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Non-standard formats, resolutions and versions may not in the future be included in the preservation pathways that will enable long term access and future format migration. When producing digital copies of analogue material IASA recommends a minimum sampling rate of 48 kHz for any material. However, higher sampling rates are readily available and may be advantageous for many content types. Although the higher sampling rates encode audio outside of the human hearing range, the net effect of higher sampling rate and conversion technology improves the audio quality within the ideal range of human hearing.
The unintended and undesirable artefacts in a recording are also part of the sound document, whether they were inherent in the manufacture of the recording or have been subsequently added to the original signal by wear, mishandling or poor storage. Both must be preserved with utmost accuracy. For certain signals and some types of noise, sampling rates in excess of 48 kHz may be advantageous. IASA recommends 96 kHz as a higher sampling rate, though this is intended only as a guide, not an upper limit; however, for most general audio materials the sampling rates described should be adequate.
For audio digital-original items, the sampling rate of the storage technology should equal that of the original item. IASA recommends an encoding rate of at least 24 bit to capture all analogue materials. For audio digital-original items, the bit depth of the storage technology should at least equal that of the original item. It is important that care is taken in recording to ensure that the transfer process takes advantage of the full dynamic range. It is the most critical component in the digital preservation pathway.
The external synchronistation circuit must reject incoming jitter so that the synchronised sample rate clock is free from artefacts and disturbances. Both systems are successfully implemented as audio transmission interfaces across the major personal computer platforms, and can reduce the requirement to install a specialised, high-quality soundcard interface in the computer chassis.
Audio quality is generally independent of the bus technology in use. When choosing a convertor, and before any further evaluation is undertaken, IASA recommends that all specifications are tested against the reference standards described above. Any converter which does not meet the basic IASA technical specifications will produce less than accurate conversions. In conjunction with technical evaluation, statistically valid blind listening tests should be carried out on short listed converters to determine overall suitability and performance.
All the specifications and testing described above are stringent and complex, and these specifications are highly important in selecting and evaluating analogue to digital convertors. The published specifications from the equipment manufacturers are sometimes challenging to compare, often incomplete and occasionally difficult to reconcile with the performance of the device they purport to represent.
It may suit certain communities or groups to undertake group or panel testing to maximise resources. Certain institutions, such as state archives, libraries or academic science departments may be in a position to assist with testing.
Care should be taken in choosing a card that accepts the appropriate sampling and bit rates, and does not inject noise or other extraneous artefacts. IASA recommends the use of a high quality sound card that meets the following specification: 2. Once in the digital domain, the integrity of the audio files should be maintained.
However, some systems truncate the word length of an item in order to process it, resulting in a lower effective bit rate and others may only process compressed file formats, such as MP3, neither of which is acceptable. IASA recommends that a professional audio computer based system be used whose processing word length exceeds that of the file i.
Transfers employing such data reduction mean that parts of the primary information are irretrievably lost. The results of such data reduction may sound identical or very similar to the unreduced linear signal, at least for the first generation, but the further use of the data reduced signal will be severely restricted and its archival integrity has been compromised. Wave files are widely used in the professional audio industry. The benefit of BWF for both archiving and production uses is that metadata can be incorporated into the headers which are part of the file.
In most basic exchange and archiving scenarios this is advantageous; however, the fixed nature of the embedded information may become a liability in large and sophisticated data management systems see discussion chapter 3 Metadata and Ch 7 Small Scale Approaches to Digital Storage Systems. This, and other limitations with BWF, can be managed by using only a minimal set of data within BWF and maintaining other data with external data management systems. As these are all still under development, one pragmatic approach may be to create multiple time coherent mono BWF files wrapped in the tar tape archive format.
The replay equipment, audio path, target format and standards must exceed that of the original carrier.
Recommandations pour la production et la conservation des objets audionumériques
IASA TC04 PDF
2: Key Digital Principles