Objects Principle 2

Objects Principle 2: A good object is preservable.

That is, the object will not raise unnecessary barriers to remaining accessible over time despite changing technologies.

Anyone who has ever used a Wang Word Processor or a 5.25" floppy disk knows the life spans of media, hardware, software platforms, and digital file formats are notoriously short. A digital object that is perfectly usable today may be unusable in the future unless some preservation action is taken. There are many strategies being tested for use in the preservation of digital objects. Two of the most widely discussed are migration and emulation.

Migration involves transforming objects so they can move between technical regimes as those regimes change. Migration occurs at all levels, as objects are moved:

  • across media as media evolve (e.g., from CD to DVD); 
  • across software products as the products become outmoded (e.g., from one version of a spreadsheet to another); and, 
  • across formats or encodings as new standards emerge (e.g., from SGML to XML, or from JPEG to JPEG2000). 

Emulation involves reproducing on contemporary systems the computer environment in which digital objects were originally created and used. Programs can be written to emulate obsolete hardware, device drivers, and operating systems. Emulation strategies may be particularly appropriate for executables and complex multimedia objects such as interactive learning modules.

Migration and emulation should be seen as complementary approaches. Neither is appropriate for all types of materials, and many preservation strategies combine aspects of emulation and migration.

Master objects should be created whenever possible with digital preservation in mind. Although no digital format will last forever, certain qualities will improve the chances that a digital object can be successfully carried forward into the future. When possible, choose formats that are non-proprietary and do not contain patented technologies. Formats that are widely used and have published specifications are most likely to have migration paths. Prefer formats that allow embedded metadata and have few external dependencies. 

Preservation masters of retrospectively digitized materials should be as close to the source version as possible. This generally means using a high resolution or sampling rate. Master files should not contain access inhibitors like watermarks or encryption, and should not be compressed with proprietary or lossy compression schemes. Whenever possible, embed everything needed to render the object in the object itself; for example, PDF files should always embed their fonts. Selecting file formats for preservability:

The actual provision of migration or emulation services is expected to require detailed information about the format characteristics and the hardware and software environment required to support it. Because the process of gathering and maintaining this information is so complex, a distributed network of authoritative format information registries is expected to emerge.

Format registries:

  • The National Archives (U.K.), PRONOM website http://www.nationalarchives.gov.uk/pronom/. PRONOM is an online registry of information about file formats, and the hardware platforms and software applications that support them. 
  • Global Digital Format Registry website http://hul.harvard.edu/gdfr/about.html. The GDFR, still under development, “will provide sustainable distributed services to store, discover, and deliver representation information about digital formats.” The website links to the project’s wiki, with detailed working documents. 
  • Digital Curation Center, Representation Information Registry Repository website http://registry.dcc.ac.uk/omar/

There is a large and growing body of literature on the preservation of digital material. Here are a few starting points:


Last updated: 09/03/2008