Comparing Discs

Disc Comparison

If you place a CD, a DVD, and a Blu-ray Disc™ next to each other, they look nearly identical. But if you try to play a DVD in your CD player, or a Blu-ray Disc in your DVD player, it won't read it. It can't because each type of disc has a different physical structure, data format, and error correction system. Below we'll discuss what makes a DVD different from a CD, and we'll talk about what makes Blu-ray unique.

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The Compact Disc (CD)

1. A polycarbonate disc layer has the data encoded by using bumps.
2. A shiny layer reflects the laser.
3. A layer of lacquer helps keep the shiny layer shiny.
4. Artwork is screen printed on the top of the disc.

A Compact Disc (also known as a CD) is an optical disc used to store digital data. It was originally developed to store sound recordings exclusively, but later it also allowed the preservation of other types of data. Audio CDs have been commercially available since October 1982. In 2010, they remain the standard physical storage medium for audio.

Standard CDs have a diameter of 120 mm and can hold up to 80 minutes of uncompressed audio (700 MB of data). The Mini CD has various diameters ranging from 60 to 80 mm; they are sometimes used for CD singles or device drivers, storing up to 24 minutes of audio. The technology was eventually adapted and expanded to encompass data storage CD-ROM, write-once audio and data storage CD-R, rewritable media CD-RW, Video Compact Discs (VCD), Super Video Compact Discs (SVCD), PhotoCD, PictureCD, CD-i, and Enhanced CD.

CD-ROMs and CD-Rs remain widely used technologies in the computer industry. The CD and its extensions are successful: in 2004, worldwide sales of CD audio, CD-ROM, and CD-R reached about 30 billion discs. By 2007, 200 billion CDs had been sold worldwide.

The DVD

DVD, also known as Digital Video Disc or Digital Versatile Disc, is an optical disc storage media format, and was invented and developed by Philips, Sony, TOSHIBA, and Time Warner in 1995. Its main uses are video and data storage. DVDs are of the same dimensions as compact discs (CDs), but store more than six times as much data.

Where a CD is a single 1.2-millimeter-thick disc, all DVDs use a "sandwich" design — two 0.6-millimeter discs bonded together. This sandwich construction allows DVD discs to have information on both sides and on one or two layers per side. This design is also more structurally stable and resistant to disc warping.

Compared to CD, DVD uses smaller data pits and more closely-spaced pit rows of "tracks," as illustrated above. This increased data density makes it possible for each layer on a DVD to hold more than six times the amount of data on a CD. DVD players use a red laser in place of the infrared laser found in CD players. The red laser's shorter wavelength, combined with a special lens design, result in a narrower, more tightly focused laser beam that can easily read the smaller pits.

Variations of the term DVD often indicate the way data is stored on the discs: DVD-ROM (read only memory) has data that can only be read and not written; DVD-R and DVD+R (recordable) can record data only once, and then function as a DVD-ROM; DVD-RW (re-writable), DVD+RW, and DVD-RAM (random access memory) can all record and erase data multiple times. The wavelength used by standard DVD lasers is 650 nm; thus, the light has a red color.

The Blu-Ray Disc

Blu-ray Disc (official acronym BD, also known as BR or Blu-ray) is an optical disc storage medium designed to supersede the standard DVD format. Its main uses are for storing high-definition video, PlayStation 3 video games, and other data, with up to 25 GB per single layered, and 50 GB per dual layered disc. Although these numbers represent the standard storage for Blu-ray Disc drives, the specification is open-ended, with the upper theoretical storage limit left unclear. 200 GB discs are available, and 100 GB discs are readable without extra equipment or modified firmware. The disc has the same physical dimensions as standard DVDs and CDs.

The name Blu-ray Disc derives from the blue-violet laser used to read the disc. While a standard DVD uses a 650 nanometer red laser, Blu-ray Disc uses a shorter wavelength, a 405 nm blue-violet laser, and allows for almost ten times more data storage than a DVD.

Even though high-definition video requires so much more data, high-def discs can easily hold even the longest movies on a single disc. Blu-ray discs can hold multiple hours of HD content, with plenty of room to spare for the bonus features you may have grown accustomed to with DVD. The developers of Blu-ray couldn't make the disc physically larger, so in order to significantly increase the information storage capacity, they increased the data density. The information pits got smaller, and the spacing of the pit rows got tighter. The discs also have a super-thin transparent protective coating, which places the data layer closer to the disc's surface and thus closer to the laser. In order to read these much smaller data pits, Blu-ray players use a blue-violet laser, which has a shorter wavelength and a smaller "beam spot" than the red laser used in DVD players. The players also spin the discs at higher speeds for even faster data transfer.

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