CD-ROM
A CD-ROM ( /ˌsiːˌdiːˈrɒm/, an acronym of
"Compact Disc Read-only memory") is a pre-pressed compact disc that
contains data accessible to, but not writable by, a computer for data storage
and music 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 video 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, while data (such as software or digital video) is
only usable on a computer (such as ISO 9660 format 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. At the time of the technology's
introduction it had more capacity than computer hard drives common at the time.
The reverse is now true, with hard drives far exceeding CDs, DVDs and Blu-ray,
though some experimental descendants of it such as HVDs may have more space and
faster data rates than today's biggest hard drive.
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 CD-ROM 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 shone 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
Several
formats are used for data stored on compact discs, known as the Rainbow Books. These include the
original Red Book standards for CD audio, White Book and Yellow Book CD-ROM.
The ISO/IEC 10149 / 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 (EFM), can be downloaded
from ISO[1] or ECMA.
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 (703 MiB) of data with error correction (or 847 MB total). In
comparison, a single-layer DVD-ROM can hold 4.7 GB of error-protected data,
more than 6 CD-ROMs.
ROM
can easily store the entirety of a paper encyclopedia's words and images, plus
audio & video clips
|
Type
|
Sectors
|
Data
max. size
|
Data
max. size
|
Audio
max. size
|
Time
|
|
|
|
(MB)
|
Approx.
(MiB)
|
|
|
|
8
cm
|
94,500
|
193.536
|
184.570
|
222.264
|
21
|
|
|
283,500
|
580.608
|
553.711
|
666.792
|
63
|
|
650
MB
|
333,000
|
681.984
|
650.391
|
783.216
|
74
|
|
700
MB
|
360,000
|
737.280
|
703.125
|
846.720
|
80
|
|
800
MB
|
405,000
|
829.440
|
791.016
|
952.560
|
90
|
|
900
MB
|
445,000
|
912.384
|
870.117
|
1,047.816
|
99
|
Note:
megabyte (MB) and minute (min) values are exact; MiB values are approximate
Compact
Disc (CD)
The
Compact Disc (also known as a CD) is an optical disc used to store digital
data. It was originally developed to store and playback sound recordings exclusively,
but later expanded to encompass data storage (CD-ROM), write-once audio and
data storage (CD-R), rewritable media (CD-RW), Video Compact Discs (VCD), Super
Video Compact Discs (SVCD), PhotoCD, PictureCD, CD-i, and Enhanced CD. Audio
CDs and audio CD players have been commercially available since October 1982.
Standard
CDs have a diameter of 120 millimetres (4.7 in) and can hold up to 80 minutes
of uncompressed audio or 700 MB (700 × 220 bytes) of data. The Mini CD has
various diameters ranging from 60 to 80 millimetres (2.4 to 3.1 in); they are
sometimes used for CD singles, storing up to 24 minutes of audio or delivering
device drivers.
CD-ROMs
and CD-Rs remain widely used technologies in the computer industry. The CD and
its extensions are successful: in 2004, worldwide sales of CD audio, CD-ROM,
and CD-R reached about 30 billion discs. By 2007, 200 billion CDs had been sold
worldwide.[1] Compact Discs are increasingly being replaced or supplemented by
other forms of digital distribution and storage, such as downloading and flash
drives, with audio CD sales dropping nearly 50% from their peak in 2000
Manufacturing
tolerances
Current manufacturing
processes allow an audio CD to contain up to 80 minutes (variable from one
replication plant to another) without requiring the content creator to sign a
waiver releasing the plant owner from responsibility if the CD produced is
marginally or entirely unreadable by some playback equipment. Thus, in current
practice, maximum CD playing time has crept higher by reducing minimum
engineering tolerances; by and large, this has not unacceptably reduced
reliability.
