DVD

Technical information

DVDs are made from a 0.6 mm thick disc of polycarbonate plastic coated with a much thinner (reflective) aluminium layer. Two such discs are glued together to form a 1.2 mm double-sided disc. The substrates are half as thick as a CD to make it possible to use a lens with a higher numerical aperture and therefore use smaller pits and narrower tracks.

Related Topics:
Polycarbonate - Aluminium - CD - Numerical aperture

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A single-layer DVD can store 4.7 Gbyte, which is around seven times as much a standard CD-ROM. By employing a red laser at 650 nm (was 780 nm) wavelength and a numerical aperture of 0.6 (was 0.45), the read-out

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resolution is increased by a factor 1.65. This holds for two dimensions, so that the actual physical data density increases by a factor of 3.5. DVD uses a more efficient coding method in the physical layer. CD's error correction, CIRC, is replaced by a powerful Reed-Solomon product code, RS-PC; Eight-to-Fourteen Modulation (EFM) is replaced by a more efficient version, EFMPlus, which has the same characteristics as classic EFM. The CD subcode is removed. As a result, the DVD format is 47 percent more efficient with respect to CD-ROM, which uses a 'third' error correction layer.

Related Topics:
CIRC - Reed-Solomon - Eight-to-Fourteen Modulation

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A DVD can contain:

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• DVD-Video (containing movies (video and sound))
• DVD-Audio (containing high-definition sound)
• DVD-Data (containing data)

The disc medium can be:

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• DVD-ROM (read only, manufactured by a press)
• DVD-R/RW (R=Recordable once, RW = ReWritable)
• DVD-RAM (random access rewritable)
• DVD+R/RW (R=Recordable once, RW = ReWritable)
• DVD-R DL (double layer)
• DVD+R DL (double layer)

The disc may have one or two sides, and one or two layers of data per side; the number of sides and layers determines the disc capacity.

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• DVD-5: single sided, single layer, 4.7 gigabytes (GB), or 4.38 gibibytes (GiB)
• DVD-9: single sided, double layer, 8.5 GB (7.92 GiB)
• DVD-10: double sided, single layer on both sides, 9.4 GB (8.75 GiB)
• DVD-14: double sided, double layer on one side, single layer on other, 13.3 GB (12.3 GiB)
• DVD-18: double sided, double layer on both sides, 17.1 GB (15.9 GiB)

The capacity of a DVD-ROM can be visually determined by noting the number of data sides, and looking at the data side(s) of the disc. Double-layered sides are sometimes gold-colored, while single-layered sides are silver-colored, like a CD. One additional way to tell if a DVD contains one or two layers is to look at the center ring on the underside of the disc. If there are two barcodes, it is a dual layer disc. If there is one barcode, there is only one layer.

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Each medium can contain any of the above content and can be any layer type. Double layer DVD+R discs are already on the market.

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The DVD Forum created the official DVD-ROM/R/RW/RAM standards and the DVD+RW Alliance created the DVD+R/RW standards. Since DVD+R/RW discs are not technically DVDs as per the DVD Forum standards, they are not allowed to display the DVD logo; instead, they display an "RW" logo. However, they are readable by most DVD drives, so they are referred to as DVD+R and DVD+RW.

Related Topics:
DVD Forum - DVD+RW Alliance

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The "+" (plus) and "-" (dash) are similar technical standards and are partially compatible. As of 2004, both formats are equally popular, with about half of the industry supporting "+", and the other half "-". All DVD readers are supposed to read both formats, though real-world compatibility is around 90% for both formats, with DVD-R having the best overall compatibility in independent tests. Most new DVD writers can write both formats and carry both the RW and DVD logos.

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Unlike compact discs, where sound (CDDA, Red Book) is stored in a fundamentally different fashion than data (Yellow book et al.), a properly authored DVD will always contain data in the UDF filesystem.

Related Topics:
Compact discs - CDDA - Red Book - Yellow book - UDF

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The data transfer rate of a DVD drive is given in multiples of 1350 kB/s, which means that a drive with 16x speed designation allows a data transfer rate of 16 x 1350 = 21600 kB/s (21.09 MB/s). As CD drive speeds are given in multiples of 150 kB/s, one DVD "speed" equals nine CD "speeds", i.e. 8x DVD drive should have data transfer rate similar to a 72 x CD drive. In physical rotation terms (spins per second), one DVD "speed" equals three CD "speeds", so an 8x DVD drive has the same rotational speed as 24x CD drive.

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Early CD and DVD drives read data at a constant rate. The data on the disc is passed under the read head at a constant rate (Constant Linear Velocity, or CLV). As linear (meters per second) track speed grows at outer parts of the disc proportionally to the radius, the rotational speed of the disc was adjusted according to which portion of the disc was being read. Most current CD and DVD drives have a constant rotation speed (Constant Angular Velocity, or CAV). The maximum data rate specified for the drive/disc is achieved only at the end of the disc's track (discs are written from inside). The average speed of the drive therefore equals to only about 50-70% of the maximum nominated speed. While this seems a disadvantage, such drives have a lower seek time as they do not have to change the disc's speed of rotation.

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