Deciding how to archive
The significant properties of video files are discussed in detail in the JISC report ‘The Significant Properties of Moving Images’ (Coyne & Stapleton 2008) as well as in the AHDS preservation handbook for moving images (Knight & McHugh 2005, 7). To summarise, the properties of digital video files that should remain unchanged when preserving or storing data are:
- the length and size of the file (e.g. 5min 31secs / 150MB)
- the frame rate in frames per second (e.g. 25 for PAL or 30 for NTSC)
- the frame size / video resolution (e.g. 720 x 576 pixels)
- the bit-rate (in kbps)
- audio bit-rate (kbps)
- audio frequency (kHz)
- audio channels used (e.g. stereo)
- associated metadata and documentation, and file size.
Prior to conversion it is advisable to check the video file against any documentation provided to ensure that the file is the specified length (minutes and seconds) and that its significant properties are correctly documented. This ensures that the file is complete and that it will no suffer any unplanned degradation while undergoing any planned file conversion.
The actual process of transcoding digital video – i.e. migrating data from one digital format to another – is described in detail on the JISC Digital Media site.
The formats described in the table below are recommended for the long-term preservation of digital video:
|MPEG 1 (.mpg,.mpeg)||A published open standard ISO/IEC standard (11172) suitable for preservation and dissemination.|
|MPEG 2 (.mpg,.mpeg)||As with MPEG-1, a published open standard (ISO/IEC (13818)) particularly suitable for preservation.|
|MPEG 4 (.mp4)||A published open standard ISO/IEC 14496-14:2003 suitable for data preservation and dissemination although a higher quality MPEG format should be used where appropriate.|
Motion JPEG 2000, as discussed in detail by Pearson & Gill (2005) may also provide a useful preservation format for digital video, especially in the case of digitisation projects, though research into the suitability of the format is still currently ongoing.