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Optical discs and drives
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Optical discs can store data at much higher densities than magnetic disks. They are therefore ideal for multimedia applications where images, animations and sound occupy a lot of disc space. Furthermore, optical discs are not affected by magnetic fields, meaning that they are secure and stable, and can be transported through airport metal detections without damaging the data. However, optical drives are slower that hard drives.
CDs and DVDs
At first sight, a DVD is similar to a CD. Both discs are 120 mm in diameter and 1.2 mm thick. They also both use a laser beam to read data. However, they are very different in internal structure and data capacity. In a DVD, the tracks are very close together, thus allowing more tracks. The pits in which data is stored are also smaller, so there are more pits per track. As a result, a CD can hold 650-700MB, whereas a basic DVD can hold 4.7GB. In addition, a DVD can be double-sided and dual layer, with a capacity of 17GB.
CDs come in three different formats:
- CD-ROMs are read-only units, meaning you cannot change the data stored on them.
- CD-R discs are write-once devices which let you duplicate music CDs and other data CDs.
- CD-RW discs enable you to write onto them many times, just like a hard disk. 
DVDs also come in several formats:
- DVD-ROMs are used in DVD computer drives. They allow for data archiving as well as interactive content (for example, an encyclopedia or a movie).
- DVD-R or DVD+R can only be recorded once.
- DVD-RW or DVD+RW discs can be erased and reused many times. They are used to back up data files and to record audio and video.
The DVD drive used in computers is also called a DVD burner because it records information by burning via a laser to a blank DVD discs. However, a DVD recorder typically refers to a standalone unit which resembles a video cassette recorder. New DVD recorder can play all CD and DVD formats. There are also portable DVD players – handheld devices which let you watch movies or TV, play games and listen to music, wherever you are. They come with a built-in DVD drive and widescreen (rectangular 16: 9 format) LCD display. They usually support multi-format playback – that is, they can play many file formats, including DVD-video, DivX, CD audio discs, MP3 music and JPEG images.
HD –DVD and Blu-ray discs
These two competing formats are expected to replace current DVD as the standard for watching movies at home. On one side are Toshiba, Microsoft and the DVD Forum, who support the High Definition-DVD (HD-DVD). Sony, Panasonic, Samsung, JVC and many movie studios are behind the Blu-ray format.
A Blu-ray disc has a capacity of 25GB (single layer), 50GB (dual layer) and 100 GB (four layer). Unlike DVDs, which use a red laser to read and write data, Blu-ray uses a blue-violet laser, hence its name. Blu-ray discs can record and play back high-definition television and digital audio, as well as computer data.
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