AHDS Preservation Handbook Arts and Humanities Data Service http://www.ahds.ac.uk/ Page 1 of 8 File: presVideo_d4.doc File Author: Gareth Knight File Date: 28/07/2005 18:14 Preservation Handbook Moving Image Author Gareth Knight & John McHugh Version 1 Date 27 July 2005 Change HistoryAHDS Preservation Handbook Arts and Humanities Data Service http://www.ahds.ac.uk/ Page 2 of 8 File: video-preservation-handbook.rtf File Author: Gareth Knight File Date: 28/07/2005 18:14 Definition Moving image data is composed of a sequence of still images that, when shown in succession, create the impression of motion. It is often accompanied by audio that may be synchronised with a specific time point. Examples of moving images include animations, movies, television programs, and other visual output. This document describes platform-independent formats, such as MPEG and Quicktime. Desktop Video Moving image formats for use in the desktop video market are developed within particular emphasis upon playback and compression rates. The internal structure is a deciding factor in assessing the capabilities of a file format. Th e majority of formats share a common structure, consisting of four strata: moving image; audio data; and information necessary to decode (header) and describe (metadata) the data stream. Some formats constrain the type of information that may be encoded within the file (MPEG-1, MPEG-2), while others (QuickTime, AVI, RealMedia, MPEG4) serve as ‘wrappers’ that allow a degree of flexibility regarding the encoding method. Compression rates often vary according to the algorithm in use, using temporal, spatial, lossy, lossless, symmetrical or asymmetrical calculations to compress video to a much smaller size. The majority of common video encoding formats are ‘lossy’, that operate by storing a key frame and recording subsequent changes to the image. This produces highly compressed data files, for example, a 450Mb MPEG-1 video may be compressed to a 180Mb Divx file. However, these files are difficult to edit and result in some loss of quality. Unlike analogue videotape, that stores audio and video data concurrently, the distinction between the audio and video elements is important. Synchronicity is an issue when editing these files, often resulting in the video becoming out of sync with the audio – a character will move their mouth, but the words may be delayed by two seconds. However, the independence has one benefit – it enables developers to use their own video compression formats without adverse effects upon the audio data, and vice versa. Preservation Policy The rendering of moving image data is often associated with end-user requirements – a moving image file may be played through appropriate software, rendering the byte stream as a series of images to be displayed and with accompanying sound. This should not be limited to specific hardware devices and must be feasible for users of current desktop computers. Guidance on the preservation of digital video should, by necessity, change over time. MPEG-1 is suitable for low-quality video. It is documented, may be manipulated using free software tools, and can be played on current computers and digital video players without additional software. However, it imposes restrictions in terms of resolution and dimensions, which may prove restrictive for born-digital content. The MPEG-2 and MPEG-4 formats are better suited to high-quality digital video. MPEG-2 is better known for its use as a format for DVD-Video, which encourages confidence when considering the likelihood that the format will be readable in the long-term. The format has an average transfer rate of 2-5 megabits per second, but there may be disk space restraints and the software tools necessary to convert and store this format are costly. MPEG-4 has a lower transfer rate of 1-2 megabits per second and is intended for streaming video. Other codecs, such as QuickTime, Windows Media, Real Video and Open DIVX, are useful for specific purposes, but not suitable for preservation. For details on the preservation of audio data, please consult the relevant handbook. If audio data is contained within the same digital file as the video data, it should be migrated to a format suitable for preservation and resaved. If this is not possible (i.e. the digital wrapper is capable of using only its own proprietary codec), the digital audio should be output as a separate digital audio file. This should be noted in the work log, along with details of any sequencing issues (e.g. audio must begin on the 5129 frame of the video) necessary to recombine it at a later date.AHDS Preservation Handbook Arts and Humanities Data Service http://www.ahds.ac.uk/ Page 3 of 8 File: video-preservation-handbook.rtf File Author: Gareth Knight File Date: 28/07/2005 18:14 Additional Information • An introduction to Digital Video < http://www.doc.ic.ac.uk/~nd/surprise_96/journal/vol1/sab/article1.html > Last checked 25/07/2005 • The DV, DVCAM, & DVCPRO Formats < http://www.adamwilt.com/DV.html > Last checked 25/07/2005 • DV format information < http://www.adamwilt.com/DV -tech.html > Last checked 25/07/2005 • Moving Picture Experts Group (MPEG) website < http://www.chiariglione.org/mpeg/ > Last checked 25/07/2005 • Divx < http://www.divx.com > Last checked 25/07/2005 • NINCH Guide to Good Practice < http://www.nyu.edu/its/humanities/ninchguide/VII/ > Last checked 25/07/2005 • General info on Video File Extensions < http://graphics.csail.mit.edu/~tbuehler/video/extensions.html > Last checked 25/07/2005 • Worldwide TV standards: A Web Guide < http://www.ee.surrey.ac.uk/Contrib/WorldTV/ > Last checked 25/07/05AHDS Preservation Handbook Arts and Humanities Data Service http://www.ahds.ac.uk/ Page 4 of 8 File: video-preservation-handbook.rtf File Author: Gareth Knight File Date: 28/07/2005 18:14 Technical Environment Desktop video data may be played on many current operating systems. The level of entry, in terms of hardware requirements necessary to decode the data and play it in real time, increases according to the complexity of the format. Migration and editing of these formats will require a faster machine, in order to complete the operation in a timely fashion. Common Formats Contemporary codecs, such as MPEG-4 offer significantly improved capabilities, in terms of smaller size and higher quality audio and video. However, a faster machine is