Also, HD-DVD offers the interactive capabilities of Blu-ray, with HDi. The read-only versions hold slightly less data. A rewritable, single-layer HD-DVD can hold 15 GB of data, a double-layer disc can hold 30 GB, and a triple-layer disc can hold 45 GB (that's compared to 27 GB and 50 GB for Blu-ray). HD-DVD matches the storage capacity of Blu-ray. The advantage to HD-DVD is that it uses the same basic format as the traditional DVD and can therefore be manufactured with the same equipment, saving on costs. Learn how Blu-ray overcomes these obstacles in the next section. All of these issues lead to a very involved manufacturing process. Also, if the DVD surface is not exactly flat, and is therefore not exactly perpendicular to the beam, it can lead to a problem known as disc tilt, in which the laser beam is distorted. If the beam is split too widely, the disc cannot be read. Having a polycarbonate layer on top of the data can cause a problem called birefringence, in which the substrate layer refracts the laser light into two separate beams. In a DVD, the data is sandwiched between two polycarbonate layers, each 0.6-mm thick. But the two types of discs store data differently. The smaller pits, smaller beam and shorter track pitch together enable a single-layer Blu-ray disc to hold more than 25 GB of information - about five times the amount of information that can be stored on a DVD.Įach Blu-ray disc is about the same thickness ( 1.2 millimeters) as a DVD. Plus, Blu-ray has reduced the track pitch from 0.74 microns to 0.32 microns. The smaller beam focuses more precisely, enabling it to read information recorded in pits that are only 0.15 microns (µm) (1 micron = 10 -6 meters) long - this is more than twice as small as the pits on a DVD. A blue laser has a shorter wavelength ( 405 nanometers) than a red laser (650 nanometers). Unlike current DVDs, which use a red laser to read and write data, Blu-ray uses a blue laser (which is where the format gets its name).
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