What are digital objects?

A digital object is 'an object composed of a set of bit sequences' [CCSDS, 2002]. A digital object can be either 'born digital' or a 'digital surrogate' which is the result of digitising an analogue or physical object.

There are numerous benefits/incentives of digitizing analogue objects, namely:

• Improving access. Digitised items which are available on the Digital Mirror section of the Library’s website can be accessed anywhere at any time. Here you can find a wealth of material digitised by the Library.
• Assisting research. For example, by making the text in magazines and newspapers searchable, researchers can quickly identify sections of interest without having to browse through every issue. A recent Library project that has incorporated this functionality is the Welsh Journals Online.
• Preserving items. Digitisation means that original material may not have to be used as often by researchers. The Library holds many very old and fragile items and therefore digitising them acts to preserve and reduce access to the original item. Access to the original will only be allowed in exceptional cases. Examples of such material include the Black Book of Carmarthen, Welsh Bible 1588 and the National Anthem. There are also items that are more volatile than paper and therefore are at higher risk of degradation and potential loss such as negatives on cellulose acetate film bases. These often suffer from vinegar syndrome over time and pose a great threat to the negatives. Therefore in order to safeguard these images they are digitised and the negatives are then vacuum-sealed and frozen in order to slow down this effect. An example of such material is the photographs by Geoff Charles.

Digital objects can take 3 different forms:

• Simple digital objects, which primarily consist of a single file, that is intended to be viewed as one conceptual object, e.g. a Word document or a TIFF image. In some cases they are accompanied by metadata.

• Digital object groups, which consist of a set of independent but related files that have been collectively described, e.g. a floppy disk containing 100 letters. Each file is accessible independently (as a Simple object), but its relationship to other objects in the group provides valuable context.  An example is the photographs of Geoff Charles.

• Complex digital objects, which consist of a group of dependent files, that are intended to be viewed as a single conceptual object, e.g. a web site or a CD-ROM. 

File formats and Significant Properties

A file format is a particular way to encode information for storage in a computer file. The particular way in which a file format is structured and organized is often laid out in a document called a file format specification. This document provides the details necessary to construct a valid file of a particular type and to develop software applications that can decode and render such files. For example the TIFF 6.0 format specification can be seen here. 

Digital objects come in a wide variety of formats but in terms of digital preservation these digital objects need to be widely accepted and generally an open format standard for long-term preservation. The Library therefore takes great care in selecting a file format for preservation and is working on creating guidelines for depositors of accepted and preferred file formats. For images created by the Library as part of its digitisation programme the Library currently creates 3 copies of the image - a master TIFF file for preservation purposes and two smaller derivative files, a JPG for the reference or screen display and a GIF for the thumbnail version. In some instances where the resource benefits from zooming capabilities a PFF file is also created. 

Significant properties are essential attributes of a digital object that affect its appearance, behaviour, quality and usability. They can be grouped into categories such as:

• content
• context (metadata)
• appearance (e.g. layout, colour)
• behaviour (interaction, functionality)
• structure (e.g. pagination, sections) 

Significant properties must be preserved over time for the digital object to remain accessible and meaningful. 

Why is digital preservation important?

There has been a tremendous growth in the amount of digital material being created, stored and made available to us. The Library as custodian of this wealth of material must therefore take responsibility for their preservation. Digital materials, whether 'born digital' or converted to digital form, are at risk from technology obsolescence and physical damage or deterioration.

It is important that material be preserved so that it remains accessible for current and future generations, be that in analogue or digital format. Preserving digital material however is far more complex than preserving analogue material such as paper and consequently the preservation community is facing new challenges regarding the preservation of digital material. The following have to be taken in to consideration with digital material:

Technological obsolescence – can affect hardware, software and even the arrangement of the data in a stored file. Some examples of obsolescence include:

• File format is superseded by newer versions, which may no longer be supported by the current vendor or relevant standards body.

• Storage medium may be superseded by newer and denser version of that medium, or by new types of media.

• Device needed to read a storage medium may no longer be manufactured.

• Software used to create, manage or access digital content may be superseded by newer versions.

• Computers are continually being superseded by faster, more powerful machines with more capabilities.

Physical damage/deterioration – can affect hardware and the media carriers on which the digital material is stored. Some examples of physical damage/deterioration include:

• Hardware and media carriers can stop working due to human error, natural events and often just the passing of time.

Strategies for overcoming these obstacles include refresh, migration, replication and emulation. The Library uses a mixture of both refresh and migration in order to address these problems.

Links

• Timeline: Digital Technology and Preservation
• Obsolete and endangered media
• Survey of Digital Preservation in Wales: Summary of findings
  • Appendix 2 - Spreadsheet of responses