As stated throughout this guide and by Knight (2005), "the moment of creation is the most important stage of digital audio preservation" and, where possible the original file should be preserved in a format which captures and maintains its significant properties (discussed below) at an equal or higher value. In addition to the physical characteristics of the file, any associated metadata and documentation (either embedded or stored separately) should also be preserved and maintained.
The significant properties of audio files that should be maintained throughout preservation activities are discussed in detail in 'Significant Properties Testing Report: Audio Recordings.' (Knight 2010, 8-10). These are summarised below:
The file formats outlined below are those recommended for the preservation of audio files. As highlight by the JISC Digital Media document 'Uncompressed Audio File Formats ', uncompressed audio formats provide "the most accurate digital representation of a soundwave, but can also be the most resource-intensive method of recording and storing digital audio". In light of this, uncompressed formats/codecs are highly recommended for the preservation of audio files but, where this is not possible, high quality lossless compression may be used (e.g. FLAC).
|Waveform Audio (.wav)||Recommended for preservation using uncompressed audio (PCM). The format can also be tagged with metadata and can embed metadata in the XMP format.|
|Broadcast Wave Format (BWF) (.bwf .wav)||Recommended for preservation, BWF also extends the WAV format with an additional 'chunk' for metadata although users should be aware that compatibility between WAV and BWF files can be problematic, particularly when migrating WAV to BWF.|
|Audio Interchange File Format (.aif, .aiff)||Suitable for preservation when used for storing uncompressed PCM audio files. The format is also capable of storing embedded metadata.|
|FLAC (.flac)||An open and free lossless compression codec, suitable for preservation when uncompressed audio is not desirable.|
A number of different metadata schemes exist for audio files and the data type is notable in that many formats allow metadata to be embedded within the file itself. The metadata elements descibed below cover primaril technical aspects and form a minimum of what should be recorded. Generic Project Metadata should also be recorded.
|Software||The software (or device) used to create the file.|
|Bit Depth||E.g. 16 or 24 bit.|
|Bit Rate||Optional, often recorded as kbps|
|Sample Rate (KHz)||E.g. 44.1kHz|
|Codec Used||The codec used in creating the file e.g. FLAC or AAC.|
|Length of Recording||Preferably recorded in hours, minutes, seconds (hh:mm:ss).|
|Copyright Clearances||These are very important for audio files, especially oral histories/interviews.|
|Transcriptions of interviews||Transcripts of interviews can be important documentation particularly in clarifying those involved in recordings and allowing specific individuals to be identified.|
As highlighted in Section 2, data creators should be aware of the various rights involved in recording people and should ensure that the relevant rights and clearances are attained. These issues are discussed in detail in the JISC Digital Media document 'Copyright and Other Rights for Creating Time-based Media Resources'.