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Transcoding Digital Video

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‘Digitisation' is a process in which analogue materials become digital, while ‘transcoding' is the shift from one digital format to another digital format. This brief paper looks at some common situations which call for transcoding, such as when ‘migrating' data to new formats in order to avoid obsolescence.

The need to transcode

With vast amounts of moving image materials still sitting on fragile and rapidly deteriorating analogue videotape it is easy to see why much of the available literature focuses on analogue-to-digital conversion. Digitisation is our best, perhaps only, hope at conserving the content of magnetic tapes for future generations. Until relatively recently digitised material or material which was born-digital (was never housed on analogue media such as magnetic tape) was seen as low priority or even as ‘successfully preserved'. This is far from accurate. It is true that analogue formats are not ideal long-term storage media. Analogue media typically require tightly controlled environmental conditions, relative humidity, temperature; even magnetic fields can pose a threat to some analogue magnetic materials. Digitisation then, if planned and carried out with best practice in mind can seem to be a magical elixir, prolonging the life of moving image content indefinitely.

But there is no evidence yet to prove that digital media are any more robust, over the long-term than their analogue counterparts, in fact in some ways digital is almost certainly less robust. Obsolescence is a particularly prominent threat to digital formats, with manufacturers seemingly producing new technologies every day, technologies which make no promise of backward compatibility.

Transcoding video

The importance of regular digital-to-digital transcoding to moving image collections cannot be overstated. Digitising analogue materials can be a costly and prolonged exercise, rendered pointless unless the digitised results are properly maintained.

Born-digital video collections may be equally in need of regular transcoding. Video was an early adopter of digital technologies and many different formats have been in common use since the 1980s. Typically digital video media will be an analogue/digital hybrid; with digital data stored on magnetic tape (as in mini-DV). In some ways this combination has the vulnerabilities of both analogue and digital media. Current best practice suggests that that tape-based technology is avoided altogether and hard drives are used for storage; this is at last becoming a financially viable option, even for modest institutions. Error checking and transcoding actions can then be automated, saving valuable time and resources.

The following list looks at the most common digital video media formats and assesses their current status.

Digital video cassette formats

D-1 (introduced 1988)

D-1 was a groundbreaking digital videotape format but was extremely expensive when it was introduced. D-1 cassettes are still available for purchase but the age of the D-1 format makes it an unpopular choice for production and an unsuitable medium for archival use. Consider migrating to a hard drive based system.

D-2 (introduced 1988)

D-2 was a low cost alternative to D-1 aimed at TV broadcasters. Again cassettes are available for purchase but the age of the D-2 makes it unpopular and unsuitable for archival use; consider migrating to a hard drive based system.

D-3 (introduced 1990) and D-5 / D-5 HD (introduced 1994)

D-3 was a competitor to D-2. Cassettes were made available in small, medium and large sizes. Later, the same tapes were used with higher performance systems and marketed as the D-5 format. D-5 player/recorders are still available but the format suffers from the disadvantages of any tape-based format. Automated checking and immediate access are not possible. Though not a migration priority, consider transcoding to a hard drive based system when possible.

Ampex DCT (introduced 1992)

The DCT format was a fully proprietary format produced by Ampex. DCT was never as popular as Digibeta. Players are still available but Ampex are widely considered to have now ceased development of video systems altogether and this may be a deciding factor when considering migration.

Digibeta (Digital Betacam) (introduced 1993)

Very popular format, used extensively in television production and until relatively recently considered a strong format within the archival community. Digibeta is however, a compressed format with the disadvantages of any tape-based system; automated error checking and immediate access are not possible. It may be desirable to migrate materials to a hard disc based system but this action should not be considered a migration priority, as equipment is still readily available.

D-9 Digital-S / D-9 HD (introduced 1995)

D-9 was also launched as a competitor to Digibeta. Cassettes are still available but the age of the D-9 format makes it an unpopular production choice and unsuitable for archival use. Consider migrating D-9 cassettes to a hard drive based system.

DV / Mini-DV (DVC) / DVCAM (introduced 1995)

The DV format has enjoyed wide consumer uptake since its introduction. However, the format now shows signs of declining popularity as solid-state camcorders come to significance. High compression ratios and (in the case of mini-DV) fragile cassette housing make the format unsuitable for archival use. It may be desirable to migrate materials to a hard disc based system but this action should not be considered a priority as players and media are still widely available.

D-6 HDTV VTR (introduced 1996)

The D-6 format was aimed squarely at the media production industry. The cassettes look like the D-1 or D-2 format but are not compatible with these technologies. D-6 was an expensive format and was never widely adopted. Consider migrating to a hard drive based system.

D-7 / DVCPRO / DVCPRO 50 / DVPRO HD (DVCPRO100) (introduced 1996)

D-7 is a very similar but slightly more robust adaptation of the DV format. DVCPRO was aimed at electronic newsgathering and television production industries. Later developments offered yet higher performance. The D-7 format was adopted by the BBC (as DVCPRO50) amongst others and is still in use. The DVCPRO data structure (rather than the physical carrier) is also used to record onto P2 solid-state cards in some video cameras. It may be desirable to migrate D-7 materials to a hard disc based system but this action should not be considered a priority as players and media are still widely available.

Betacam SX (introduced 1996)

Betacam SX was introduced by Sony as a cheaper alternative to Digibeta. Although tapes are fairly robust, this format is no longer manufactured. Consider migrating to a hard drive based system.

HDCAM (introduced 1997)

HDCAM is a high definition version of Digibeta. The format is commonly used for Sony's CineAlta range of professional cameras. HDCAM is a robust and currently well-supported format. Ultimately though, rapid advancements in HD digital videocassette technology make HDCAM an uncertain format for long-term archival use. Although low priority, consider migrating to a hard drive based system.

D-VHS / Digital VHS (introduced 1998)

This final format in the VHS family was aimed at the domestic user but was not able to compete with DVD. D-VHS cassettes look identical to the analogue S-VHS format but have a more robust tape inside. As D-VHS was not a commercial success and never widely adopted cassettes should be treated as a migration priority, consider migrating to a hard drive based system.

Digital8 (D-8) (introduced 1999)

Digital8 was a consumer camcorder format which built on the popularity of the analogue Hi8 format, in fact the two media types are physically identical. Although cassettes are still available the format largely lost out to mini-DV. This factor, coupled with a fragile cassette design makes migration a priority. Consider migrating to a hard drive based system.

MicroMV (introduced 2001)

A now unsupported format intended to rival mini-DV and aimed at consumer camcorder use. As with Digital8, the unpopularity of the format coupled with a fragile cassette design makes migration a priority. Consider migrating to a hard drive based system.

MPEG IMX (introduced 2001)

A later development of the Digibeta format aimed at the television production industry. Cassettes and players are still readily available but as with HDCAM, rapid advancements in HD digital videocassette technology make MPEG IMX an uncertain format for long-term archival use. Although low priority, consider migrating to a hard drive based system.

HDCAM SR (introduced 2003)

HDCAM SR is a higher performance derivative of HDCAM (see above) similarly aimed at the television production industry. HDCAM technology is still widely used and supported. Again, rapid advancements in HD digital videocassette technology make HDCAM SR an uncertain format for long-term archival use. Another low priority, but consider migrating to a hard drive based system.

Optical discs

VCD (SVCD (Super VCD), XSVCD, RSVCD, MVCD, CVD, Chaoji Video CD, EVD, HVD, FVD, MovieCD) (introduced 1985)

A whole host of data structures have been employed in order to store and playback video data on compact disc (CD). Beginning with Video Compact Disc (VCD) in the 1980s the development of the format was largely abandoned when DVD-video was introduced. Although video content can often be retrieved from VCD family discs, some data structures offered interactivity. This functionality may now be difficult to emulate on contemporary systems.
CD-based formats can suffer from de-lamination, physical damage and ‘CD-rot' (chemical deterioration). Consider migrating to a hard disc based system.

DVD-video / MiniDVD (introduced 1995)

Successor to the VCD family of discs, the DVD (Digital Versatile Disc) has been a great success and largely replaced both VCD and analogue formats such as VHS as the preferred domestic video format. While the data structure has remained intact through successive generations of DVD-videos, the actual physical substrate has developed considerable from the DVD cartridge (DVD-ROM) through single layered DVD+R and DVD-R to dual-layered discs. Later generations are theoretically backwards compatible with the exclusion of DVD-ROM cartridges. As with CDs, DVDs can suffer from de-lamination, physical damage and ‘DVD-rot'. Migrating to a hard disc based system will allow instant access and automated error checking. As with VCDs some interactivity maybe difficult to preserve in the long-term. Although many early digitisation projects used DVD technology as an archival storage medium, the above factors coupled with a typically high compression rate make DVD an unsuitable format for preservation purposes but an effective local, off-line delivery format.

HD DVD (introduced 2006)

High definition DVDs were high capacity optical discs almost solely used for video content. The format was launched as a competitor to Blu-ray but is now obsolete. HD DVD suffers from all of the same vulnerabilities and disadvantages of DVD (see above) but the need for migration is more pressing due to lack of manufacturer's support.

Blu-ray (introduced 2006)

Blu-ray looks set to become the dominant high definition optical disc video format. Although Blu-ray technology is currently well supported there are clear advantages to migrating video data to a hard drive system, though this action should not be seen as a priority. As with DVD, Blu-ray makes an excellent local delivery system but relatively high compression rates make it unsuitable as an archival preservation storage medium.

XDCAM (Professional Disc) / XDCAM HD (introduced 2003)

The Professional Disc format was adopted by Sony as the medium for several high-end video cameras. Little is know about the physical robustness of Professional Discs but these re-writable discs are not primarily intended for long-term storage. This suggests the need to migrate content to a hard rive based system.

UMD (Universal Media Disc) (introduced 2005)

This 64mm optical disc video format was launched by Sony for use with the Play Station Portable video game system. The disc is housed inside a plastic caddy which, some users have reported to be brittle and easily broken. High compression, a proprietary nature and possible fragility mean UMD is not suitable for archival usage.

HD VMD (Versatile Multilayer Disc) (introduced 2006)

HD VMD is a third contender attempting to become the high definition domestic format of choice. Although not discontinued like HD DVD, the format has little take-up when compared to Blu-ray. HD VMD is not a suitable archival format with all of the vulnerabilities of the DVD format.

Solid-state

XDCAM EX (SxS Solid-state) (introduced 2003)

While the data structure of XDCAM EX is similar to XDCAM (Professional Disc, see above) and the target audience the same, XDCAM EX is a solid-state alternative. While solid-state technology is not yet proven for long-term storage, it is likely to prove an excellent archival medium. XDCAM EX though is an extremely specialised solid-state video format intended for content gathering and subsequent transfer.

Digital Cinema (introduced 2005)

Digital cinema is a recently agreed upon set of standards which is fast achieving prominence as traditional film-based cinemas turn to digital alternatives. ‘Digital cinema packages' of moving image, audio and metadata are distributed either via broadcast or on hard drive. These hard drives, while undoubtedly of excellent robustness and performance are formatted specifically for distribution and as such are not intended to be used for archival purposes.

Of course, any digital storage media could potentially be used to hold video data, just as it could be used to hold documents or digital photographs. As with media used solely for digital video, such ‘data carrier' formats (such as floppy disks or Zip disks) should be closely monitored for obsolescence and condition problems and appropriate remedial action taken.

Conclusion

Failure to regularly inspect a digital video collection can result in having to source obsolete playback equipment and carry out a large-scale transcoding project which is virtually indistinguishable from a large-scale digitisation project.
The strength of digital material lies in its ability to be replicated indefinably, each generation (theoretically) identical to the proceeding one. In order to take advantage of this strength, digital preservation must remain an active process, drawing together regular cycles of obsolescence and error checking, refreshment (copying data to the same media type e.g. CD-ROM to CD-ROM) and migration (copying data to newer media types e.g. CD-ROM to DVD-ROM) into a cohesive policy backed by institutional financial commitment.
While this task can seem daunting, JISC Digital Media has produced a series of advice documents, intended to help staff sustainably manage a digital video collection, whether it be digitised, or born-digital.

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