Introduction to the First Edition
Digital audio has, over the past few years, reached a level of development that makes it both effective and affordable for use in the preservation of audio collections of every magnitude. The integration of audio into data systems, the development of appropriate standards, and the wide acceptance of digital audio delivery mechanisms have replaced all other media to such an extent that there is little choice for sound preservation except digital storage approaches. Digital technology offers the potential to provide an approach that addresses many of the concerns of the archiving community through lossless cloning of audio data through time. However, the processes of converting analogue audio to digital, transferring to storage systems, of managing and maintaining the audio data, providing access and ensuring the integrity of the stored information, present a new range of risks that must be managed to ensure that the benefits of digital preservation and archiving are realised. Failure to manage these risks appropriately may result in significant loss of data, value and even audio content.
This publication of the Technical Committee of the International Association of Sound and Audiovisual Archives (IASA) "Guidelines on the Production and Preservation of Digital Audio Objects" is intended to provide guidance to audiovisual archivists on a professional approach to the production and preservation of digital audio objects. It is the practical outcome of the previous IASA Technical Committee paper, IASA-TC 03 "The Safeguarding of the Audio Heritage: Ethics, Principles and Preservation Strategy, Version 2, September 2001". The Guidelines addresses the production of digital copies from analogue originals for the purposes of preservation, the transfer of digital originals to storage systems, as well as the recording of original material in digital form intended for long-term archival storage. Any process of digitisation is selective, the audio content itself supplies more information to potential users than is contained in the intended signal and the standards of analogue to digital conversion fix the limits of the resolution of the audio permanently and, unless carefully undertaken, partially.
There are three main parts to the content of the Guidelines:
- Standards, Principles and Metadata
- Signal Extraction from Originals
- Target Formats
Standards, Principles and Metadata: Of the four basic tasks that are performed by all archives - acquisition, documentation, access, preservation, the primary task is to preserve the information placed in the care of the collection (IASA-TC 03, 2001). However, if the tasks of acquisition and documentation are undertaken in combination with a well planned digital preservation strategy that adheres to adequate standards, the task of providing access is facilitated by the process. Long term access is a product of appropriate preservation.
Adherence to widely used and accepted standards suitable for the preservation of digital audio is a fundamental necessity of audio preservation. The IASA Guidelines recommend linear PCM (pulse code modulation) (interleaved for stereo) in a .wav or preferably BWF .wav file (EBU Tech 3285) for all two track audio. The use of any perceptual coding (“lossy compression") is strongly discouraged. It is recommended that all audio be digitised at 48 kHz or higher, and with a bit depth of at least 24 bit. Analogue to Digital (A/D) conversion is a precision process, and low cost converters integrated into computer sound cards cannot meet the demands of archival preservation programs.
Once encoded as a data file, the preservation of the audio faces many of the same issues as those of all digital data, and foremost in managing these is the assigning of a unique Persistent Identifier (PI) and providing appropriate metadata. Metadata is not just the descriptive information that allows the user or archive to identify the content, but also includes the technical information that enables the recognition and replaying of the audio, and the preservation metadata that retains information about the processes that went to generate the audio file. It is only thus that the integrity of the audio content can be guaranteed. The digital archive will depend on comprehensive metadata to maintain its collection. A well planned digital archive will automate the production of much of the metadata and should include the original carrier, its format and state of preservation, replay equipment and parameters, the digital resolution, format, all equipment used, the operators involved in the process and any processes or procedures undertaken.
Signal Extraction from Originals: “Sound archives have to ensure that, in the replay process, the recorded signals can be retrieved to the same, or a better, fidelity standard as was possible when they were recorded... (also) carriers are the bearers of information: desired or primary information, consisting of the intended sonic content, and ancillary or secondary information which may take manifold forms. Both primary and secondary information form part of the Audio Heritage.” (IASA-TC 03, 2001).
To take full advantage of the potential offered by digital audio it is necessary to adhere to the above principles and ensure that the replay of the audio original is made with a full awareness of all the possible issues. This requires knowledge of the historic audio technologies and a technical awareness of the advances in replay technology.Where appropriate, the Guidelines provide advice on the replay of historical mechanical and other obsolete formats, including, cylinders and coarse groove discs, steel wire and office dictation systems, vinyl LP records, analogue magnetic tape, cassette and reel, magnetic digital carriers such as DAT and its video tape based predecessors, and optical disk media such as CD and DVD. For each of the formats there is advice on selection of best copy, cleaning, carrier restoration, replay equipment, speed and replay equalisation, corrections for errors caused by misaligned recording equipment and removal of storage related signal artefacts and the time required to undertake a transfer to digital. All of these are important topics which are addressed in the Guidelines with a consideration of the associated ethical issues, though the latter issue is particularly significant as many digitisation plans fail to budget for the considerable time constraints of an audio transfer process.
All the parameters mentioned above must be determined objectively, and appropriate records kept of every process. Digital storage and associated technologies and standards enable an ethical approach to sound archiving by enabling the production of documentation and its storage in linked, related metadata fields.
Target Formats: Data can be stored in many ways and on many carriers and the appropriate type of technology will be dependant on the circumstances of the institution and its collection. The Guidelines provide advice and information on various suitable approaches and technologies including Digital Mass Storage Systems (DMSS) and small scale manual approaches to digital storage systems, data tape, hard disks, optical disks including CD and DVD recordable and magneto optical disks (MO).
No target format is a permanent solution to the issue of digital audio preservation, and no technological development will ever provide the ultimate solution; rather they are a step in a process whereby institutions will be responsible for maintaining data through technological changes and developments, migrating data from the current system to next for as long as the data remains valuable. The DMSS with suitable management software are the most appropriate for the long term maintenance of audio data.”Such systems permit automatic checking of data integrity, refreshment, and, finally, migration with a minimum use of human resources” (IASA-TC 03, 2001). These systems can be scaled to suit smaller archives, though this will most often result in an increased responsibility for manual data checking. Discrete storage formats such as CD and DVD recordable, and magneto optical disks (MO) are inherently less reliable. The Guidelines suggest standards and approaches to maintaining the data on these carriers, while recommending that the more reliable solutions found in integrated storage systems are to be substantially preferred.