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DVD-ROM

Like DVD discs, there is little to distinguish a DVD-ROM drive from an ordinary CD-ROM drive as the only giveaway is the DVD logo on the front. Even inside the drive there are more similarities than differences: the interface is ATAPI or SCSI for the more upmarket drives, and the transport is much like any other CD-ROM drive. CD-ROM data is recorded near the top surface of a disc. DVD’s data layer is right in the middle so that the disc can be double-sided. Therefore, the laser assembly of a DVD-ROM drive needs to be more complex than its CD-ROM counterpart, to enable it to read from both CD and DVD media. An early solution to entailed having a pair of lenses on a swivel: one to focus the beam onto the DVD data layers and the other for reading ordinary CDs. Subsequently, more sophisticated designs have emerged that eliminate the need for lens switching. For example, Sony’s dual discrete optical pickup design has separate lasers optimised for CD (780nm wavelength) and DVD (650 nm). Many Panasonic drives employ an even more elegant solution which avoids the need to switch either lenses or laser beams by use of a holographic optical element capable of focusing a laser beam at two discrete points.

DVD-ROM drives spin the disc a lot slower than their CD-ROM counterparts. However, since the data is packed much closer together on DVD discs, the throughput is substantially better than a CD-ROM drive at equivalent spin speed. While a 1x CD-ROM drive has a maximum data rate of only 150 KBps, a 1x DVD-ROM drive can transfer data at 1,250 KBps, which is just over the speed of an 8x CD-ROM drive.

DVD-ROM drives became generally available in early 1997 and these early 1x devices were also capable of reading CD-ROM discs at 12x speed – sufficient for full-screen video playback. As with CD-ROM, higher speed drives appeared as the technology matured. By the beginning of 1998, multispeed DVD-ROM drives had already reached the market, capable of reading DVD media at double-speed, producing a sustained transfer rate of 2,700 KBps, and of spinning CDs at 24-speed and by the end of that year DVD read performance had been increased to 5-speed. A year later performance had improved to six-speed (8,100 KBps) reading of DVD media and 32-speed reading of CD-ROMs. By late-2001 performance levels had reached 16-speed/40-speed for DVD-ROM/CD-ROM respectively.

There is no standard terminology to describe the various generations of DVD-ROM drive. However, second generation (or DVD II) is usually used to refer to 2x drives also capable of reading CD-R/CD-RW media and third generation (or DVD III) usually means 5x (or sometimes 4.8x or 6x) drives, some of which are capable of reading DVD-RAM media.

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