Optical Drives

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CD-ROM, CD-R and CD-RW Drives

It is an acronym of “Compact Disc Read-only memory” and is a disc that contains data accessible but not writable. The 1985 “Yellow Book” standard developed by Sony and Philips adapted the format to hold any form of binary data. They are popularly used to distribute computer software and audio. Discs are made from a 1.2 mm thick disc of polycarbonate plastic, with a thin layer of aluminum to make a reflective surface. The most common size of CD-ROM disc is 120 mm in diameter, though the smaller Mini CD standard with an 80 mm diameter, as well as numerous non-standard sizes and shapes (e.g., business card-sized media) are also available. ISO 9660 defines the standard file system of a CD-ROM and the bootable CD specification, to make a CD emulate a hard disk or floppy, is called El Torito. CD-ROM drives are rated with a speed factor relative to music CDs (1× or 1-speed which gives a data transfer rate of 150 KB/s) and maximum 52x is achieved. A standard 120 mm, 700 MB CD-ROM can actually hold about 737 MB. CD-ROM discs are read using CD-ROM drives. A CD-ROM drive may be connected to the computer via an IDE (ATA), SCSI, SATA, FireWire, or USB interface.

Data is stored on the disc as a series of microscopic indentations. A laser is shone onto the reflective surface of the disc to read the pattern of pits and lands (“pits”, with the gaps between them referred to as “lands”).CD-ROM drives employ a near-infrared 780 nm laser diode. The laser beam is directed onto the disc via an opto-electronic tracking module, which then detects whether the beam has been reflected or scattered.

CD-recordable (CD-R) and CD-rewritable (CD-RW) drives (also known as CD burners) are Similar to CD-ROM drives but we can create (or burn) CD-ROMs. They look similar to CD-ROM drives, except the front panel has been written with to either CD-R or CD-RW. CD-R drives can write to a CD only once. A CD-RW can erase information from a disc and rewrite to it multiple times. CD-RW drives are rated according to their read, write, and rewrite times. So instead of a single rating like 40X, they have a rating of 32X-16X-4X, which means it reads at 32X,

writes at 16X, and rewrites at 4X. A CD-RW is a rewritable optical disc introduced in 1997. It needs more sensitive laser optics. CD-RW discs need to be blanked before reuse. They have a shorter rewriting cycle life (about 1,000) as compared to other storage of media types (above 10,000). Their usage is in creation of test disks, temporary short or mid-term backups, and in general, where an intermediate solution between online and offline storage schemes is required.

DVD Drives

DVD (digital video disc) technology is mainly used in many home theater systems. They can store up to 4.3GB of data. A single DVD is a double-sided, double-layered configuration and can hold as much as 17GB (approx 26 CD-ROMs). A DVD burner operates in a similar manner to a CD-R or CD-RW drive but can store large amounts of data onto a DVD. Single-sided, double-layered (DL) discs can be burned PC to hold 8.5GB. Common names for the variations of DVD burning technologies include DVD+R, DVD+RW, DVD-R, DVDRW, DVD-RAM, DVD-R DL, and DVD+R DL. It was invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995.Blank recordable DVDs (DVD-R and DVD+R) can be recorded once using a DVD recorder and then function as a DVD-ROM. Rewritable DVDs (DVD-RW, DVD+RW, and DVD-RAM) can be recorded and erased multiple times. DVDs are used in DVD-Video consumer digital video format and in DVD-Audio consumer digital audio format, as well as for authoring AVCHD discs. DVDs containing other types of information may be referred to as DVD data discs.

Blu-ray Disc

It is a 12 cm plastic optical disc, the same size as DVDs and CDs. Blu-ray Discs contain 25 GB per layer, with dual layer discs (50 GB). The name Blu-ray Disc refers to the blue laser used to read the disc, which allows information to be stored at a greater density than is possible with the longer-wavelength red laser used for DVDs.

Disc	BD-ROM	DVD ROM
Laser wavelength	405 Nm	650 Nm
Storage capacity single layer	25 GB	4.7 GB
Storage capacity dual layer	50 GB	8.5 GB
Maximum data transfer rate	54.0 Mbit/s	11.08 Mbit/s

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