Sustainability of Digital Formats
 Planning for Library of Congress Collections

The illustrative tables presented here are intended to suggest how a curator of moving image content would determine format preferences. The first table illustrates a planning matrix that would result from analyzing the significant or essential characteristics for content subcategories. The second table illustrates how this analysis of significant features would be combined with technical information about formats to produce a set of format-preference statements for the subcategories.

Table 1: Significant characteristics of moving image content subcategories

1. Generally speaking, this characteristic is associated with surround sound, although it may also pertain to multi-channel audio (e.g., narration available in English and French). There is related interest in metadata that offers a map of the channels.

2. Reduction to two-loudspeaker rendering should be feasible with appropriate software. Normalization to stereo may be appropriate for content that has a more complex sound field originally but where this particular characteristic is not deemed necessary for retention. 

3. Normal rendering means playback with picture on a single screen and sound in mono or stereo through one or two speakers (or equivalent headphones). Associated software provides user control over some picture elements (brightness, hue, contrast) and over some sound elements (volume, tone, balance) and also navigation features (fast forward, go-to-segment, etc.). Normal rendering would also allow playback through software that allows sound picture and sound analysis and excerpting. Normal rendering must not be limited to specific hardware models or devices and must be feasible for current users and future users and scholars.

4. Normal functionality for custodians includes the ability to preserve digital content and provide service to users and designated communities now and decades. Thus custodians must be able to replicate the content on new media, migrate and normalize it in the face of changing technology, and disseminate it to users at a resolution consistent with network bandwidth constraints.

5. Technical protection must not prevent custodians from taking appropriate steps to preserve the digital content and make it accessible to future generations. See Notes 3 and 4.

6. In contrast, moving image files harvested from the Web through a program targeted specifically at video capture would be considered as M2 through M5.

7. Guidelines from custodians and user communities will be needed to determine when the functionality inherent in a encoded representation is an essential characteristic or when the composition should be preserved as bitmapped-image video. If saved as bitmapped video, computer-generated animations fall into categories M2 or M3.

Table 2: Format preferences for moving image subcategories

1. Some of the formats listed here include elements that may be employed for technological protection. If implemented, such protections may defeat preservation. Thus the assignment of preferred or acceptable status to these formats assumes that protections have not been implemented or that the Library is in a position to overcome them.

This table excludes media-dependent formats (e.g., DVD disks) as inappropriate as part of a general strategy for long-term preservation of digital content.

2. Surround sound is important to retain when it is a key element of the artist's intent (e.g. sounds intended to move around in the performance space). However, for most content, reduction to stereo is appropriate.

3. The production and management of uncompressed and losslessly compressed video is very much an evolving field, and this preference is, in fact, tentative and provisional.

4. At this writing, MXF (Media Exchange Format) and MJP2_FF are listed illustratively. Comments welcome.

5. Quality will depend upon the degree of compression and specific encoding algorithms applied to moving image content. Thus the same compressed-video formats appear in this table under both "high-quality " and "low-quality" labels; the distinction is revealed by the file's data rate, not the format. Examples of the range of choices include the Public Broadcasting Service plan to considering archiving standard definition MPEG2 files encoded at about 50 mbps, for possible re-broadcasting; the U.S. Senate recording studio consideration of high definition MPEG2 files at about 100 mbps, as masters for many types of repurposing; and the Vanderbilt University's Television News Archive's decision to create standard definition MPEG2 files encoded at about 6 mbps, as service copies for researchers to consult.

6. Guidelines from custodians and user communities will be needed to determine when the functionality inherent in a encoded representation is an essential characteristic as compared to the determination that the content can be preserved in "frozen" or flattended form as bitmapped-image video. If saved as bitmapped video, computer-generated animations fall into categories M2 or M3.