Blu Ray Discs
Blu Ray disc refers to an optical media that is capable of storing large amount data, about 10 times more than a DVD. Single layer blu ray disc can hold 25GB of data and dual layer can store 50 GB of data. Hence, they are used to store high definition videos, data and play station games. Although the storage capacity of the blu ray disc is very large compared to the DVD/CD, the physical dimensions are same. The difference in size is due to use of different wavelength of laser light. A DVD uses 650 nanometer red laser light and blu ray discs use blue-violet laser light of wavelength 405 nm. Shorter wavelength means more storage space, since the laser can be controlled easily. The name Blu-Ray comes from the blue laser light that is used for reading/writing such discs. Blu ray disc was first developed by Blu-ray disc association. There was competition between Blu Ray disc and HD DVD, but Blu-Ray turned out be a superior media compared to HD DVD.
The Blu ray disc is similar in size with respect to CDs and DVDs, but because of the shorter wavelength, more data can be stored. The spot size on which the laser light will be incident decides the storage capacity of the optical media, which in turn depends on the diffraction and the numerical aperture used to focus the light on the media. It is not easy to focus laser on smaller spots and requires lens that have larger numerical aperture size. In addition, the cover layer should be made thinner to avoid other optical effects, so that light can be focused on the smaller spot. The encoding techniques have also been increased which has aided in increased capacity of the discs.
Unlike DVDs, the data layer of the blu ray discs is closer to the surface, making it vulnerable to damages. There is need to protect the data layer and cartridges were to used to do it. Cartridges made the expensive blu-ray discs more expensive. Later, hard coating of pick-up surface was introduced to prevent damage of the data layer. Custom made hard- coat technologies were introduced by the blu-ray disc manufacturers like Sony, LG, Verbatim etc. These materials had antistatic and scratch resistant properties. Although DVDs are safer compared to Blu-Ray discs, many companies introduced scratch-free coatings for expensive DVD discs.
Blu Ray is popular for storing HD videos and PS3 games. The BD ROM players must support the common codec like MPEG2, SMPTE VC1, and H.264/MPEG-4.The advantage of using blu-ray discs to store multimedia is that large files can fit into one Blu-Ray disc and also high resolution videos can be watched.
In any optical media, the data is stored in a particular form called pits. In a disc burner, laser beams are used to heat the dye present on the disc to form certain pattern which represents digital data – 0s and 1s.The distance between the pits is known as land and it is also known as marks in writable media. In readable media, liquid plastic is used to stamp on the discs through a molding machine and then a coat of protective metallic reflective layer is added over the disc. On the other hand, writable discs are so designed that marks can be created by heating the disc using laser beams at specific points. While the disc spins, the laser beam hits these patterns or marks and the intensity of the beams get modified and reflected back. Then they are converted into electrical signals through and the data is shown on the output device.
The process of creating pattern is not simple. Many assume that zeroes and ones are etched directly as pits and lands, but in reality this does not happen. Both pits and lands represent a series of 0s of varying lengths and the transition between them is treated as one. The patterns are designed such that data can be accurately read as string of pulses. Modulation keeps the strings of 0s in a particular range ( >=2 & <10) and the transition between them is 1. The thickness of a human hair can be equivalent to 150 pits and a small dust particle can clog thousands of pits. But the laser light focuses past the surface and hence these particles won’t affect in any way. Error correction is done in the case of slightly damaged discs and that appear as glitches. Extremely damaged discs cannot be read by the BD player and the error is reported to the computer.
100 GB blu ray discs are being produced and the future for blu ray discs look very bright. Blu-ray discs – the next generation optical media have superseded DVD data format and they are here to stay!