CD-ROM
A CD-ROM is a type of read-only memory consisting of a pre-pressed optical compact disc that contains data computers can read, but not write or erase. Some CDs, called enhanced CDs, hold both computer data and audio with the latter capable of being played on a CD player, while data is only usable on a computer.
During the 1990s and early 2000s, CD-ROMs were popularly used to distribute software and data for computers and fifth generation video game consoles. DVDs as well as downloading started to replace CD-ROMs in these roles starting in the early 2000s, and the use of CD-ROMs for commercial software is now rare.
History
The earliest theoretical work on optical disc storage was done by independent researchers in the United States including David Paul Gregg and James Russel. In particular, Gregg's patents were used as the basis of the LaserDisc specification that was co-developed between MCA and Philips after MCA purchased Gregg's patents, as well as the company he founded, Gauss Electrophysics. The LaserDisc was the immediate precursor to the CD, with the primary difference being that the LaserDisc encoded information through an analog process whereas the CD used digital encoding.Key work to digitize the optical disc was performed by Toshi Doi and Kees Schouhamer Immink during 1979–1980, who worked on a taskforce for Sony and Philips. The result was the Compact Disc Digital Audio, defined in 1980. The CD-ROM was later designed as an extension of the CD-DA, and adapted this format to hold any form of digital data, with an initial storage capacity of 553 MB. Sony and Philips created the technical standard that defines the format of a CD-ROM in 1983, in what came to be called the Yellow Book. The CD-ROM was announced in 1984 and introduced by Denon and Sony at the first Japanese COMDEX computer show in 1985. The first CD-ROM drive released to the public, the CM 100, was introduced in July 1985 by Philips. In November 1985, several computer industry participants, including Microsoft, Philips, Sony, Apple and Digital Equipment Corporation, met to create a specification to define a file system format for CD-ROMs. The resulting specification, called the High Sierra format, was published in May 1986. It was eventually standardized, with a few changes, as the ISO 9660 standard in 1988. One of the first products to be made available to the public on CD-ROM was the Grolier Academic Encyclopedia, presented at the Microsoft CD-ROM Conference in March 1986.
CD-ROMs began being used in home video game consoles starting with the PC Engine CD-ROM2 in 1988, while CD-ROM drives had also become available for home computers by the end of the 1980s. In 1990, Data East demonstrated an arcade system board that supported CD-ROMs, similar to 1980s LaserDisc video games but with digital data, allowing more flexibility than older LaserDisc games. By early 1990, about 300,000 CD-ROM drives were sold in Japan, while 125,000 CD-ROM discs were being produced monthly in the United States. Some computers that were marketed in the 1990s were called "multimedia" computers because they incorporated a CD-ROM drive, which allowed for the delivery of several hundred megabytes of video, picture, and audio data. The first laptop to have an integrated CD-ROM drive as an option was 1993's CF-V21P by Panasonic; however, the drive only supported mini CDs up to 3.5 inches in diameter. The first notebook to support standard 4.7-inch-diameter discs was IBM's ThinkPad 755CD in 1994.
On early audio CD players that were released prior to the advent of the CD-ROM, the raw binary data of CD-ROM was played back as noise. To address this problem, the subcode channel Q has a "data" flag in areas of the disc that contain computer data rather than playable audio. The data flag instructs CD players to mute the audio.
CD-ROM discs
Media
CD-ROMs are identical in appearance to audio CDs, and data are stored and retrieved in a very similar manner. Discs are made from a 1.2 mm thick disc of polycarbonate plastic, with a thin layer of aluminium to make a reflective surface. The most common size of CD-ROM is 120 mm in diameter, though the smaller Mini CD standard with an 80 mm diameter, as well as shaped compact discs in numerous non-standard sizes and molds, also exist.Data is stored on the disc as a series of microscopic indentations called "pits", with the non-indented spaces between them called "lands". A laser is shone onto the reflective surface of the disc to read the pattern of pits and lands. Because the depth of the pits is approximately one-quarter to one-sixth of the wavelength of the laser light used to read the disc, the reflected beam's phase is shifted in relation to the incoming beam, causing destructive interference and reducing the reflected beam's intensity. This is converted into binary data.
Standard
Several formats are used for data stored on compact discs, known as the Rainbow Books. The Yellow Book, created in 1983, defines the specifications for CD-ROMs, standardized in 1988 as the ISO/IEC 10149 standard and in 1989 as the ECMA-130 standard. The CD-ROM standard builds on top of the original Red Book CD-DA standard for CD audio. Other standards, such as the White Book for Video CDs, further define formats based on the CD-ROM specifications. The Yellow Book itself is not freely available, but the standards with the corresponding content can be downloaded for free from ISO or ECMA.There are several standards that define how to structure data files on a CD-ROM. ISO 9660 defines the standard file system for a CD-ROM. ISO 13490 is an improvement on this standard which adds support for non-sequential write-once and re-writeable discs such as CD-R and CD-RW, as well as multiple sessions. The ISO 13346 standard was designed to address most of the shortcomings of ISO 9660, and a subset of it evolved into the UDF format, which was adopted for DVDs. A bootable CD specification, called El Torito, was issued in January 1995, to make a CD emulate a hard disk or floppy disk.
Manufacture
Pre-pressed CD-ROMs are mass-produced by a process of stamping where a glass master disc is created and used to make "stampers", which are in turn used to manufacture multiple copies of the final disc with the pits already present. Recordable and rewritable discs are manufactured by a different method, whereby the data are recorded on them by a laser changing the properties of a dye or phase transition material in a process that is often referred to as "burning".CD-ROM format
Data stored on CD-ROMs follows the standard CD data encoding techniques described in the Red Book specification. This includes cross-interleaved Reed–Solomon coding, eight-to-fourteen modulation, and the use of pits and lands for coding the bits into the physical surface of the CD.The structures used to group data on a CD-ROM are also derived from the Red Book. Like audio CDs, a CD-ROM sector contains 2,352 bytes of user data, composed of 98 frames, each consisting of 33 bytes. Unlike audio CDs, the data stored in these sectors corresponds to any type of digital data, not audio samples encoded according to the audio CD specification. To structure, address and protect this data, the CD-ROM standard further defines two sector modes, Mode 1 and Mode 2, which describe two different layouts for the data inside a sector. A track inside a CD-ROM only contains sectors in the same mode, but if multiple tracks are present in a CD-ROM, each track can have its sectors in a different mode from the rest of the tracks. They can also coexist with audio CD tracks, which is the case of mixed mode CDs.
Sector structure
Both Mode 1 and 2 sectors use the first 16 bytes for header information, but differ in the remaining 2,336 bytes due to the use of error correction bytes. Unlike an audio CD, a CD-ROM cannot rely on error concealment by interpolation; a higher reliability of the retrieved data is required. To achieve improved error correction and detection, Mode 1, used mostly for digital data, adds a 32-bit cyclic redundancy check code for error detection, and a third layer of Reed–Solomon error correction using a Reed-Solomon Product-like Code. Mode 1 therefore contains 288 bytes per sector for error detection and correction, leaving 2,048 bytes per sector available for data. Mode 2, which is more appropriate for image or video data, contains no additional error detection or correction bytes, having therefore 2,336 available data bytes per sector. Both modes, like audio CDs, still benefit from the lower layers of error correction at the frame level.Before being stored on a disc with the techniques described above, each CD-ROM sector is scrambled to prevent some problematic patterns from showing up. These scrambled sectors then follow the same encoding process described in the Red Book in order to be finally stored on a CD.
The following table shows a comparison of the structure of sectors in CD-DA and CD-ROMs:
The net byte rate of a Mode-1 CD-ROM, based on comparison to CD-DA audio standards, is 44,100 Hz × 16 bits/sample × 2 channels × 2,048 / 2,352 / 8 = 150 KB/s . This value, 150 Kbyte/s, is defined as "1× speed". Therefore, for Mode 1 CD-ROMs, a 1× CD-ROM drive reads 150/2 = 75 consecutive sectors per second.
The playing time of a standard CD is 74 minutes, or 4,440 seconds, contained in 333,000 blocks or sectors. Therefore, the net capacity of a Mode-1 CD-ROM is 650 MB. For 80 minute CDs, the capacity is 703 MB.
CD-ROM XA extension
CD-ROM XA is an extension of the Yellow Book standard for CD-ROMs that combines compressed audio, video and computer data, allowing all to be accessed simultaneously. It was intended as a bridge between CD-ROM and CD-i and was published by Sony and Philips, and backed by Microsoft, in 1991, first announced in September 1988. "XA" stands for eXtended Architecture.CD-ROM XA defines two new sector layouts, called Mode 2 Form 1 and Mode 2 Form 2. XA Mode 2 Form 1 is similar to the Mode 1 structure described above, and can interleave with XA Mode 2 Form 2 sectors; it is used for data. XA Mode 2 Form 2 has 2,324 bytes of user data, and is similar to the standard Mode 2 but with error detection bytes added. It can interleave with XA Mode 2 Form 1 sectors, and it is used for audio/video data. Video CDs, Super Video CDs, Photo CDs, Enhanced Music CDs and CD-i use these sector modes.
The following table shows a comparison of the structure of sectors in CD-ROM XA modes: