DVD-R read/write side
|Media type||Optical disc|
|Capacity||4.7 GB (single-sided, single-layer – common)
8.5–8.7 GB (single-sided, double-layer)
9.4 GB (double-sided, single-layer)
17.08 GB (double-sided, double-layer – rare)
|Read mechanism||650 nm laser, 10.5 Mbit/s (1×)|
|Write mechanism||10.5 Mbit/s (1×)|
|Standard||DVD Forum’s DVD Books and DVD+RW Alliance specifications|
Pre-recorded DVDs are mass-produced using molding machines that physically stamp data onto the DVD. Such discs are known as DVD-ROM, because data can only be read and not written nor erased. Blank recordable DVD discs (DVD-R and DVD+R) can be recorded once using a DVD recorder and then function as a DVD-ROM. Rewritable DVDs (DVD-RW, DVD+RW, and DVD-RAM) can be recorded and erased multiple times.
DVDs are used in DVD-Video consumer digital video format and in DVD-Audio consumer digital audio format, as well as for authoring AVCHD discs. DVDs containing other types of information may be referred to as DVD data discs.
- 1 Etymology
- 2 History
- 3 Specifications
- 4 DVD recordable and rewritable
- 5 Capacity
- 6 DVD drives and players
- 7 DVD-Video
- 8 DVD-Audio
- 9 Successors
- 10 Use as backup medium
- 11 See also
- 12 References
- 13 Further reading
- 14 External links
Etymology[edit source | edit]
When first invented in 1995, DVD was an initialism for the unofficial term digital videodisk.
Within the next few years, certain members of the DVD Forum proposed the backronym digital versatile disc to express that DVD goes beyond video. However, according to Jim Taylor, a prominent industry figure, the forum never universally accepted this name and a 1999 report decreed that DVD was “simply three letters” and stood for “nothing”.
Today, usage varies and there is no universal agreement. Digital versatile disc has gained wide acceptance and is used by the official DVD charter. However, others such as Toshiba (who maintain the DVD Forum Web site) still use the original digital video disc, while Jim Taylor’s FAQ still maintains that DVD has never officially stood for anything.
History[edit source | edit]
Development[edit source | edit]
Before the advent of DVD in 1995, Video CD (VCD) became the first format for distributing digitally encoded films on standard 120 mm (4.7 in) optical discs. (Its predecessor, CD Video, used analog video encoding.) VCD was on the market in 1993. In the same year, two new optical disc storage formats were being developed. One was the Multimedia Compact Disc (MMCD), backed by Philips and Sony, and the other was the Super Density (SD) disc, supported by Toshiba, Time Warner, Matsushita Electric, Hitachi, Mitsubishi Electric, Pioneer, Thomson, and JVC.
Representatives from the SD camp asked IBM for advice on the file system to use for their disc, and sought support for their format for storing computer data. Alan E. Bell, a researcher from IBM’s Almaden Research Center, got that request, and also learned of the MMCD development project. Wary of being caught in a repeat of the costly videotape format war between VHS and Betamax in the 1980s, he convened a group of computer industry experts, including representatives from Apple, Microsoft, Sun Microsystems, Dell, and many others. This group was referred to as the Technical Working Group, or TWG.
The TWG voted to boycott both formats unless the two camps agreed on a single, converged standard. They recruited Lou Gerstner, president of IBM, to pressure the executives of the warring factions. In one significant compromise, the MMCD and SD groups agreed to adopt proposal SD 9, which specified that both layers of the dual-layered disc be read from the same side—instead of proposal SD 10, which would have created a two-sided disc that users would have to turn over. As a result, the DVD specification provided a storage capacity of 4.7 GB for a single-layered, single-sided disc and 8.5 GB for a dual-layered, single-sided disc. The DVD specification ended up similar to Toshiba and Matsushita’s Super Density Disc, except for the dual-layer option (MMCD was single-sided and optionally dual-layer—whereas SD was single-layer, but optionally double-sided) and EFMPlus modulation.
Philips and Sony decided that it was in their best interests to avoid another format war over their Multimedia Compact Disc, and agreed to unify with companies backing the Super Density Disc to release a single format, with technologies from both. After other compromises between MMCD and SD, the computer companies through TWG won the day, and a single format, now called DVD, was agreed upon. The TWG also collaborated with the Optical Storage Technology Association (OSTA) on the use of their implementation of the ISO-13346 file system (known as Universal Disk Format) for use on the new DVDs.
Adoption[edit source | edit]
Movie and home entertainment distributors adopted the DVD format to replace the ubiquitous VHS tape as the primary consumer film distribution format. They embraced DVD because it produces superior moving pictures and sound, provides superior durability, and can be interactive. Interactivity on LaserDiscs had proven desirable to consumers, especially collectors. When LaserDisc prices dropped from approximately $100 per disc to $20 per disc at retail, this luxury feature became available for mass consumption. Simultaneously, the movie studios decided to change their home entertainment release model from a rental model to a for purchase model, and large numbers of DVDs were sold.
At the same time, a demand for interactive design talent and services was created. Movies in the past had uniquely designed title sequences. Suddenly every movie being released required information architecture and interactive design components that matched the film’s tone and were at the quality level that Hollywood demanded for its product.
DVD as a format had two qualities at the time that were not available in any other interactive medium: enough capacity and speed to provide high quality, full motion video and sound, and low cost delivery mechanism provided by consumer products retailers. Retailers would quickly move to sell their players for under $200, and eventually for under $50 at retail. In addition, the medium itself was small enough and light enough to mail using general first class postage. Almost overnight, this created a new business opportunity and model for business innovators to re-invent the home entertainment distribution model. It also gave companies an inexpensive way to provide business and product information on full motion video through direct mail.
Specifications[edit source | edit]
The DVD specifications created and updated by the DVD Forum are published as so-called DVD Books (e.g. DVD-ROM Book, DVD-Audio Book, DVD-Video Book, DVD-R Book, DVD-RW Book, DVD-RAM Book, DVD-AR Book, DVD-VR Book, etc.).
Some specifications for mechanical, physical and optical characteristics of DVD optical discs can be downloaded as freely available standards from the ISO website. There are also equivalent Ecma standards for some of these specifications, such as Ecma-267 for DVD-ROMs. Also, the DVD+RW Alliance publishes competing recordable DVD specifications such as DVD+R, DVD+R DL, DVD+RW or DVD+RW DL. These DVD formats are also ISO standards.
Some of DVD specifications (e.g. for DVD-Video) are not publicly available and can be obtained only from the DVD Format/Logo Licensing Corporation for a fee of US $5000. Every subscriber must sign a non-disclosure agreement as certain information in the DVD Book is proprietary and confidential.
DVD recordable and rewritable[edit source | edit]
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, 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 CSS encrypted video content but General discs may not.
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 play both kinds of discs, though older models can have trouble with the “plus” variants.
Some first generation DVD players would cause damage to DVD±R/RW/DL when attempting to read them.
The form of the spiral groove that makes up the structure of a recordable DVD encodes unalterable identification data known as Media Identification Code (MID). The MID contains data such as the manufacturer and model, byte capacity, allowed data rates (also known as speed), etc.
Dual-layer recording[edit source | edit]
Dual-layer recording (sometimes also known as double-layer recording) allows DVD-R and DVD+R discs to store significantly more data—up to 8.5 gigabytes per disc, compared with 4.7 gigabytes for single-layer discs. Along with this, DVD-DLs have slower write speeds as compared to ordinary DVDs. When played, a slight transition can sometimes be seen in the playback when the player changes layers. DVD-R DL was 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).
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. 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. 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 DVDs, 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.
Capacity[edit source | edit]
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 SI sense (i.e., 109, or 1,000,000,000 bytes). For distinction, gibibyte (with symbol GiB) is used (i.e., 10243 (230), or 1,073,741,824 bytes).
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:
|DVD-R||SS SL (1.0)||1||1||12||3.95||3.68|
|DVD-R||SS SL (2.0)||1||1||12||4.70||4.37|
|DVD-RAM||SS SL (1.0)||1||1||12||2.58||2.40|
|DVD-RAM||SS SL (2.0)||1||1||12||4.70||4.37|
|DVD-RAM||DS SL (1.0)||2||2||12||5.16||4.80|
|DVD-RAM||DS SL (2.0)||2||2||12||9.40||8.75*|
DVD drives and players[edit source | edit]
Laser and optics[edit source | edit]
DVD uses 650 nm wavelength laser diode light, as opposed to 780 nm for CD. This shorter wavelength etches a smaller pit on the media surface compared to CDs (0.74 µm for DVD versus 1.6 µm for CD), allowing in part for DVD’s increased storage capacity.
In comparison, Blu-ray Disc, the successor to the DVD format, uses a wavelength of 405 nm, and one dual-layer disc has a 50 GB storage capacity.
Transfer rates[edit source | edit]
Read and write speeds for the first DVD drives and players were of 1,385 kB/s (1,353 KiB/s); this speed is usually called “1×”. More recent models, at 18× or 20×, have 18 or 20 times that speed. Note that for CD drives, 1× means 153.6 kB/s (150 KiB/s), about one-ninth as swift.
|Drive speed||Data rate||~Write time (minutes)|
DVD-Video[edit source | edit]
DVD-Video is a standard for storing and distributing video/audio content on DVD media. The format went on sale in Japan in 1996, in the United States in 1997, in Europe in 1998 and in Australia in 1999. DVD-Video became the dominant form of home video distribution in Japan when it first went on sale in 1996, but did not become the dominant form of home video distribution in the United States until June 15, 2003, when weekly DVD-Video in the United States rentals began outnumbering weekly VHS cassette rentals, reflecting the rapid adoption rate of the technology in the U.S. marketplace. Currently, DVD-Video is the dominant form of home video distribution worldwide, although in Japan it was surpassed by Blu-ray Disc when Blu-ray first went on sale in Japan on March 31, 2006.
Security[edit source | edit]
The Content Scramble System (CSS) is a Digital Rights Management (DRM) and encryption system employed on almost all commercially produced DVD-video discs. CSS utilizes a proprietary 40-bit stream cipher algorithm. The system was introduced around 1996 and was first compromised in 1999.
The purpose of CSS is twofold:
- CSS prevents byte-for-byte copies of an MPEG (digital video) stream from being playable since such copies do not include the keys that are hidden on the lead-in area of the restricted DVD.
- CSS provides a reason for manufacturers to make their devices compliant with an industry-controlled standard, since CSS scrambled discs cannot in principle be played on noncompliant devices; anyone wishing to build compliant devices must obtain a license, which contains the requirement that the rest of the DRM system (region codes, Macrovision, and user operation prohibition) be implemented.
While most CSS-decrypting software is used to play DVD videos, other pieces of software (such as DVD Decrypter, AnyDVD, DVD43, Smartripper, and DVD Shrink) can copy a DVD to a hard drive and remove Macrovision, CSS encryption, region codes and user operation prohibition.
Consumer restrictions[edit source | edit]
The rise of filesharing and “piracy” has prompted many copyright holders to display notices on DVD packaging or displayed on screen when the content is played that warn consumers of the illegality of certain uses of the DVD. It is commonplace to include a 90 second advert warning that most forms of copying the contents are illegal. Many DVDs prevent skipping past or fast-forwarding through this warning.
Arrangements for renting and lending differ by geography. In the U.S., the right to re-sell, rent, or lend out bought DVDs is protected by the first-sale doctrine under the Copyright Act of 1976. In Europe, rental and lending rights are more limited, under a 1992 European Directive that gives copyright holders broader powers to restrict the commercial renting and public lending of DVD copies of their work.
DVD-Audio[edit source | edit]
DVD-Audio is a format for delivering high fidelity audio content on a DVD. It offers many channel configuration options (from mono to 5.1 surround sound) at various sampling frequencies (up to 24-bits/192 kHz versus CDDA‘s 16-bits/44.1 kHz). Compared with the CD format, the much higher-capacity DVD format enables the inclusion of considerably more music (with respect to total running time and quantity of songs) and/or far higher audio quality (reflected by higher sampling rates and greater sample resolution, and/or additional channels for spatial sound reproduction). This DVD-Audio technique was first used by the brothers Lagkind.
Despite DVD-Audio’s superior technical specifications, there is debate as to whether the resulting audio enhancements are distinguishable in typical listening environments. DVD-Audio currently forms a niche market, probably due to the very sort of format war with rival standard SACD that DVD-Video avoided.
Security[edit source | edit]
Although CPPM was supposed to be much harder to crack than a DVD-Video CSS, it too was eventually cracked, in 2007, with the release of the dvdcpxm tool. The subsequent release of the libdvdcpxm library (based on dvdcpxm) allowed for the development of open source DVD-Audio players and ripping software, such as DVD-Audio Explorer. As a result, making 1:1 copies of DVD-Audio discs is now possible with relative ease, much like DVD-Video discs.
Successors[edit source | edit]
In 2006, two new formats called HD DVD and Blu-ray Disc were released as the successor to DVD. HD DVD competed unsuccessfully with Blu-ray Disc in the format war of 2006–2008. A dual layer HD DVD can store up to 30GB and a dual layer Blu-ray disc can hold up to 50GB.
However, unlike previous format changes, e.g., audio tape to Compact Disc or 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 75% of video sales and approximately one billion DVD player sales worldwide as of 3 April 2011. 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.
Consumers initially were also slow to adopt Blu-ray due to the cost. By 2009, 85% of stores were selling Blu-ray Discs. A high-definition television 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. The DVD succeeded because it offered a compelling alternative to VHS. In addition, the uniform media size let manufacturers make Blu-ray players and now defunct format HD DVD players backward-compatible, so they can play older DVDs. This stands in contrast to the change from vinyl to CD, and from tape to DVD, which involved a complete change in physical medium. As of 2012[update] it is still commonplace for studios to issue major releases in “combo pack” format, including both a DVD and a Blu-ray disc (as well as, in many cases, a third disc with an authorized digital copy). Also, some multi-disc sets use Blu-ray for the main feature, but DVDs for supplementary features (examples of this include the Harry Potter “Ultimate Edition” collections, the 2009 re-release of the 1967 The Prisoner TV series, and a 2007 collection related to Blade Runner). Another reason cited (July 2011) for the slower transition to Blu-ray from DVD is the necessity of and confusion over “firmware updates” and needing an internet connection to perform updates.
This situation is similar to the changeover from 78 rpm shellac recordings to 45 rpm and 33⅓ rpm vinyl recordings. Because the new and old mediums were virtually the same (a disc on a turntable, played by a needle), phonograph player manufacturers continued to include the ability to play 78s for decades after the format was discontinued.
Manufacturers continue to release standard DVD titles as of 2012[update], and the format remains the preferred one for the release of older television programs and films. Some programs, such as Star Trek: The Original Series must be re-scanned to produce a high definition version from the original film recordings. (Certain special effects were also updated to appear better in high-definition.) In the case of Doctor Who, a series primarily produced on standard definition videotape between 1963 and 1989, BBC Video reportedly intends to continue issuing DVD-format releases of that series until at least November 2013.
DVDs are also facing competition from video on demand services. With increasing numbers of homes having high speed internet links, many people now have the option to either rent or buy video from an online service, and view it by streaming it directly from that service’s servers, meaning that the customer need not have any form of permanent storage media for video at all.
Use as backup medium[edit source | edit]
DVD durability is measured by how long the data remains readable 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.
The longevity of the ability to read from a DVD+R or DVD-R largely depends on manufacturing quality, ranging from two to 15 years, and is considered an unreliable backup medium unless great care is taken for storage conditions and handling.
According to the Optical Storage Technology Association (OSTA), “Manufacturers claim lifespans 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.”
See also[edit source | edit]
References[edit source | edit]
- DVD FLLC (2009-02) DVD Book Construction – list of all available DVD Books, Retrieved on 2009-07-24
- DVD FLLC DVD Format Book – History of Supplements for DVD Books, Retrieved on 2009-07-24
- MPEG.org, DVD Books overview, Retrieved on 2009-07-24
- “How much does a DVD weigh?”. Answers.com. Retrieved December 27, 2012.
- Markoff, John (1996-01-11). “A Battle for Influence Over Insatiable Disks”. The New York Times. Retrieved 2007-04-09.
- “DVD FAQ”. DVD Demystified. 2006-09-12.
- “DVD Primer”. DVD Forum. 2004-11-14. Retrieved 2008-01-23.
- Super Video Compact Disc, A Technical Explanation (PDF) (PDF). Philips System Standards and Licensing. 1998. p. 2. Retrieved 2008-02-13
- “E-commerce and Video Distribution: DVD and Blu-ray”.
- “DVD: coming soon to your PC?”. Computer Shopper 16 (3): 189. March 1, 1996.
- ISO ISO Freely Available Standards, Retrieved on 2009-07-24
- Standard ECMA-267 – 120 mm DVD – Read-Only Disk
- ISO ISO/IEC 17344:2009, Data interchange on 120 mm and 80 mm optical disc using +R format – Capacity: 4,7 Gbytes and 1,46 Gbytes per side (recording speed up to 16X), Retrieved on 2009-07-26
- ISO ISO/IEC 25434:2008, Data interchange on 120 mm and 80 mm optical disc using +R DL format – Capacity: 8,55 Gbytes and 2,66 Gbytes per side (recording speed up to 16X), Retrieved on 2009-07-26
- ISO ISO/IEC 17341:2009, Data interchange on 120 mm and 80 mm optical disc using +RW format – Capacity: 4,7 Gbytes and 1,46 Gbytes per side (recording speed up to 4X), Retrieved on 2009-07-26
- ISO ISO/IEC 26925:2009, Data interchange on 120 mm and 80 mm optical disc using +RW HS format – Capacity: 4,7 Gbytes and 1,46 Gbytes per side (recording speed 8X), Retrieved on 2009-07-26
- DVD FLLC (2009) DVD Format Book, Retrieved on 2009-08-14
- DVD FLLC (2009) How To Obtain DVD Format/Logo License (2005–2009), Retrieved on 2009-08-14
- “DVD Media / DVD-R Media”. Tape Resources. Retrieved 2011-08-09.
- Robert DeMoulin. “Understanding Dual Layer DVD Recording”. BurnWorld.com. Retrieved 2007-07-06.
- “DVD players benchmark”. hometheaterhifi.com. Retrieved 2008-04-01.[dead link]
- “DVD Book A: Physical parameters”. Mpeg.org. Retrieved 2009-08-22.
- Cinram: DVD in Detail[dead link]
- Taylor, Jim. “DVD Demystifed FAQ”. Dvddemystified.com. Retrieved 2009-08-22.
- “DVD-14”. AfterDawn Ltd. Retrieved 2007-02-06.
- “Understanding DVD -Recording Speed”. Osta.org. Retrieved 2011-08-09.
- The write time is wildly optimistic for higher (>4x) write speeds, due to being calculated from the maximum drive write speed instead of the average drive write speed.
- “Discount stores are a video lover’s channel of choice”. Discount Store News. findarticles.com. 1998-08-10. Retrieved 2008-03-06.[dead link]
- Bakalis, Anna (2003-06-20). “It’s unreel: DVD rentals overtake videocassettes”. Washington Times.
- IEEE – Copy Protection for DVD Video p.2[dead link]
- “DVD-Audio ripper”. Retrieved 2008-04-09.
- “What is Blu-ray Disc?”. Sony. Retrieved 2008-11-25.
- “DVD FAQ: 3.13 – What about the new HD formats?”. 2008-09-21. Retrieved 2008-11-25.
- “High-Definition Sales Far Behind Standard DVD’s First Two Years”. Movieweb.com. 2008-02-20. Retrieved 2009-08-22.
- “Blu-ray takes 25% Market share:”. 2008-09-21. Retrieved 2011-06-28.
- Robert Martorana (2009-11-04). “Slow Blu-ray Adoption: A Threat to Hollywood’s Bottom Line?”. Seeking Alpha. Retrieved 2011-08-09.
- “Gates And Ballmer On “Making The Transition””. BusinessWeek. 2004-04-19. Retrieved 2009-08-22.
- “Kirk/Spock STAR TREK To Get All-New HD Spaceships”. Aintitcool.com. Retrieved 2009-08-22.[unreliable source?]
- “How To Choose CD/DVD Archival Media”. 2006-10-30. Retrieved 2009-03-14.
- “Optical Media Longetivity”. Retrieved 2010-02-26.
- “DVD Durability”. Retrieved 2010-02-26.
- “The Life-Span of DVDs”. January 2006. Retrieved 2010-02-26.
- “How long will data recorded on writable DVD discs remain readable?”. Retrieved 2009-03-14.
Further reading[edit source | edit]
- Bennett, Hugh (April 2004). “Understanding Recordable and Rewritable DVD”. Optical Storage Technology Association. Retrieved 2006-12-17.
- Labarge, Ralph (2001). DVD Authoring and Production. Gilroy, California: CMP Books. ISBN 1-57820-082-2.
- Taylor, Jim (2000). DVD Demystified (2nd ed.). New York: McGraw-Hill Professional. ISBN 0-07-135026-8.
[edit source | edit]
|Wikimedia Commons has media related to: DVD|
|Wikibooks has a book on the topic of: Inside DVD-Video/MPEG Format|
- All About Converting From Several Video Formats To DVD at Wikibooks
- DVD at the Open Directory Project
- Dual Layer Explained – Informational Guide to the Dual Layer Recording Process
- DVD Frequently Asked Questions (and Answers)
- YouTube: Segment from 1997 Toshiba DVD demo disc: Technical information about the then-new DVD format, from “DVD Gallery,” an in-store demonstration disc from Toshiba
This article uses material from the Wikipedia article DVD, which is released under the Creative Commons Attribution-Share-Alike License 3.0.