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The compact disc (CD) is a digital optical disc data storage format for storing and playing digital audio recordings that was co-developed by Philips and Sony. The first compact disc was made in August 1982. It was later renamed Digital Audio Compact Disc and introduced in October 1982.
Later, the format was developed for data storage (CD-ROM). Write-once audio and data storage (CD-R), rewritable media (CD-RW), Video CD (VCD), Super Video CD (SVCD), Photo CD, Picture CD, Compact Disc-Interactive (CD-i), and Enhanced Music CD were all evolved from these.
CDs have a diameter of 120 millimetres (4.7 inches) and can carry up to 74 minutes of uncompressed stereo digital audio or 650 megabytes of data. By cramming more data onto the same sized disc, capacity may be increased to 80 minutes and 700 MB. Mini CDs come in a variety of sizes, ranging from 60 to 80 millimetres (2.4 to 3.1 inches), and are occasionally used to store up to 24 minutes of audio or to provide device drivers.
A CD could store significantly more data than a personal computer hard disk drive, which could only hold 10 MB at the time of its release in 1982. By 2010, hard drives had dropped in price to the point that they could hold the same amount of data as a thousand CDs. Around 30 billion audio CDs, CD-ROMs, and CD-Rs were sold globally in 2004. Globally, 200 billion CDs have been sold by 2007.
A CD is constructed of polycarbonate plastic that is 1.2 millimetres thick (0.047 mm) and weighs 14″33 grams. The center spindle hole (15 mm), the first-transition region (clamping ring), the clamping area (stacking ring), the second-transition area (mirror band), the program (data) area, and the rim are the components from the center outward. The radius of the inner program area ranges from 25 to 58 mm.
The surface is coated with a small layer of aluminum or, more rarely, gold to make it reflective. A film of lacquer is generally spin coated directly on the reflective layer to preserve the metal. Screen printing or offset printing is used to print the label on the lacquer layer.
A compact disc’s Pits and Lands under a microscope
CD data is recorded in a spiral track moulded into the top of the polycarbonate layer as small indentations known as pits. Lands are the places in between mines. Each hole is around 100 nm deep, 500 nm broad, and ranges in length from 850 nm to 3.5 m. The pitch (distance between the tracks) is 1.6 meters.
When you play an audio CD, a motor within the player rotates the disc at 1.2″1.4 m/s (constant linear velocity, CLV)”roughly 500 RPM on the inside and 200 RPM on the outer edge. The CD track starts on the inside and spirals outward, slowing the rotation rate of the disc when played from beginning to end.