An optical disc is a flat, usually circular disc which encode binary data in the form of pits and lands on a special material on one of its flat surfaces. The lands represent "1" and the pits represent "0" in binary computing. The bits are read by the disc drive that uses a laser beam to distinguish between the lands and pits based on the amount of scattering or deflection that occurs when the beam of light hits the surface of the disc.
But it is not simply so that a land is a "1" data bit, and a pit is a "0" data bit. A data bit is a "1" or "0" from the original data, but on an optical disc there are no data-bits but channel-bits. A channel bit is the smallest time unit used on a disc, for a CD it equals 1/4,321,800 sec.
In a pressed or mass-replicated CD or DVD, the bumps and grooves that represent the binary data on a disc's substrate are pressed into it during manufacture. CD/DVD-R discs do not have true pits and lands, but the unmelted, clear areas and melted, opaque places in the dye layer fulfill the same function as pits and lands on a pressed disc.
To write data onto a disc, the optical drive uses this type of laser to make a series of microscopic marks in the dye. The resulting sequence of light and dark spots (pits and lands) represent the digital ones and zeros that comprise your data.
DVD uses 650 nm wavelength laser diode light as opposed to 780 nm for CD. This permits a smaller pit to be etched on the media surface compared to CDs (0.74 µm for DVD versus 1.6 µm for CD), allowing for a DVD's increased storage capacity. In comparison, Blu-ray disc, the successor to the DVD format, uses a wavelength of 405 nm, and one dual-layer disc has a 50 GB storage capacity.
Unlike the floppy disk, most optical discs do not have an integrated protective casing and are therefore susceptible to data transfer problems due to scratches, fingerprints, and other environmental problems. An optical disc is designed to support one of three recording types: read-only (e.g. CD and CD-ROM), recordable (write-once, e.g. CD-R), or re-recordable (rewritable, e.g. CD-RW).
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