Deciding how to archive
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:
- Duration – i.e. the length of the audio file in Timecode character format (TCF). Checks should be made to ensure that the file matches its intended length.
- Bit Depth – indicates the number of bits of information stored per sample and is an indicator of audio quality e.g. 16 or 24 bit.
- Sample Rate – an indicator of the number of samples per second, sample rate is usually expressed in hertz e.g. 44.1 kHz (a common sample rate) and is, like bit depth, an indicator of the quality of the file.
- Channels – A descriptive or numeric value that indicates the number of distinct streams within an audio object or a description of their configuration e.g. “2 (stereo)”.
- Channel Assignment: Channel Number and Sound Map Location – Two values that allow specific channels to be mapped to specific output locations to configure the output sound.
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.|