|Introduction | Sustainability Factors | Content Categories | Format Descriptions | Contact|
Table of Contents
• External dependencies
• Impact of patents
• Technical protection mechanisms
Overview of factors
Additional factors will come into play relating to the ability to represent significant characteristics of the content. These factors reflect the quality and functionality that will be expected by future users. These factors will vary by genre or form of expression for content. For example, significant characteristics of sound are different from those of still pictures, whether digital or not, and not all digital formats for images are appropriate for all genres of still pictures. These factors are discussed in the sections of this Web site devoted to particular Content Categories.
A spectrum of disclosure levels can be observed for digital formats. Non-proprietary, open standards are usually more fully documented and more likely to be supported by tools for validation than proprietary formats. However, what is most significant for this sustainability factor is not approval by a recognized standards body, but the existence of complete documentation, preferably subject to external expert evaluation. The existence of tools from various sources is valuable in its own right and as evidence that specifications are adequate. The existence and exploitation of underlying patents is not necessarily inconsistent with full disclosure but may inhibit the adoption of a format, as indicated below. In the future, deposit of full documentation in escrow with a trusted archive would provide some degree of disclosure to support the preservation of information in proprietary formats for which documentation is not publicly available. Availability, or deposit in escrow, of source code for associated rendering software, validation tools, and software development kits also contribute to disclosure.Back to top
Evidence of wide adoption of a digital format includes bundling of tools with personal computers, native support in Web browsers or market-leading content creation tools, including those intended for professional use, and the existence of many competing products for creation, manipulation, or rendering of digital objects in the format. In some cases, the existence and exploitation of underlying patents may inhibit adoption, particularly if license terms include royalties based on content usage. A format that has been reviewed by other archival institutions and accepted as a preferred or supported archival format also provides evidence of adoption.Back to top
Transparency is enhanced if textual content (including metadata embedded in files for non-text content) is encoded in standard character encodings (e.g., UNICODE in the UTF-8 encoding) and stored in natural reading order. For preserving software programs, source code is much more transparent than compiled code. For non-textual information, standard or basic representations are more transparent than those optimized for more efficient processing, storage or bandwidth. Examples of direct forms of encoding include, for raster images, an uncompressed bit-map and for sound, pulse code modulation with linear quantization. For numeric data, standard representations exist for signed integers, decimal numbers, and binary floating point numbers of different precisions (e.g., IEEE 754-1985 and 854-1987, currently undergoing revision).
Many digital formats used for disseminating content employ encryption or compression. Encryption is incompatible with transparency; compression inhibits transparency. However, for practical reasons, some digital audio, images, and video may never be stored in an uncompressed form, even when created. Archival repositories must certainly accept content compressed using publicly disclosed and widely adopted algorithms that are either lossless or have a degree of lossy compression that is acceptable to the creator, publisher, or primary user as a master version.
The transparency factor relates to formats used for archival storage of content. Use of lossless compression or encryption for the express purpose of efficient and secure transmission of content objects to or from a repository is expected to be routine.Back to top
The value of richer capabilities for embedding metadata in digital formats has been recognized in the communities that create and exchange digital content. This is reflected in capabilities built in to newer formats and standards (e.g., TIFF/EP, JPEG2000, and the Extended Metadata Platform for PDF [XMP]) and also in the emergence of metadata standards and practices to support exchange of digital content in industries such as publishing, news, and entertainment. Archival institutions should take advantage of, and encourage, these developments. The Library of Congress will benefit if the digital object files it receives include metadata that identifies and describes the content, documents the creation of the digital object, and provides technical details to support rendering in future technical environments. For operational efficiency of a repository system used to manage and sustain digital content, some of the metadata elements are likely be extracted into a separate metadata store. Some elements will also be extracted for use in the Library's catalog and other systems designed to help users find relevant resources.
Many of the metadata elements that will be required to sustain digital objects in the face of technological change are not typically recorded in library catalogs or records intended to support discovery. The OAIS Reference Model for an Open Archival Information System recognizes the need for supporting information (metadata) in several categories: representation (to allow the data to be rendered and used as information); reference (to identify and describe the content); context (for example, to document the purpose for the content's creation); fixity (to permit checks on the integrity of the content data); and provenance (to document the chain of custody and any changes since the content was originally created). Digital formats in which such metadata can be embedded in a transparent form without affecting the content are likely to be superior for preservation purposes. Such formats will also allow metadata significant to preservation to be recorded at the most appropriate point, usually as early as possible in the content object's life cycle. For example, identifying that a digital photograph has been converted from the RGB colorspace, output by most cameras, to CMYK, the colorspace used by most printing processes, is most appropriately recorded automatically by the software application used for the transformation. By encouraging use of digital formats that are designed to hold relevant metadata, it is more likely that this information will be available to the Library of Congress when needed.Back to top
This factor is primarily relevant for categories of digital content beyond those considered in more detail in this document, for which static media-independent formats exist. It is however worth including here, since dynamic content is likely to become commonplace as part of electronic publications. The challenge of sustaining dynamic content with such dependencies is more difficult than sustaining static content, and will therefore be much more costly.Back to top
Impact of patents
The core components of emerging ISO formats such as JPEG2000 and MPEG4 are associated with "pools" that offer licensing on behalf of a number of patent-holders. The license pools simplify licensing and reduce the likelihood that one patent associated with a format will be exploited more aggressively than others. However, there is a possibility that new patents will be added to a pool as the format specifications are extended, presenting the risk that the pool will continue far longer than the 20-year life of any particular patent it contains. Mitigating such risks is the fact that patents require a level of disclosure that should facilitate the development of tools once the relevant patents have expired.
The impact of patents may not be significant enough in itself to warrant treatment as an independent factor. Patents that are exploited with an eye to short-term cash flow rather than market development will be likely to inhibit adoption. Widespread adoption of a format may be a good indicator that there will be no adverse effect on the ability of archival institutions to sustain access to the content through migration, dynamic generation of service copies, or other techniques.Back to top
Technical protection mechanisms
No digital format that is inextricably bound to a particular physical carrier is suitable as a format for long-term preservation; nor is an implementation of a digital format that constrains use to a particular device or prevents the establishment of backup procedures and disaster recovery operations expected of a trusted repository.
Some digital content formats have embedded capabilities to restrict use in order to protect the intellectual property. Use may be limited, for example, for a time period, to a particular computer or other hardware device, or require a password or active network connection. In most cases, exploitation of the technical protection mechanisms is optional. Hence this factor applies to the way a format is used in business contexts for particular bodies of content rather than to the format.
The embedding of information into a file that does not affect the use or quality of rendering of the work will not interfere with preservation, e.g., data that identifies rights-holders or the particular issuance of a work. The latter type of data indicates that this copy of this work was produced for an specific individual or other entity, and can be used to trace the movement of this copy if it is passed to another entity.
1 For examples of Lorie's treatment of this subject, see his "Long Term Preservation of Digital Information" in E. Fox and C. Borgman, editors, Proceedings of the First ACM/IEEE Joint Conference on Digital Libraries (JCDL'01), pages 346-352, Roanoke, VA, June 24-28 2001, http://doi.acm.org/10.1145/379437.379726; and The UVC: a Method for Preserving Digital Documents: Proof of Concept (December 2002), http://www.kb.nl/hrd/dd/dd_onderzoek/reports/4-uvc.pdf.
Back to top