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CD-ROM (an initialism of "Compact Disc Read-Only Memory") is a
pre-pressed Compact Disc that contains data accessible to, but not writable
by, a computer. While the Compact Disc format was originally designed for
music storage and playback, the 1985 "Yellow Book" standard developed by Sony
and Philips adapted the format to hold any form of binary data.
CD-ROMs are popularly used to distribute computer software, including
games and multimedia applications, though any data can be stored (up to the
capacity limit of a disc). Some CDs hold both computer data and audio with
the latter capable of being played on a CD player, whilst data (such as
software or digital video) is only usable on a computer (such as PC CD-ROMs).
These are called Enhanced CDs.
Although many people use lowercase letters in this acronym, proper
presentation is in all capital letters with a hyphen between CD and ROM. It
was also suggested by some, especially soon after the technology was first
released, that CD-ROM was an acronym for "Compact Disc read-only-media", or
that it was a more "correct" definition. This wasn't the intention of the
original team who developed the CD-ROM, and common acceptance of the "memory"
definition is now almost universal. This is probably in no small part due to
the widespread use of other "ROM" acronyms such as Flash-ROMs and EEPROMs
where "memory" is usually the correct term.
Contents
1 Media
1.1 Standard
1.2 CD-ROM format
1.2.1 CD sector contents
1.3 Manufacture
1.4 Capacity
2 CD-ROM drives
2.1 Laser and Optics
2.2 Transfer rates
3 Copyright issues
Media
CD-ROM discs are identical in appearance to audio CDs, and data are
stored and retrieved in a very similar manner (only differing from audio CDs
in the standards used to store the data). 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 this disc 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 (e.g. business card-sized media) are
also available. Data is stored on the disc as a series of microscopic
indentations. A laser is shown onto the reflective surface of the disc to
read the pattern of pits and lands ("pits", with the gaps between them
referred to as "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
There are several formats used for data stored on compact discs,
known collectively as the Rainbow Books, or CD's. These include the original
Red Book standards for CD audio, White Book and Yellow Book CD-ROM. The
ECMA-130 standard, which gives a thorough description of the physics and
physical layer of the CD-ROM, inclusive of Cross-interleaved Reed-Solomon
coding CIRC and Eight-to-Fourteen Modulation.
ISO 9660 defines the standard file system of a CD-ROM, although it is
due to be replaced by ISO 13490. UDF format is used on user-writeable CD-R
and CD-RW discs that are intended to be extended or overwritten. The bootable
CD specification, to make a CD emulate a hard disk or floppy, is called El
Torito. Apparently named this because its design originated in an El Torito
restaurant in Irvine, California.
CD-ROM format
A CD-ROM sector contains 2352 bytes, divided into 98 24-byte frames.
The CD-ROM is, in essence, a data disk, which can't rely on error concealment
or interpolation, and therefore requires a higher reliability of the
retrieved data. In order to achieve improved error correction and detection,
a CD-ROM has a third layer of Reed-Solomon error correction. A Mode-1 CD-ROM,
which has the full three layers of error correction data, contains a net 2048
bytes of the available 2352 per sector. In a Mode-2 CD-ROM, which is mostly
used for video files, there are 2336 user-available bytes per sector. The net
byte rate of a Mode-1 CD-ROM, based on comparison to CDDA audio standards, is
44.1k/s * 4B * 2048/2352 = 153.6 kB/s. The playing time is 74 minutes, or
4440 seconds, so that the net capacity of a Mode-1 CD-ROM is 682 MB.
A 1x speed CD drive reads 75 consecutive sectors per second.
CD sector contents
A standard 74 min CD contains 333,000 blocks or sectors. Each sector
is 2352 bytes, and contains 2048 bytes of PC (MODE1) Data, 2336 bytes of
PSX/VCD (MODE2) Data, or 2352 bytes of AUDIO.
The difference between sector size and data content are the Headers
info and the Error Correction Codes, that are big for Data (high precision
required), small for VCD (standard for video) and none for audio.
If extracting the disc in RAW format (standard for creating images)
always extract 2352 bytes per sector, not 2048/2336/2352 bytes depending on
data type (basically, extracting the whole sector). This fact has two main
consequences:
a) Recording data CDs at very high speed (40x) can be done without losing
information. However, as audio CDs don't contain a third layer of error
correction codes, recording these at high speed may result in more
unrecoverable errors or 'clicks' in the audio.
b) On a 74 minute CD, one can fit larger images using RAW mode, up to 333,000
* 2352 = 783,216,000 bytes (747~ MB). This is the upper limit for RAW
images created on a 74 min or 650~ MB Red Book CD. The 14.8% increase is
due to the discarding of error correction data.
The sync pattern for Mode 1 CDs is 0xff00ffffffffffffffff00ff. Please
note that an image size is always a multiple of 2352 bytes (the size of a
block) when extracting in RAW mode.
Layout Type ? 2,352 bytes block ?
CD Digital Audio: 2,352 bytes of Digital Audio
CD-ROM (MODE1): 12 4 2,048 bytes of user data 4 8 276
CD-ROM (MODE2): 12 4 2,336 bytes of user data
Legend (bytes)
12 sync
4 sector ID
data
4 error detection
8 blank/null
276 error correction
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 (CD-R) and rewritable (CD-RW) discs are manufactured by a
similar method, but the data are recorded on them by a laser changing the
properties of a dye or phase change material in a process that is often
referred to as "burning".
Capacity
The CD-ROM can easily contain all the encyclopaedia's words and
images, plus audio & video clips. 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 hold about 847 MB of data, or 737 MB
(703 MiB) with error correction. In comparison, a single-layer DVD-ROM can
hold 4.7 GB of error-protected data, more than 6 CD-ROMs.
Capacities of Compact Disc types
Type Sectors Data max size Audio max size Time
(MB) (MB) (min)
8 cm 94,500 193.536 222.264 21
283,500 580.608 666.792 63
650 MB 333,000 681.984 783.216 74
700 MB 360,000 737.280 846.720 80
800 MB 405,000 829.440 952.560 90
900 MB 445,500 912.384 1,047.816 99
Note: Megabyte (MB) and minute (min) values are exact.
CD-ROM drives
Old 4x CD-ROM DriveCD-ROM discs are read using CD-ROM drives, which
are now almost universal on personal computers. A CD-ROM drive may be
connected to the computer via an IDE (ATA), SCSI, S-ATA, Firewire, or USB
interface or a proprietary interface, such as the Panasonic CD interface.
Virtually all modern CD-ROM drives can also play audio as well as Video CDs
and other data standards 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
The rate at which CD-ROM drives can transfer data from the disc is
gauged by a speed factor relative to music CDs: 1x or 1-speed which gives a
data transfer rate of 150 kilobytes per second in the most common data format.
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 8x speed spins
the disc at up to 4000 rpm (compared to the 500 rpm maximum for 1x speed),
giving a transfer rate of 1.2 megabytes per second. Above 12x speed,
vibration and heat can become a problem. CD-ROM drives above this speed
tackle the problem in several ways. Constant angular velocity (CAV) drives
spin the disc at a constant rate, leading to faster data transfer when
reading from the outer parts of the disc, but slower towards the centre. 20x
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 52x or 10,350 rpm and 7.62 megabytes per second, though this is only
when reading information from the outer parts of a disc. Future speed
increases based simply upon spinning the disc faster are particularly limited
by the strength of polycarbonate plastic used in CD manufacturing, though
improvements can still be obtained by the use of multiple laser pickups as
demonstrated by the Kenwood TrueX 72x which uses seven laser beams and a
rotation speed of approximately 10x.
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; ie a 12x/10x/32x CD drive can, CPU and media permitting, write to CD-R
discs at 12x speed (1.80 MB/s), write to CD-RW discs at 10x (1.50 MB/s), and
read from CD discs at 32x (4.80 MB/s).
The 1x speed rating for CD-ROM (150 kB/s) is different than 1x speed
rating for audio CD (172.3 kB/s) and isn't to be confused with the 1x speed
rating for DVDs (1.32 MB/s).
Common transfer speeds:
Data Transfer Speeds Transfer Speed
KiB/s Mb/s
1x 150 1.2288
2x 300 2.4576
4x 600 4.9152
8x 1200 9.8304
10x 1500 12.2880
12x 1800 14.7456
20x 3000 24.5760
32x 4800 39.3216
36x 5400 44.2368
40x 6000 49.1520
48x 7200 58.9824
50x 7500 61.4400
52x 7800 63.8976
Copyright issues : CD/DVD copy protection
There has been a move by the recording industry to make audio CDs
(CDDAs, Red Book CDs) unplayable on computer CD-ROM drives, to prevent the
copying of music. This is done by intentionally introducing errors onto the
disc that the embedded circuits on most stand-alone audio players can
automatically compensate for, but which may confuse CD-ROM drives. Consumer
rights advocates are as of October 2001 pushing to require warning labels on
compact discs that don't conform to the official Compact Disc Digital Audio
standard (often called the Red Book) to inform consumers of which discs don't
permit full fair use of their content.
In 2005, Sony BMG Music Entertainment was criticised when a copy
protection mechanism known as Extended Copy Protection (XCP) used on some of
their audio CDs automatically and surreptitiously installed copy-prevention
software on computers. Such discs aren't legally allowed to be called CDs or
Compact Discs because they break the Red Book standard governing CDs, and
Amazon.com for example describes them as "copy protected discs" rather than
"compact discs" or "CDs".
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 (CD-R or CD-RW) 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 SID,
the Source Identification Code, an eight character code beginning with "IFPI"
that is usually stamped on discs produced by CD recording plants.
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