About CD-ROMs
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 1.2mm thick disc of polycarbonate plastic,
with a thin layer of aluminium to make a reflective surface.
The most usual size of CD-ROM disc is 120mm in diameter,
though the smaller Mini CD standard with an 80mm diameter,
as well as a lot of non-standard sizes and shapes
(for example,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 alluded to as"lands").
Because the depth of the pits is about 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.
CD-ROM drives employ a close to-infrared 780nm 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.
A CD-ROM 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 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 dubbed enhanced CDs.
Even though 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 quite a few, 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 was not the intention of the original team who developed the CD-ROM,
and usual acceptance of the"memory"definition is at present nearly universal.
This is most likely in no small part because of the widespread use of other "ROM"acronyms
such as Flash-ROMs and EEPROMs where "memory" is most commonly the correct term.
At the time of the technology's introduction it had more capacity than computer hard drives usual at the time.
The reverse is at present true, with hard drives far surpassing 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 largest 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 1.2mm thick disc of polycarbonate plastic, with a thin layer of aluminium to make a reflective surface.
The most usual size of CD-ROM disc is 120mm in diameter,though the smaller Mini CD standard with an 80 mm diameter,
as well as a lot of non-standard sizes and shapes(for example, 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 alluded to as "lands").
Because the depth of the pits is about 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.
CD-ROM drives employ a close to-infrared 780nm 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.