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CD ROM

A compact disc with read only memory is called a CD-ROM. It is a pre-pressed disc that contains data to be stored on a computer. CD-ROMs may also contain music play lists or other data. While the content on them is accessible, it is not writable by the computer.

CD-ROMs are widely used to distribute computer software such as games and multimedia applications; however, other forms of data can be stored depending on how much free space is on the CD-ROM. Some CDs are able to be played using a CD player while holding added computer data along with audio. Such CDs are called enhanced CDs. When CD-ROMs were first introduced, they were capable of holding more data than computer hard drives. Today, however, hard drives far exceed present-day CD-ROMs.

CD-ROM discs look and operate very similarly to basic audio CDs. The data is both stored and retrieved in very similar methods. The actual discs are, on average, 120 mm in diameter. They have a thin layer of reflective aluminum, with a 1.2 mm thick disc of polycarbonate plastic as the base. Data is stored on the disc as a series of microscopic indentations. The pattern of pits and lands is read by a laser shining onto the reflective surface. The depth of the pits is approximately one-quarter to one-sixth of the wavelength of the laser light used to read the disc, thus causing the reflected beam’s phase to shift in relation to the incoming beam. This in turn causes destructive interference and reduces the reflected beam’s intensity. The pattern of changing intensity of the reflected beam is converted into binary data. Rainbow books are various formats used to store data on CD-ROMs. For example, the Red Book is standard for an audio CD, while the White Book or Yellow Book is standard for a CD-ROM.

The rate of speed by which CD-ROM drives operate is relative to music CDs. For example, 12× drives were common in the late 1990s. Once they started reaching speeds above 12×, problems with vibration and heat arose. Constant angular velocity (CAV) drives can reach speeds as high as 30× at the outer edge of the disc. However, due to the fact that the CAV stays relatively the same at the inner edge while increasing slightly, the actual throughput increase is small, roughly 20× average for a completely full disc, and even less for a partially filled one. Due to problems with vibration, CD-ROM drive speeds have not massively increased in the last decade. Today, the most commonly available drives may have speed varying between 24× and 52×, and they all use CAV to achieve their claimed “max” speeds. Even so, these speeds can cause poor reading and even shattering of feebly made or physically damaged media, with small cracks quickly developing into disastrous breakages when centripetally stressed at 10,000 – 13,000rpm. High rotational speeds also cause air to blast and the spindle motor to produce undesirable noises. Thankfully, most 21st century drives allow forced low speed modes in order to be able to accurately read or silence, and will automatically fall back if a large number of sequential read errors and retries are run into.

Most standard CD-ROMs contain 333,000 blocks or sectors. Each sector is 2,352 bytes, and contains 2,048 bytes of PC data, 2,336 bytes of PSX/VCD (mode 2) data, or 2,352 bytes of audio. The difference between sector size and data content are the header information and the error-correcting codes, which are big for data (high precision required) yet small for VCD (standard for video) and none for audio. An image size is always a multiple of 2,352 bytes (the size of a block) when extracting in raw mode.

Pre-pressed CD-ROMs are mass-produced by a stamping process. First, a glass master disc is created to make stamps. These 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 similar method. The only difference is that the data is recorded on them using a laser that changes the properties of a dye or phase transition material. This process is commonly referred to as “burning a disc.”

CD-ROMs generally have large capacities. They can quite easily store the entirety of an English encyclopedia’s words and images, in addition to audio and video data. CD-ROM capacities are normally expressed through binary prefixes, which subtract 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.

CD-ROM discs are read using CD-ROM drives that are connected to the computer. Just about all modern CD-ROM drives have the ability to play audio CDs as well as video CDs and other data standards when used in conjunction with the right software.

CD-ROM drives utilize a near-infrared 780 nm laser diode. The laser beam is directed onto the disc via an electronic tracking module, which then detects whether the beam has been reflected or scattered. If a CD-ROM is read at the same rotational speed as an audio CD, then the data transfer rate is 150 KiB/s (1×). At this rate, the track moves along beneath the laser spot at about 1.2 m/s. This velocity is maintained by varying the angular velocity from 500 rpm at the inner edge to 200 rpm at the outer edge while the optical head moves to different positions. By increasing the spinning speed of the disc, data can be transferred at much faster rates.

The recording industry has proposed to make audio CDs unplayable on computer CD-ROM drives, so as to prevent the copying of music. To do this, intentional errors would have to be introduced onto the disc. Such errors would have to be made to confuse CD-ROM drives, because most stand-alone audio players can automatically compensate for smaller errors. Software distributors, and in particular distributors of computer games, often utilize various copy protection schemes to avert software from running any media besides the original CD-ROMs. This differs somewhat from audio CD protection in that it is usually applied in both the media and the software itself. The CD-ROM itself makes copying difficult by having weak sectors along with additional data that may be difficult or impossible to copy to a CD or disc image. However, the software check for these each time it is run to ensure an original disc and not an unauthorized copy is present in the computer’s CD-ROM drive.

For preventative reasons, 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.

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CD ROM


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