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2.2 Content media

2.2.2 Digital mode: Video

 

At first glance, it might appear that there is no need to include digital works (photo, video, computer, etc.) in a preservation study, the assumption being that they only need be copied to be adequately preserved. Computer users perform this action many times a day, usually without considering the possibility of a file becoming corrupted; users burn photo DVDs, transfer text documents, save e-mails, and copy files to USB sticks. However, within a preservation context, these activities carry risks. For example, when a file is zipped, its composition changes, and when one format is migrated to another, metadata can be lost.

It should be noted that audiovisual images and documents stored on digital media can originate from two different sources. They may have been created on an analog medium, such as silver-gelatine film, then digitized, but more commonly they are created directly in digital format. In the latter case, they can be referred to as “born digital.”

While it is recommended to digitize analog documents to preserve them (due to their fragility or obsolescence), digital documents must also be preserved. It is therefore important to establish preservation strategies that will assure the longevity of these documents, be they born digital or not.

Digital documents primarily face risks associated with technological changes to the equipment on which they are stored (including players) and the ongoing advancement of their encoding and compression formats.

 

Digital Video

 

Digital video records and encodes moving images in a series of zeros and ones, which means that the data recorded is stable. If any data is lost, it can be easily monitored by error correcting systems transparent to the user. Unlike the analog format, duplications are identical to the original, free of noise and distortion. If an attempt is made to copy material that was originally of poor quality to a digital medium, this will in no way improve the quality of the video, but it will make the material more stable over the long term. There are many advantages to digital preservation, but it should not be forgotten that the physical image and its restitution are analog and nothing can change this. Digitization converts an analog object to digital format; if the conversion is of poor quality, the results will reflect this, and precise sampling at a high frequency is therefore recommended.

While digital data is reduced to numbers (digits), cassettes and tapes that store the signal remain material objects that are subject to wear and mechanical breakdowns. They should be handled with care.

At the present time, the digital formats most used by artists that merit study for preservation purposes are those in the DV and Digital8 family.

 

DV and MiniDV

 

MiniDV cassette

Also known as DVC (Digital Video), DV was the first digital format to make real inroads into the domestic and professional markets, becoming a universal standard. This format records video data through a standardized compression algorithm, such as M-JPEG in a 5:1 ratio. The DV format was the technological foundation for the popular MiniDV, DVCAM, DVCPRO and Digital8 formats. In fact, the video specifications and processing of the MiniDV and DVCAM are the same; the difference lies only in the videotape and its management.

 

The MiniDV format (¼ inch tape, small size) was introduced in 1995, and players are still widely available. The MiniDV cassette uses the same tape and signal compression as DVCAM cassettes, but it records at varying speeds. The MiniDV cassette can be played on most DVCAM and DVCPRO players, but tapes recorded on DVCAM or DVCPRO cannot be read by a MiniDV player or camera. The MiniDV format was developed for the industrial and education markets as well as for the general public. It was widely used by artists and activists, both in the education sector and for independent productions. Its small size (65 × 48 × 12 mm) and high visual quality made this format popular, particularly for videotaping outdoors.

 

From an equipment perspective, the digital tape is susceptible to the same deterioration problems as an analog tape. Its small tape size makes it more fragile than other formats, as it can easily become creased when play is constantly stopped and reversed. Damage to a MiniDV tape can corrupt a higher volume of data than similar damage to a DVCAM or DVCPRO tape, as the data is more densely recorded on the MiniDV tape. The MiniDV format is not used for video archiving, and migration to more recent formats (remastering) should therefore be considered.

 

DVCAM

 

DVCAM cassettes

DVCAM, a variation of the DV format, was introduced in 1996 and is still easy to acquire. The DVCAM format was developed by Sony for the industrial, education and professional markets. It has been widely used for news gathering, cable television and video production. It is also used by artists and independent producers, particularly for documentaries. The maximum length of the tape on a single cassette is 184 minutes, and cassettes come in two formats—small (S) and large (L)—with capacity relative to their size (40 minutes for small cassettes).

 

From an equipment perspective, digital tape is susceptible to the same deterioration problems as analog tape. However, having been designed for professional use, the DVCAM cassette is more robust. [1] The tape run is faster than with the DV, and data is therefore not as dense. This means that damage to the tape will have less impact on data integrity than would damage to the DV format. The DVCAM is backward compatible (most devices) with the DV and DVCPRO. And while it may not have all of the archiving features desired, the DVCAM is nevertheless a format robust and stable enough to not require immediate migration.

 

DVCPRO

 

DVCPro cassette

DVCPRO is another improvement on the DV format; it offers more enhancements than the DVCAM format had over the DV. Philippe Bellaïche has written that its reliability and performance are on a par with those in the television industry. [2] The only ways it differs from the DVCAM are in the sampling structure, data density, and magnetic coating of the tape, [3] which contribute to its robustness and make it more resistant to damage and data loss than the DV and DVCAM. The DVCPRO comes in two formats: DVCPRO25 and DVCPRO50. The latter is an upgrade of the former made distinctive through 4:2:2 sampling (compared to 4:1:1) and a compression ratio increased to 3.3:1. The quality of the DVCPRO50 image is comparable to that obtained with the digital Betacam. [4] The DVCPRO is backward compatible with the DV and DVCAM. The DVCPRO 50 is the best format in the DV family.

 

Digital8

 

Hi8 cassette

Digital8, introduced by Sony in 1999, can be considered as a transition between the analog and digital formats. This format, whose specifications are similar to DV, differs in its use of Hi8 cassettes. This single feature reduced the cost of the format over DV to such a degree that its popularity increased among artists, who also appreciated its backward compatibility to the 8 mm and Hi8 formats. As the cassettes are larger than those used with the DV, they are also more robust. Sony no longer sells Digital8 camcorders. [5] And, while it is possible to use 8 mm cassettes in a Digital8 taping, Sony recommends the exclusive use of Hi8 cassettes. [6]

 

Compression

 

Compression of files optimizes the storage of data on a given medium. While sacrificing a certain amount of data and thus to some extent altering image and sound quality, it also allows a greater volume of readable data to be stored (in terms of recording time for a video or audio file), whether on a dedicated medium or not. In addition, compression reduces the data flow. The greater the data flow, the more expensive and powerful the machine that must decode, process and read the data. The common practice among professionals, conservators and archivists is to find a healthy balance between low data flow and a high-quality image.

To decrease the size and weight of certain digital formats, the DV format in particular uses compression to save and reduce data. Compression effectively reproduces certain unalterable components of the image rather than storing what appears to be the same data. This allows more data to be stored in a smaller space on the tape and the signal to be easily interrupted.

Ideally, conservation should never involve compression. However, conservation formats that offer minimum compression are not viable within a museum context. One uncompressed example is D-VHS (Digital-VHS), which never took hold commercially and is becoming obsolete. D5 and high definition D6 (VooDoo) are other examples; these uncompressed formats have never found their place in commerce or industry due to their prohibitive cost. They are rarely if ever used and are seriously threatened with obsolescence.

 

The relative necessity of compressing

Over time, the capacity of storage media has tended to increase, regardless of the device used. For example, the hard drives of 1979 might have offered 5 MB capacity for approximately $1,500 ($300 per megabyte) [7]; 30 years later, hard drives of 1.5 TB (1,500,000 MB) can be purchased for approximately $180 ($0.00012 per megabyte). [8] In the interim, hard drives have seen almost linear growth, and it is highly likely this trend will continue. This type of storage medium has become very affordable, allowing users to store multiple voluminous files.

 

Archiving formats

 

When working with digital video, it is important to copy or “migrate” the video to Digital Betacam, known also as Digibeta. [9] Digital Betacam compresses slightly, although considerably less than the DVCAM format, and its tape is physically more stable and robust (½ inch). A digital Betacam archiving policy has been adopted by many galleries with video art collections, notably the Tate in the U.K., the Foundation of the Conservation of Modern Art, [10] and the National Gallery of Canada. Digital Betacam tapes and players are, however, very expensive. Many archives opt instead for analog Betacam SP; this format allows for high-quality recording, although its lifespan is difficult to predict.

 

Digital Betacam

Digital Betacam, which offers an excellent compression ratio (2:1), is the benchmark for television production and postproduction. It poses no problems of obsolescence. Introduced in 1993, this format also reads Betacam and Betacam SP. It is a robust format and is recommended for archiving. According to institutional experts, migrations from a format at risk of obsolescence can be done on digital Betacam or on a server system (storage on hard discs).

 

Should it not be possible to record on digital Betacam, DVCAM or DVCPRO are better choices than MiniDV, and the difference is considerable. However, these formats (whose tapes are small) are vulnerable to physical damage; it is therefore recommended to make a second copy and store it in a different location. To make copies from analog tapes for conservation purposes, whether on digital Betacam, DVCAM or DVCPRO, a digitization process must first be carried out. [11]

Whatever format is used, it will likely be necessary to migrate the video to a new tape and perhaps even a new format after a number of years have passed. It is normally recommended that videos be transferred to new tapes approximately every seven years. [12]

 

Preventive conservation

 

Generally accepted conservation practices dictate that tapes be stored vertically on metal shelving in a dark room, far removed from any magnetic fields. The storage area should be clean, secure, cool and dry. Storage standards for videotape have been established by the International Organization for Standardization (ISO 18923:2000) and are summarized in the document Videotape Preservation Fact Sheets. [13]

“Acceptable extended-term storage conditions for polyester-based magnetic tape, such as videotape, are: 20°C (68°F) and 20-30% RH; 15°C (59°F) and 20-40% RH; or 10°C (50°F) and 20-50% RH. The best long-term storage temperature is approximately 8°C (46°F) (never below) and 25% RH. Humidity variation should be less than ±5% RH and the temperature variation should be less than +-2°C (+-4°F) within a 24-hour period.” [14]

 

Optimum videotape storage conditions

 

Maximum temperature Relative humidity

20 °C / 68 °F

20 % - 30 %
15 °C / 59 °F 20 % - 40 %
10 °C / 50 °F 20 % - 50 %

 

Projections for the future

 

Information and recommendations concerning video preservation and archiving are constantly evolving. The DVCAM format was introduced in 1995; does this mean it is already obsolete and users should be considering replacing it soon? Will the digital Betacam format soon become outmoded? Will current developments in high definition (HD) [15] and Blu-ray [16] technologies deliver the expected changes?

Such is the present state of digital video: change, development and instability. Given this context, it would be unwise to offer specific recommendations on this subject. One can, however, discuss current trends and strategies and learn to work with the information available today, without losing sight of possible changes on the horizon. The decisions that must be made should include potential future changes and how they could influence the overall approach to preserving these technologies.

 

 


[1] The DVCAM was designed for editing conditions that involve the videotape being pushed to the limit, with rapid tape runs and repeated pauses.
[2] Philippe Bellaïche, Les secrets de l’image video (Paris: Eyrolles, 2007), 399.
[3] The magnetic coating on the metal evaporated tape of the DV and DVCAM has given way to metal articles (television industry standard) for the DVCPRO format. Metal particle coating improves the retentivity and coercivity. In addition, the tape is thicker and therefore more resistant.
[4] Philippe Bellaïche, op. cit., 399.
[5] Sony, "Digital 8", http://www.sony.co.uk/product/sdh-digital-8.
[6] Sony, "eSupport", http://www.docs.sony.com/.
[7] PC INpact, "Seagate passe la barre symbolique du milliard de disques durs", http://www.pcinpact.com/actu/news/43289-Seagate-symbole-1-milliard-disques-durs.htm.
[8] Prices in effect in big box store in Montreal, July 2009.
[9] Digital Betacam (commonly referred to as Digibeta, d-beta, dbc or simply Digi) was launched in 1993.

http://en.wikipedia.org/wiki/Betacam
[10] Keep Moving Images. Preservation information for artists working with the moving image, "Video: Formats", http://kmi.lux.org.uk/video/formats.htm.
[11] To digitise video is to use a digital videotape to record the information produced by an analogue video, encoding as it goes.

http://kmi.lux.org.uk/video/digitization.htm
[12] Keep Moving Images. Preservation information for artists working with the moving image, "Video: Formats", op.cit.
[13] AMIA, "Videotape Preservation Fact Sheets", http://www.amianet.org/resources/guides/fact_sheets.pdf.
[14] Ibid.
[15] High-definition video or HD video refers to any video system of higher resolution than standard-definition (SD) video, and most commonly involves display resolutions of 1280×720 pixels (720p) or 1920×1080 pixels (1080i/1080p).

http://en.wikipedia.org/wiki/High-definition_video
[16] Blu-ray Disc (BD), sometimes called "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.

http://en.wikipedia.org/wiki/Blu-ray_Disc