Tuesday, May 11, 2010

DVD as a Backup Medium

There are two considerations for a backup medium: obsolescence and durability. If there is no device that can read the medium, it is obsolete and the data is unavailable and thus lost.

Durability of DVDs is measured by how long the data may be read from the disc, assuming compatible devices exist that can read it: that is, how long the disc can be stored until data is lost. Five factors affect durability: sealing method, reflective layer, organic dye makeup, where it was manufactured, and storage practices.[36]

The longevity of the ability to read from a DVD+R or DVD-R, is largely dependent on manufacturing quality[37] ranging from 2 to 15 years,[38][39][40] and is believed to be an unreliable medium for backup unless great care is taken for storage conditions and handling.

According to the Optical Storage Technology Association (OSTA), "manufacturers claim life spans ranging from 30 to 100 years for DVD, DVD-R and DVD+R discs and up to 30 years for DVD-RW, DVD+RW and DVD-RAM

Improvements and Succession

Blu-ray Disc

In 2006, a new format called Blu-ray Disc (BD), designed by Philips, Sony, Samsung, and Panasonic, was released as the successor to DVD. Another format, HD DVD, competed unsuccessfully with this format in the format war of 2006–08. A dual layer Blu-ray Disc can store 50 to 100 GB.[29][30]

However, unlike previous format changes (e.g., audio tape to compact disc, VHS videotape to DVD), there is no immediate indication that production of the standard DVD will gradually wind down, as they still dominate, with around 87% of video sales and approximately one billion DVD player sales worldwide. In fact experts claim that the DVD will remain the dominant medium for at least another five years as Blu-Ray technology is still in its introductory phase, write and read speeds being poor as well as the fact of necessary hardware being expensive and not readily available.[5][31]

Consumers initially were also slow to adopt Blu-ray due to the cost[citation needed]. By 2009, 85% of stores were selling Blu-ray Discs. A high-definition TV and appropriate connection cables are also required to take advantage of Blu-ray disc. Some analysts suggest that the biggest obstacle to replacing DVD is due to its installed base; a large majority of consumers are satisfied with DVDs.[32] The DVD succeeded because it offered a compelling alternative to VHS. In addition, Blu-ray players are designed to be backward-compatible, allowing older DVDs to be played since the media are physically identical; this differed from the change from vinyl to CD and from tape to DVD, which involved a complete change in physical medium.

This situation can be best compared to the changeover from 78 rpm shellac recordings to 45 rpm and 33⅓ rpm vinyl recordings; because the medium used for the earlier format was virtually the same as the latter version (a disc on a turntable, played using a needle), phonographs continued to be built to play obsolete 78s for decades after the format was discontinued. Manufacturers continue to release standard DVD titles as of 2010, and the format remains the preferred one for the release of older television programs and films, with some programs such as Star Trek: The Original Series requiring remastering and replacement of certain elements such as special effects in order to be better received in high-definition viewing.[33] In the case of Doctor Who, a series primarily produced on videotape between 1963 and 1989 (and thus expected to be difficult to upconvert to high definition), BBC Video reportedly intends to continue issuing DVD-format releases of that series until at least November 2013.[34]

Holographic Versatile Disc

The Holographic Versatile Disc (HVD) is an optical disc technology that may one day hold up to 3.9 terabytes (TB) of information, albeit the current maximum is 500GB. It employs a technique known as collinear holography.


The 5D DVD, being developed in the Swinburne University of Technology in Melbourne, Australia, uses a multilaser system to encode and read data on multiple layers. Disc capacities are estimated at up to 10 terabytes, and the technology could be commercially ready within ten years

DVD Recordable and Rewritable

Faceplate of a DVD Drive supporting both DVD+ and DVD- formats.

HP initially developed recordable DVD media from the need to store data for backup and transport.

DVD recordables are now also used for consumer audio and video recording. Three formats were developed: DVD-R/RW(hyphen), DVD+R/RW (plus), and DVD-RAM. DVD-R is available in two formats, General (650 nm) and Authoring (635 nm), where Authoring discs may be recorded with encrypted content but General discs may not.[1]

Although most DVD writers can nowadays write the DVD+R/RW and DVD-R/RW formats (usually denoted by "DVD±RW" and/or the existence of both the DVD Forum logo and the DVD+RW Alliance logo), the "plus" and the "dash" formats use different writing specifications. Most DVD readers and players will play both kinds of discs, although older models can have trouble with the "plus" variants.

Dual-layer recording

Dual-layer recording (sometimes also known as double-layer recording) allows DVD-R and DVD+R discs to store significantly more data—up to 8.54 gigabytes per disc, compared with 4.7 gigabytes for single-layer discs. Along with this, DVD-DL's have slower write speeds as compared to ordinary DVD's and when played on a DVD player, a slight transition can be seen between the layers. DVD-R DLwas developed for the DVD Forum by Pioneer Corporation; DVD+R DL was developed for the DVD+RW Alliance by Philips and Mitsubishi Kagaku Media (MKM).[24]

A dual-layer disc differs from its usual DVD counterpart by employing a second physical layer within the disc itself. The drive with dual-layer capability accesses the second layer by shining the laser through the first semitransparent layer. In some DVD players, the layer change can exhibit a noticeable pause, up to several seconds.[25] This caused some viewers to worry that their dual-layer discs were damaged or defective, with the end result that studios began listing a standard message explaining the dual-layer pausing effect on all dual-layer disc packaging.

DVD recordable discs supporting this technology are backward-compatible with some existing DVD players and DVD-ROM drives.[24] Many current DVD recorders support dual-layer technology, and the price is now comparable to that of single-layer drives, although the blank media remain more expensive. The recording speeds reached by dual-layer media are still well below those of single-layer media.

There are two modes for dual-layer orientation. With Parallel Track Path (PTP), used on DVD-ROM, both layers start at the inside diameter (ID) and end at the outside diameter (OD) with the lead-out. With Opposite Track Path (OTP), used on many Digital Video Discs, the lower layer starts at the ID and the upper layer starts at the OD, where the other layer ends; they share one lead-in and one lead-out. However, some DVDs also use a parallel track, such as those authored episodically, as in a disc with several separate episodes of a TV series—where more often than not, the layer change is in-between titles and therefore would not need to be authored in the opposite track path fashio

DVD Capacity

DVD-1[21]SS SL1181.461.36
DVD-2SS DL1282.662.47
DVD-3DS SL2282.922.72
DVD-4DS DL2485.324.95
DVD-5SS SL11124.704.38
DVD-9SS DL12128.547.95
DVD-10DS SL22129.408.75
DVD-14[22]DS DL/SL231213.2412.33
DVD-18DS DL241217.0815.90

Capacity and nomenclature of (re)writable discs
DVD-RSS SL (1.0)11123.953.68
DVD-RSS SL (2.0)11124.704.37
DVD-RWSS SL11124.704.37
DVD+RSS SL11124.704.37
DVD+RWSS SL11124.704.37
DVD-RDS DL22129.408.75
DVD-RWDS DL22129.408.75
DVD+RDS DL22129.408.75
DVD+RWDS DL22129.408.75
DVD-RAMSS SL1181.461.36*
DVD-RAMDS DL2282.652.47*
DVD-RAMSS SL (1.0)11122.582.40
DVD-RAMSS SL (2.0)11124.704.37
DVD-RAMDS DL (1.0)22125.164.80
DVD-RAMDS DL (2.0)22129.408.75*

The basic types of DVD (12 cm diameter, single-sided or homogeneous double-sided) are referred to by a rough approximation of their capacity in gigabytes. In draft versions of the specification, DVD-5 indeed held five gigabytes, but some parameters were changed later on as explained above, so the capacity decreased. Other formats, those with 8 cm diameter and hybrid variants, acquired similar numeric names with even larger deviation.

The 12 cm type is a standard DVD, and the 8 cm variety is known as a MiniDVD. These are the same sizes as a standard CD and a mini-CD, respectively. The capacity by surface (MiB/cm2) varies from 6.92 MiB/cm2 in the DVD-1 to 18.0 MiB/cm2 in the DVD-18.

As with hard disk drives, in the DVD realm, gigabyte and the symbol GB are usually used in the SIsense (i.e., 109, or 1,000,000,000 bytes). For distinction, gibibyte (with symbol GiB) is used (i.e., 230, or 1,073,741,824 bytes). Most computer operating systems display file sizes in gibibytes, mebibytes, and kibibytes, labeled as gigabyte, megabyte, and kilobyte, respectively.

Size comparison: a 12 cm DVD+RW and a 19 cm pencil.

Each DVD sector contains 2,418 bytes of data, 2,048 bytes of which are user data. There is a small difference in storage space between + and - (hyphen) formats:

Capacity differences of writable DVD formats
DVD−R SL2,298,4964,707,319,8084,707.3204,489.2504.7074.384
DVD+R SL2,295,1044,700,372,9924,700.3734,482.6254.7004.378
DVD−R DL4,171,7128,543,666,1768,543.6668,147.8758.5447.957
DVD+R DL4,173,8248,547,991,5528,547.9928,152.0008.5487.961


Internal mechanism of a DVD-ROM Drive. See text for details.
DVD-RW Drive operating with the protective cover removed.

DVD uses 650 nm wavelength laser diode light as opposed to 780 nm for CD. This permits a smaller pit to be etched on the media surface compared to CDs (0.74 µm for DVD versus 1.6 µm for CD), allowing for a DVD's increased storage capacity.

In comparison, Blu-ray, the successor to the DVD format, uses a wavelength of 405 nm, and one dual-layer disc has a 50 GB storage capacity.

Writing speeds for DVD were 1×, that is, 1350 kB/s (1,318 KiB/s), in the first drives and media models. More recent models, at 18× or 20×, have 18 or 20 times that speed. Note that for CD drives, 1× means 150 KiB/s (153.6 kB/s), approximately one ninth as fast.[21]

DVD drive speeds
Drive speedData rate~Write time (min)[23]

Internal mechanism of a drive

This mechanism is shown right side up; the disc is above it. The laser and optical system "looks at" the underside of the disc.

Referring to the photo, just to the right of image center is the disc spin motor, a gray cylinder, with its gray centering hub and black resilient drive ring on top. A clamp (not in the photo, retained in the drive's cover), pulled down by a magnet, clamps the disc when this mechanism rises, after the disc tray stops moving inward. This motor has an external rotor – every part of it that you can see spins.

The gray metal chassis is shock-mounted at its four corners to reduce sensitivity to external shocks, and to reduce drive noise when running fast. The soft shock mount grommets are just below the brass-colored washers at the four corners (the left one is obscured). Running through those grommets are screws to fasten them to the black plastic frame that's underneath.

Two parallel precision guide rods that run between upper left and lower right in the photo carry the "sled", the moving optical read-write head. As shown, this "sled" is close to, or at the position where it reads or writes at the edge of the disc.

A dark gray disc with two holes on opposite sides has a blue lens surrounded by silver-colored metal. This is the lens that's closest to the disc; it serves to both read and write by focusing the laser light to a very small spot. It's likely that this disc rotates half a turn to position a different set of optics (the other "hole") for CDs vs. DVDs.

Under the disc is an ingenious actuator comprising permanent magnets and coils that move the lens up and down to maintain focus on the data layer. As well, the actuator moves the lens slightly toward and away from the spin-motor spindle to keep the spot on track. Both focus and tracking are relatively quite fast and very precise. The same actuator rotates the lens mount half.a turn as described.

To select tracks (or files) as well as advancing the "sled" during continuous read or write operations, a stepping motor rotates a coarse-pitch leadscrew to move the "sled" throughout its total travel range. The motor, itself, is the gray cylinder just to the left of the most-distant shock mount; its shaft is parallel to the support rods. The leadscrew, itself, is the rod with evenly-spaced darker details; these are the helical groove that engages a pin on the "sled".

The irregular orange material is flexible etched copper foil supported by thin sheet plastic; these are "flexible printed circuits" that connect everything to the electronics (which is not shown).