Use of CD

CD-ROM, 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 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.

COMPONENTS IN CD
Discs are made from a 1.2 mm thick disc of polycarbonate plastic, with a thin layer of aluminum 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 are also available. Data is stored on the disc as a series of microscopic indentations.

WORKING OF CD
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.

SPEED OF CD
CD-ROM drives are rated with a speed factor relative to music CDs (1× or 1-speed which gives a data transfer rate of 150 KiB/s). 12× drives were common beginning in early 1997. Above 12× speed, there are problems with vibration and heat. Constant angular velocity (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 (CLV) 12×, or 32× with a slight increase. However due to the nature of CAV (linear speed at the inner edge is still only 12×, increasing smoothly in-between) 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.

Compact Disc manufacturing
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 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
A CD-ROM can easily store the entirety of an English encyclopedia's words and images, plus audio & video clipsCD-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. Capacities of Compact Disc types (90 and 99 minute discs are not standard) Type	Sectors	Data max. size	Audio max. size	Time (MB)

Optical disc drive
A 4× CD-ROM Drive. CD-ROM discs are read using CD-ROM drives. 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 CDs as well as Video CDs and other data standards when used in conjunction with the right software. CD-ROM drive can sometimes be a misnomer for newer drives that are capable for reading and burning DVDs, the CD's successor which is now the standard optical disc drive.

PURPOSE OF CD
'CD-MIDI' CD-MIDI is a format used to store music-performance data which upon playback is performed by electronic instruments that synthesize the audio. Hence, unlike Red Book, these recordings are not audio. For the first few years of its existence, the Compact Disc was a medium used purely for audio. However, in 1985 the Yellow Book CD-ROM standard was established by Sony and Philips, which defined a non-volatile optical data computer data storage medium using the same physical format as audio Compact Discs, readable by a computer with a CD-ROM drive. 'Video CD (VCD)' Video CD (VCD, View CD, and Compact Disc digital video) is a standard digital format for storing video media on a CD. VCDs are playable in dedicated VCD players, most modern DVD-Video players, personal computers, and some video game consoles. 'Super Video CD' Super Video CD (Super Video Compact Disc or SVCD) is a format used for storing video media on standard Compact Discs. SVCD was intended as a successor to VCD and an alternative to DVD-Video, and falls somewhere between both in terms of technical capability and picture quality.

Photo CD
Photo CD is a system designed by Kodak for digitizing and storing photos on a CD. '''

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Green Book (CD-interactive standard)
The Philips "Green Book" specifies the standard for interactive multimedia Compact Discs designed for CD-i players. This format is unusual because it hides the initial tracks which contains the software and data files used by CD-i players by omitting the tracks from the disc's TOC (table of contents). This causes audio CD players to skip the CD-i data tracks. This is different from the CD-i Ready format, which puts CD-i software and data into the pregap of track 1. CDi was the leading format of its time but was supplanted by the politics of competition. Philips Interactive Media lead the way in producing breakthrough titles, including the first interactive coloring book, Sesame Street Disc and children's programs, Groliers and Comptoms encyclopedias and many more pathbreaking programs.

Enhanced CD
Enhanced CD, also known as CD Extra and CD Plus, is a certification mark of the Recording Industry Association of America for various technologies that combine audio and computer data for use in both Compact Disc and CD-ROM players. 'VinylDisc' VinylDisc is the hybrid of a standard Audio CD and the vinyl record. The vinyl layer on the disc's label side can hold approximately three minutes of music.