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FAQ General FAQs How does DVD technology differ from CD?
FAQ General FAQs How does DVD technology differ from CD?
How does DVD technology differ from CD?
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Like the CD technology from which it is derived, DVD and CD discs are the same size (120mm) and thickness (1.2mm). Like CDs, DVD-ROM discs are "stamped" in a production facility using sophisticated injection molding equipment, and DVD recordable discs are "burned" using a recorder. Also like CD discs, DVD discs store data in the dye layer using microscopic flat areas (called lands) and in non-reflective holes (called pits). All CD and DVD drive types use laser beams to scan these lands and pits. Light is transmitted through the transparent lands and off of the reflective layer above the dye, back to the optical pickup. The light is scattered by the pits, which are "stamped" in the polycarbonate during production or burned in the dye by a recorder. The transitions of the reflected/non-reflected light from the disc represent the zeros and ones of digital information.
But that's where the similarities end. DVD discs use a tighter spiral (also known as a track or helix) with only 0.74 microns between the tracks, compared to 1.6 microns on CDs. DVD recorders use a laser with a smaller wavelength, 635nm or 650 nm (visible red light) vs. 780nm (infrared) for CDs, to produce a smaller "burn" or pit in the translucent dye layer (0.4 microns minimum vs. 0.83 microns minimum on CDs). Improvements in the logarithms used for error correction allow a much greater data accuracy using smaller Error Correction Codes (ECC) than those used in CD technology, resulting in more effective use of the track space. These technologies allow DVD discs to store large amounts of data-up to seven times that of a CD.
Focusing a smaller spot on a shallower dye layer would allow vendors to reduce the thickness of the substrate, but this would make the disc too flexible and fragile. A second non-data disc is instead bonded to the single-sided disc to make it 1.2mm thick. A single-sided disc will store up to 4.7GB of information.
DVD players can also use dual-sided discs. Two pre-mastered discs are actually bonded together to form one disc. The disc must be turned over to access the data on the reverse side, and will hold about 9.4GB of information.
A dual-layer DVD-ROM disc is also two pre-mastered discs bonded together. But, instead of back-to-back layers like "dual-sided" discs, the DVD-ROM "dual-layer" disc contains a lower pre-mastered translucent opaque layer and an upper pre-mastered normal layer. By changing the laser's focus, the layer closest to the laser becomes transparent, and the laser is able to read the pits on the second layer without having to be turned over. In single-layer technology, the data is written from the hub to the outer edge of the disc. In dual-layer technology, the spiral on both layers can either start at the hub, or the top layer can start at the outer edge and track inwards toward the hub. This reverse tracking allows the laser to focus faster and synchronize to the data on the reverse-track layer faster than tracking on two parallel spirals. A dual layer disc uses about a 10% increase in the length of the pits and lands, so the resulting capacity is reduced by 10% to about 8.5GB.
But that's where the similarities end. DVD discs use a tighter spiral (also known as a track or helix) with only 0.74 microns between the tracks, compared to 1.6 microns on CDs. DVD recorders use a laser with a smaller wavelength, 635nm or 650 nm (visible red light) vs. 780nm (infrared) for CDs, to produce a smaller "burn" or pit in the translucent dye layer (0.4 microns minimum vs. 0.83 microns minimum on CDs). Improvements in the logarithms used for error correction allow a much greater data accuracy using smaller Error Correction Codes (ECC) than those used in CD technology, resulting in more effective use of the track space. These technologies allow DVD discs to store large amounts of data-up to seven times that of a CD.
Focusing a smaller spot on a shallower dye layer would allow vendors to reduce the thickness of the substrate, but this would make the disc too flexible and fragile. A second non-data disc is instead bonded to the single-sided disc to make it 1.2mm thick. A single-sided disc will store up to 4.7GB of information.
DVD players can also use dual-sided discs. Two pre-mastered discs are actually bonded together to form one disc. The disc must be turned over to access the data on the reverse side, and will hold about 9.4GB of information.
A dual-layer DVD-ROM disc is also two pre-mastered discs bonded together. But, instead of back-to-back layers like "dual-sided" discs, the DVD-ROM "dual-layer" disc contains a lower pre-mastered translucent opaque layer and an upper pre-mastered normal layer. By changing the laser's focus, the layer closest to the laser becomes transparent, and the laser is able to read the pits on the second layer without having to be turned over. In single-layer technology, the data is written from the hub to the outer edge of the disc. In dual-layer technology, the spiral on both layers can either start at the hub, or the top layer can start at the outer edge and track inwards toward the hub. This reverse tracking allows the laser to focus faster and synchronize to the data on the reverse-track layer faster than tracking on two parallel spirals. A dual layer disc uses about a 10% increase in the length of the pits and lands, so the resulting capacity is reduced by 10% to about 8.5GB.
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