In recent years, optical (as opposed to magnetic) disks have become available. They have much higher recording densities than conventional magnetic disks. Optical disks were originally developed for recording television programs, but they can be put to more esthetic use as computer storage devices. Due to their potentially enormous capacity, optical disks have been the subject of a great deal of research and have gone through an extremely rapid evolution. First-generation optical disks were invented by the Dutch electronics conglomerate Philips for holding movies. They were 30 cm across and marketed under the name LaserVision, but they did not catch on, except in Japan.

In 1980, Philips, together with Sony, developed the CD (Compact Disc), which rapidly replaced the 33 1/3-RPM vinyl record for music (except among connoisseurs, who still prefer vinyl). The precise technical details for the CD were published in an official International Standard (IS 10149), popularly called the Red Book, due to the color of its cover. (International Standards are issued by the International Organization for Standardization, which is the international counterpart of national standards groups like ANSI, DIN, etc. Each one has an IS number.) The point of publishing the disk and drive specifications as an International Standard is to allow CDs from different music publishers and players from different electronics manufacturers to work together. All CDs are 120 mm across and 1.2 mm thick, with a 15-mm hole in the middle. The audio CD was the first successful mass market digital storage medium. They are supposed to last 100 years. Please check back in 2080 for an update on how well the first batch did.

A CD is prepared in many steps. The step consists of using a high-power infrared laser to burn 0.8-micron diameter holes in a coated glass master disk. From this master, a mold is made, with bumps where the laser holes were. Into this mold, molten polycarbonate resin is injected to form a CD with the same pattern of holes as the glass master. Then a very thin layer of reflective aluminum is deposited on the polycarbonate, topped by a protective lacquer and finally a label. The depressions in the polycarbonate substrate are called pits; the unburned areas between the pits are called lands.

When played back, a low-power laser diode shines infrared light with a wavelength of 0.78 micron on the pits and lands as they stream by. The laser is on the polycarbonate side, so the pits stick out toward the laser as bumps in the otherwise flat surface. Because the pits have a height of one-quarter the wavelength of the laser light, light reflecting off a pit is half a wavelength out of phase with light reflecting off the surrounding surface. As a result, the two parts interfere destructively and return less light to the player's photodetector than light bouncing off a land. This is how the player tells a pit from a land. Although it might seem simpler to use a pit to record a 0 and a land to record a 1, it is more reliable to use a pit/land or land/pit transition for a 1 and its absence as a 0, so this scheme is used.

The pits and lands are written in a single continuous spiral starting near the hole and working out a distance of 32 mm toward the edge. The spiral makes 22,188 revolutions around the disk (about 600 per mm). If unwound, it would be 5.6 km long. The spiral is shown in Figure 1.

Recording structure of a compact disc or CD-ROM

To make the music play at a uniform rate, it is necessary for the pits and lands to stream by at a constant linear velocity. Consequently, the rotation rate of the CD must be continuously reduced as the reading head moves from the inside of the CD to the outside. At the inside, the rotation rate is 530 RPM to achieve the desired streaming rate of 120 cm/sec; at the outside it has to drop to 200 RPM to give the same linear velocity at the head. A constant linear velocity drive is quite different than a magnetic disk drive, which operates at a constant angular velocity, independent of where the head is currently positioned. Also, 530 RPM is a far cry from the 3600 to 7200 RPM that most magnetic disks whirl at.

In 1984, Philips and Sony realized the potential for using CDs to store computer data, so they published the Yellow Book defining a precise standard for what are now called CD-ROMs (Compact Disc - Read Only Memory). To piggyback on the by-then already substantial audio CD market, CD-ROMs were to be the same physical size as audio CDs, mechanically and optically compatible with them, and produced using the same polycarbonate injection molding machines. The consequences of this decision were not only that slow variable-speed motors were required, but also that the manufacturing cost of a CD-ROM would be well under one dollar in moderate volume.

What the Yellow Book defined was the formatting of the computer data. It also improved the error-correcting abilities of the system, an essential step because although music lovers do not mind losing a bit here and there, computer lovers tend to be Very Picky about that. The basic format of a CD-ROM consists of encoding every byte in a 14-bit symbol, which is enough to Hamming encode an 8-bit byte with 2 bits left over. In fact, a more powerful encoding system is used. The 14-to-8 mapping for reading is done in hardware by table lookup.

At the next level up, a group of 42 consecutive symbols forms a 588-bit frame. Each frame holds 192 data bits (24 bytes). The remaining 396 bits are used for error correction and control. Of these, 252 are the error-correction bits in the 14-bit symbols and 144 are carried in the 8-bit symbol payloads. So far, this scheme is identical for audio CDs and CD-ROMs.

What the Yellow Book adds is the grouping of 98 frames into a CD-ROM sector, as illustrated in Figure 2. Every CD-ROM sector begins with a 16-byte preamble, the first 12 of which are 00FFFFFFFFFFFFFFFFFFFF00 (hexadecimal), to allow the player to recognize the start of a CD-ROM sector. The next 3 bytes contain the sector number, needed because seeking on a CD-ROM with its single data spiral is much more difficult than on a magnetic disk with its uniform concentric tracks. To seek, the software in the drive calculates approximately where to go, moves the head there, and then starts hunting around for a preamble to see how good its guess was. The last byte of the preamble is the mode.

Logical data layout on a CD-ROM
The Yellow Book defines two modes. Mode 1 uses the layout of Figure 2, with a 16-byte preamble, 2048 data bytes, and a 288-byte error-correcting code (a crossinterleaved Reed-Solomon code). Mode 2 combines the data and ECC fields into a 2336-byte data field for those applications that do not need (or cannot afford the time to perform) error correction, such as audio and video. Note that to provide excellent reliability, three separate error-correcting schemes are used: within a symbol, within a frame, and within a CD-ROM sector. Single-bit errors are corrected at the lowest level, short burst errors are corrected at the frame level, and any residual errors are caught at the sector level. The price paid for this reliability is that it takes 98 frames of 588 bits (7203 bytes) to carry a single 2048-byte payload, an efficiency of only 28%.

Single-speed CD-ROM drives operate at 75 sectors/sec, which gives a data rate of 153,600 bytes/sec in mode 1 and 175,200 bytes/sec in mode 2. Double-speed drives are twice as fast, and so on up to the highest speed. Thus a 40x drive can deliver data at a rate of 40 x 153,600 bytes/sec, assuming that the drive interface, bus, and operating system can all handle this data rate. A standard audio CD has room for 74 minutes of music, which, if used for mode 1 data, gives a capacity of 681,984,000 bytes. This figure is generally reported as 650 MB because 1 MB is 220 bytes (1,048,576 bytes), not 1,000,000 bytes.

Note that even a 32x CD-ROM drive (4,915,200 bytes/sec) is no match for a fast SCSI-2 magnetic disk drive at 10 MB/sec, even though many CD-ROM drives use the SCSI interface (IDE CD-ROM drives also exist). When you realize that the seek time is usually several hundred milliseconds, it should be clear that CD-ROM drives are not in the same performance category as magnetic disk drives, despite their large capacity. In 1986, Philips struck again with the Green Book, adding graphics and the ability to interleave audio, video, and data in the same sector, a feature essential for multimedia CD-ROMs.

The last piece of the CD-ROM puzzle is the file system. To make it possible to use the same CD-ROM on different computers, agreement was needed on CD-ROM file systems. To get this agreement, representatives of many computer companies met at Lake Tahoe in the High Sierras on the California-Nevada boundary and devised a file system that they called High Sierra. It later evolved into an International Standard (IS 9660). It has three levels. Level 1 uses file names of up to 8 characters optionally followed by an extension of up to 3 characters (the MS-DOS file naming convention). File names may contain only upper case letters, digits, and the underscore. Directories may be nested up to eight deep, but directory names may not contain extensions. Level 1 requires all files to be contiguous, which is not a problem on a medium written only once. Any CD-ROM conformant to IS 9660 level 1 can be read using MS-DOS, an Apple computer, a UNIX computer, or just about any other computer. CD-ROM publishers regard this property as being a big plus.

IS 9660 level 2 allows names up to 32 characters, and level 3 allows noncontiguous files. The Rock Ridge extensions (whimsically named after the town in the Gene Wilder film Blazing Saddles) allow very long names (for UNIX), UIDs, GIDs, and symbolic links, but CD-ROMs  not conforming to level 1 will not be readable on all computers. CD-ROMs have become very popular for publishing games, movies, encyclopedias, atlases, and reference works of all kinds. Most commercial software now comes on CD-ROMs. Their combination of large capacity and low manufacturing cost makes them well suited to numerous applications.


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