DVD-R
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A DVD-R (properly pronounced as "DVD Are" or "DVD Dash Are", but not "DVD Minus Are") is an optical disc with a larger storage capacity than a CD-R, typically 4.7 GB (4.38 GiB) instead of 700 MiB, although the original capacity of the standard was 3.95 GB. In the recording industry, they are used to record movies in better quality than a VHS tape. A DVD-R can be written to only once, whereas a DVD-RW is rewritable (hence the name).
The DVD-R format was developed by Pioneer in autumn of 1997. It is supported by over 90% of today's DVD players, and is approved by the DVD Forum.
A competing format is DVD+R (also DVD+RW for the rewritables). Hybrid drives that can handle both formats, often labeled DVD±R, are very popular since there is no universal standard yet for recordable DVDs.
The larger storage capacity of a DVD-R or DVD+R compared to a CD-R is achieved through smaller pit size and smaller track pitch of the groove spiral which guides the laser beam. Consequently, more pits can be written on a 120 mm disc. In order to write smaller pits onto the recording dye layer (see CD-R) a red laser beam with a wavelength of 658 nm (for general use recordable DVD) is used in conjunction with a higher numerical aperture (higher quality) lens. Because of this shorter wavelength, compared to CD-R, DVD-R and DVD+R require a different dye to properly absorb this wavelength.
All DVD discs (including DVD-R and DVD+R) are composed of two 0.6 mm thick polycarbonate discs, bonded with an adhesive to each other. One contains the guiding groove and is coated with the recording dye and aluminum reflector. The other one (for single-sided discs) is an ungrooved, uncoated "dummy" disc to assure mechanical stability of the sandwich structure, and compatibility with the compact disc standard geometry which requires a total disc thickness of about 1.2 mm. Double-sided discs have two grooved, recordable disc sides, and require the user to flip the disc to access the other side (unless a dual-pickup drive is used). Compared to a CD's 1.2 mm of polycarbonate, a DVD's laser beam only has to penetrate 0.6 mm of plastic in order to reach the dye recording layer, which allows the lens to focus the beam to a smaller spot size, which is key for writing smaller pits.
In a DVD-R, the addressing (the determination of location of the laser beam on the disc) is done with special marks along the groove, called land prepits. The groove in a DVD-R has a constant wobble frequency.
In a DVD+R, addressing is achieved with no prepits, but by changing the wobble frequency.