECC uses 276 bytes, and between them are 8 bytes of unused space.
A Mode 1 sector provides 2,048 bytes of user data. This area can be divided into blocks of 512, 1024, and 2048 bytes each, but a CD-ROM typically has the same block length throughout. A block cannot be bigger than a sector, which is also the smallest addressable unit on the CD-ROM. CD-ROM Mode 2 sectors do not use additional error detection and correction, which leaves the bytes behind the sync and header bytes (2,336 bytes) as user bytes.
CD-ROM Extended Architecture (CD-ROM XA)
The Red Book CD DA and the Yellow Book CD-ROM formats soon proved too restricting to producers, so Philips, Sony, and the Microsoft Corporation combined to develop the CD-ROM Extended Architecture, or CD-ROM XA format. The CD-ROM XA format is an extension of the Yellow Book format standard.
CD-ROM XA discs can mix CD-ROM Mode 1 and Mode 2 formats to store computer data, compressed audio, graphics, and video content. CD-ROM XA does not use additional EDC/ECC capabilities, so the user gains the 288 bytes used in CD-ROM Mode 1 formats for this purpose. This format interleaves, meaning that it mixes different types of data together in different mode formats on the same CD, allowing music, data, programming, and graphics to share a single CD.
CD-ROM XA discs require a drive certified for the CD-ROM XA format. Because they usually contain compressed audio and video, these devices include hardware decoders that decompress the data as it is read.
CD-Interactive (CD 1)
In 1986, the demand and rapid growth of multimedia were the catalysts leading to the creation of the CD Interactive or CD I format. CD I discs contained text, graphics, audio, and video on a single disc format. Special hardware was used to connect CD I players to television screens for output. CD 1, like the CD-ROM XA, is a derivative of the Yellow Book, but the CD I used a proprietary and unique formatting.
Bridge CD
The term bridge CD refers to discs that support extensions of the CD-ROM XA format, defined in what is known as the White Book. These discs are called bridge CDs because they bridge the CD-ROM XA and the CD I formats and can be used for either. Using the White Book specification, CD I discs will work in CD-ROM XA drives, and CD-ROM XA discs will work in CD I drives. Examples of a bridge CD is the Kodak Photo CD and the Video CD format.
Video CD (VCD)
The video CD (VCD) is used to store compressed video information using a standard also
defined in the White Book. VCDs use MPEG (Motion Picture Experts Group) compression
to store 74 minutes of full motion video in the same space used by CD DA audio. To play
a video CD requires a CD-ROM drive or video CD player that is video CD compatible.
The compression algorithm used for VCD does not produce a high quality video; this
format will likely give way to the DVD.
Photo CD
The Photo CD standard, another standard developed by Philips this time with Kodakis adapted from the CD-ROM XA standard to hold photographs in digital form. This standard is defined in the Orange Book that also defined the CD Recordable. A photo CD uses CD-ROM Mode 2 formatting to store photographic images. Normal camera film is first developed into photo prints, which are then scanned and converted into digital images. The digitized photographs are then converted to photo CD formatting and written to the CD, using essentially the CD R process (this is covered later in the chapter). A photo CD is a type of bridge CD, which means a CD I player can read it.
CD PROM
A CD PROM (Compact Disc Programmable Read Only Memory) is a combination of the manufactured CD-ROM and the CD R disc developed by Kodak. Part of the disc can contain mastered data and another part of the disc can be recorded in a CD R drive.
CD Recordable
Each of the CD types covered to this point have been CD-ROMs, or read only discs, which means that except for during their manufacturing processes, data cannot be stored to them and they cannot be modified, other than to be destroyed. To take advantage of its large storage space, methods have been developed, along with special CD media that allow data to be written to a CD.
A CD Recordable (CD R) disc is manufactured essentially the same as a CD-ROM disc, with some slight variations. In place of the substrate is a layer of organic dye, over which is placed a reflective gold colored metallic coating. Over this is the protective lacquer layer, just like on a CD-ROM.
Two general types of CDs that can be modified in a special CD R are:
WORM (Write Once/Read Many) A special CD disc type to which data or music can be written to one time in a CD R drive, after which the data is permanent and cannot be changed.
Magneto Optical (MO) discs These versatile discs can be written to, read, and then modified. These are also referred to as CD RW (read, write) discs.
