A CD-ROM is a pre-pressed optical compact disc which contains data. The name is an acronym
which stands for "Compact Disc Read-Only Memory". Computers can read CD-ROMs, but cannot write
on the CD-ROMs which are not writable or erasable. Until the mid-2000s, CD-ROMs were popularly
used to distribute software for computers and video game consoles. 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.
The Yellow Book is the technical standard that defines the format of CD-ROMs. One of
a set of color-bound books that contain the technical specifications for all CD formats,
the Yellow Book, created by Sony and Philips in 1988, was the first extension of Compact
Disc Digital Audio. It adapted the format to hold any form of data.
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 numerous non-standard sizes and shapes are also available.
Data is stored on the disc as a series of microscopic indentations. 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 pattern of changing intensity of the reflected beam is converted into binary data.
Standard Several formats are used for data stored on
compact discs, known as the Rainbow Books. The Yellow Book, published in 1988, defines
the specifications for CD-ROMs, standardized in 1989 as the ISO/IEC 10149 / 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 separate 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. The bootable CD specification, to make a CD emulate a hard
disk or floppy disk, is called El Torito. 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 data 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, divided into 98 24-byte
frames. 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. In order 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 as well, which is the case
of mixed mode CDs. 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, and therefore requires a higher reliability of the retrieved data.
In order to achieve improved error correction and detection, a CD-ROM 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, used mostly for digital data,
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 error detection or correction bytes, having therefore 2,336 available data bytes per sector.
Note that 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 = 153.6 kB/s = 150 KiB/s.
This value, 150 KiB/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 682 MB or, equivalently,
650 MiB. For 80 minute CDs, the capacity is 737 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 in 1991. "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:
Disc images When a disc image of a CD-ROM is created,
this can be done in either "raw" mode, or obtaining only the sector's useful data. The
file size of a disc image created in raw mode is always a multiple of 2,352 bytes. Disc
image formats that store raw CD-ROM sectors include CCD/IMG, CUE/BIN, and MDS/MDF. The
size of a disc image created from the data in the sectors will depend on the type of
sectors it is using. For example, if a CD-ROM mode 1 image is created by extracting only
each sector's data, its size will be a multiple of 2,048; this is usually the case for ISO
disc images. On a 74 minute CD-R, it is possible to fit
larger disc images using raw mode, up to 333,000 × 2,352 = 783,216,000 bytes. This is the
upper limit for raw images created on a 74 min or ~650 MiB Red Book CD. The 14.8% increase
is due to the discarding of error correction data.
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".
Capacity
CD-ROM capacities are normally expressed with binary prefixes, subtracting the space used
for error correction data. A standard 120 mm, 700 MB CD-ROM can actually hold about 737 MB
of data with error correction. In comparison, a single-layer DVD-ROM can hold 4.7 GB of
error-protected data, more than 6 CD-ROMs. CD-ROM drives
CD-ROM discs are read using CD-ROM drives. A CD-ROM drive may be connected to the computer
via an IDE, SCSI, SATA, FireWire, or USB interface or a proprietary interface, such as the Panasonic
CD interface. Virtually all modern CD-ROM drives can also play audio CDs when used in
conjunction with the right software. Laser and optics
CD-ROM drives employ a near-infrared 780 nm laser diode. The laser beam is directed onto
the disc via an opto-electronic tracking module, which then detects whether the beam has been
reflected or scattered. Transfer rates
CD-ROM drives are rated with a speed factor relative to music CDs. If a CD-ROM is read
at the same rotational speed as an audio CD, the data transfer rate is 150 KiB/s, commonly
referred to as "1×". At this data rate, the track moves along under the laser spot at
about 1.2 m/s. To maintain this linear velocity as the optical head moves to different positions,
the angular velocity is varied from 500 rpm at the inner edge to 200 rpm at the outer
edge. The 1× speed rating for CD-ROM is different from the 1× speed rating for DVDs.
By increasing the speed at which the disc is spun, data can be transferred at greater
rates. For example, a CD-ROM drive that can read at 8× speed spins the disc at 1600 to
4000 rpm, giving a linear velocity of 9.6 m/s and a transfer rate of 1200 KiB/s. Above
12× speed most drives read at Constant angular velocity so that the motor is not made to
change from one speed to another as the head seeks from place to place on the disc. In
CAV mode the "×" number denotes the transfer rate at the outer edge of the disc, where
it is a maximum. 20× was thought to be the maximum speed due to mechanical constraints
until Samsung Electronics introduced the SCR-3230, a 32x CD-ROM drive which uses a ball bearing
system to balance the spinning disc in the drive to reduce vibration and noise. As of
2004, the fastest transfer rate commonly available is about 52× or 10,400 rpm and 7.62 MiB/s.
Higher spin speeds are limited by the strength of the polycarbonate plastic of which the
discs are made. At 52×, the linear velocity of the outermost part of the disk is around
65 m/s. However, improvements can still be obtained by the use of multiple laser pickups
as demonstrated by the Kenwood TrueX 72× which uses seven laser beams and a rotation
speed of approximately 10×. Faster 12× drives were common beginning in
early 1997. Above 12× speed, there are problems with vibration and heat. CAV drives give speeds
up to 30× at the outer edge of the disc with the same rotational speed as a standard constant
linear velocity 12×, or 32× with a slight increase. However due to the nature of CAV
the actual throughput increase is less than 30/12: in fact, roughly 20× average for a
completely full disc, and even less for a partially filled one.
Problems with vibration, owing to limits on achievable symmetry and strength in mass-produced
media, mean that CD-ROM drive speeds have not massively increased since the late 1990s.
Over 10 years later, commonly available drives vary between 24× and 52×, all using CAV
to achieve their claimed "max" speeds, with 32× through 48× most common. Even so, these
speeds can cause poor reading and even shattering of poorly made or physically damaged media,
with small cracks rapidly growing into catastrophic breakages when centripetally stressed at 10,000–13,000 rpm.
High rotational speeds also produce undesirable noise from disc vibration, rushing air and
the spindle motor itself. Most 21st-century drives allow forced low speed modes for the
sake of safety, accurate reading or silence, and will automatically fall back if a large
number of sequential read errors and retries are encountered.
Other methods of improving read speed were trialled such as using multiple optical beams,
increasing throughput up to 72× with a 10× spin speed, but along with other technologies
like 90~99 minute recordable media and "double density" recorders, their utility was nullified
by the introduction of consumer DVD-ROM drives capable of consistent 36× CD-ROM speeds or
higher. Additionally, with a 700 MB CD-ROM fully readable in under 2½ minutes at 52×
CAV, increases in actual data transfer rate are decreasingly influential on overall effective
drive speed when taken into consideration with other factors such as loading/unloading,
media recognition, spin up/down and random seek times, making for much decreased returns
on development investment. A similar stratification effect has since been seen in DVD development
where maximum speed has stabilised at 16× CAV and capacity at 4.3 and 8.5 GiB, with
higher speed and capacity needs instead being catered to by Blu-ray drives.
CD-Recordable drives are often sold with three different speed ratings, one speed for write-once
operations, one for re-write operations, and one for read-only operations. The speeds are
typically listed in that order; i.e. a 12×32× CD drive can, CPU and media permitting, write
to CD-R discs at 12× speed, write to CD-RW discs at 10× speed, and read from CDs at
32× speed. Copyright issues
Software distributors, and in particular distributors of computer games, often make use of various
copy protection schemes to prevent software running from any media besides the original
CD-ROMs. This differs somewhat from audio CD protection in that it is usually implemented
in both the media and the software itself. The CD-ROM itself may contain "weak" sectors
to make copying the disc more difficult, and additional data that may be difficult or impossible
to copy to a CD-R or disc image, but which the software checks for each time it is run
to ensure an original disc and not an unauthorized copy is present in the computer's CD-ROM drive.
Manufacturers of CD writers are encouraged by the music industry to ensure that every
drive they produce has a unique identifier, which will be encoded by the drive on every
disc that it records: the RID or Recorder Identification Code. This is a counterpart
to the Source Identification Code, an eight character code beginning with "IFPI" that
is usually stamped on discs produced by CD recording plants.
See also CD/DVD authoring
Compact Disc Digital Audio Computer hardware
DVD-Audio DVD-ROM
MultiLevel Recording Optical disc drive
Phase-change Dual Thor-CD
References