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best practices, the development of much needed software tools for audio preservation, the
creation or further development of audio preservation systems at each institution, and the
preservation of a number of critically endangered and highly valuable recordings. All of
the above are detailed in this publication, which we believe provides solid grounding for
institutions pursuing audio preservation either in-house or in collaboration with an outside
vendor. For institutions actively engaged in preservation transfer work themselves, the project
created a number of software tools that may be placed into service. The development of
these tools reflects both the starting points in this project and the different interests of the two
institutions. Harvard University’s experience with the vast quantities of metadata required
for preservation led them to design and develop the Harvard Sound Directions Toolkit. The
toolkit is a suite of forty open-source, scriptable, command line interface, audio preservation
software tools that streamline workflow, reduce labor costs, and reduce the potential for
human error in the creation of preservation metadata and in the encompassing preservation
package. Harvard also produced Audio Object Manager for audio object metadata creation
and Audio Processing XML Editor (APXE) for collection of digital provenance metadata. To aid
selection for preservation, Indiana University developed the Field Audio Collection Evaluation
Tool (FACET), which is a point-based, open-source software tool for ranking field collections
for the level of deterioration they exhibit and the amount of risk they carry. Indiana also
developed the Audio Technical Metadata Collector (ATMC) software for collecting and storing
technical and digital provenance metadata for audio preservation. Harvard and Indiana are
making their software tools freely available to the preservation community beginning in the
fall of 2007, with the exception of ATMC, Audio Object Manager, and APXE, all of which
will be released later after further development. A download link for these tools will be
posted on the Sound Directions website. Many of the tools are referenced throughout this
document, and a complete listing of the Harvard Sound Directions Toolkit can be found in
Appendix 5 where each tool is described, and its use and options are listed. A user’s guide
for the current version of ATMC, with details on each metadata element, can be found in
Appendix 1. All of these tools are key ingredients in the audio preservation systems at each
institution, contributing to the enduring preservation of the recordings that are processed by
these systems. If we have done our work well, these recordings will speak for our efforts far
into the future.
1.2 Introduction to Institutions
1.2.1 Indiana University
The Archives of Traditional Music (ATM)
5
fosters the educational and cultural role of Indiana
University through the preservation and dissemination of the world’s music and oral
traditions. One of the largest and oldest university-based ethnographic sound archives in
the United States, the ATM’s holdings cover a wide range of cultural and geographical areas,
and include commercial and field recordings of vocal and instrumental music, folktales,
interviews, and oral history, as well as videotapes, photographs, and manuscripts. The ATM
seeks to fulfill its mission through appropriate acquisitions and by cataloging and preserving
its collections for use by educators, researchers, and interested members of the public,
including the people from whom the material was collected. The ATM’s collections and
library contribute to the research and teaching activities of Indiana University, especially
the Departments of Folklore and Ethnomusicology, Anthropology, Linguistics; the School
of Music; and the interdisciplinary area studies programs that are associated with them. It
also serves as a research, teaching, and training center for the IU Ethnomusicology Program.
Founded in 1948, the ATM has been a recognized leader in the sound archiving community,
5 Indiana University, Archives of Traditional Music. http://www.indiana.edu/~libarchm/.
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developing in step with technological and theoretical advances in ethnographic research
and recorded sound.
At IU, the ATM’s primary partner in this project, the IU Digital Library Program (DLP)
6
is
dedicated to the selection, production, and maintenance of a wide range of high quality
networked resources for scholars and students at Indiana University and elsewhere, and
supports digital library infrastructure for the university. The DLP is a collaborative effort of
the Indiana University Libraries, the Office of the Vice President for Information Technology,
and IU’s research faculty with leadership from the School of Library and Information Science
and the School of Informatics. The DLP’s current facilities include the Digital Media and
Image Center (containing equipment for image, audio, and video capture), the Electronic
Text Development Center (supporting creation of scholarly electronic texts), and an extensive
server infrastructure for support of digital projects, with life-cycle replacement funding for
hardware and software. DLP staff provides expertise in planning, creating, and maintaining
digital projects.
1.2.2 Harvard University
The Archive of World Music (AWM) and its technological partner, Harvard College Library
Audio Preservation Services (HCL-APS), are both units of the Loeb Music Library
7
which, in
turn, is a component of the Harvard College Library that serves the Faculty of Arts and Sciences
at Harvard. The Archive of World Music was established in 1976 and, with the appointment
in 1992 of Kay Kaufman Shelemay as Harvard’s first senior professor of ethnomusicology, the
Archive moved to the Music Library to become one of its special collections. It is devoted
to the acquisition of archival field recordings of musics worldwide as well as to commercial
sound recordings, videos, and DVDs of ethnomusicological interest. The AWM developed
the HCL-APS, a state-of-the-art facility which was an early leader, and continues to provide
leadership, in the application of digital technologies to archival audio practice.
Over the past five years HCL-APS has moved toward joining its counterpart, the Harvard
College Library Digital Imaging Group (HCL-DIG) in providing top quality service and
advice for digitizing media. Both work closely with the Harvard University Library Office
for Information Systems on matters of building robust infrastructure and sustainable tools for
creating and preserving digital objects via the Digital Repository Service.
The Harvard University Office for Information Systems (OIS)
8
coordinates all of the Library’s
online catalogs (HOLLIS, its MARC catalog, OASIS for finding aids, VIA for visual images, and
so forth) as well as the highly regarded Library Digital Initiative (LDI), the Digital Repository
Service, and innumerable tools that sustain and support online resources. Led by Dale
Flecker and Tracey Robinson, OIS is home to nationally recognized experts who advised
Sound Directions. The Library Digital Initiative in some aspects parallels IU’s Digital Library
Program. Its mandate is to create the technical infrastructure to support the acquisition,
organization, delivery, and archiving of digital library materials, provide experts to advise
the community on key issues in the digital environment and enrich the Harvard University
Library collections with a significant set of digital resources.
6 Indiana University, Digital Library Program. http://www.dlib.indiana.edu/.
7 Harvard University, Loeb Music Library. http://hcl.harvard.edu/loebmusic/.
8 Harvard University Library, Office for Information Services. http://hul.harvard.edu/ois/.
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1.3 Standards and Best Practices
1.3.1 Introduction
It is critical that audio preservation systems use technologies, formats, procedures, and
techniques that conform to internationally-developed standards and best practices. These
are typically developed by technical experts and, if competently implemented, ensure that
the output of a preservation system is high-quality. Standards and best practices also provide
a philosophical and ethical foundation for preservation work by outlining expectations and
goals for the output of a preservation system along with acceptable means to achieve them.
Standards-based technologies will presumably be usable longer, fostering sustainability, and
are more likely to generate products that are interoperable. Finally, “non-standard formats,
resolutions and versions may not include preservation pathways that will enable long term
access and future format migration.”
9
In this sense we place ourselves all in the same boat by
adhering to standards, increasing the likelihood that strategies for migration and access will
be developed when it is time to move to new technologies.
Formal standards in preservation-related areas are assessed and ratified by bodies such as
the International Organization for Standardization (ISO), the National Information Standards
Organization (NISO), the World Wide Web Consortium (W3C) and others. Standards crucial
to audio preservation are also developed by organizations such as the Audio Engineering
Society (AES), the European Broadcasting Union (EBU), the Library of Congress, the Digital
Library Federation (DLF) and others that may not be official national or international standards
organizations in the strictest sense, but are charged by various constituencies with providing
leadership in this area. The publication of best or recommended practices provides guidance
in areas where standards do not yet exist or may never be created. Best practices may also
provide strategies, procedures or work plans necessary to successfully implement a standard
that has been formally adopted. The Sound Directions project has implemented and tested
the standards and best practices described below.
9 International Association of Sound and Audiovisual Archives, Technical Committee, IASA-TC 04 Guidelines
on the Production and Preservation of Digital Audio Objects: Standards, Recommended Practices, and Strategies
(Aarhus, Denmark: International Association of Sound and Audiovisual Archives, Technical Committee, 2004),
6.
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1.3.2 IASA-TC 03: The Safeguarding of the Audio Heritage: Ethics, Principles
and Preservation Strategy, Version 3, December 2005
TC 04: Guidelines on the Production and Preservation of Digital Audio
Objects
10
IASA-TC 03 provides an overview of key audio preservation topics including selection,
preservation transfer, digital archiving basic principles, preservation metadata, format
priorities for transfer, and others.
IASA-TC 04 is an important high-level recommended practices document for the preservation
of audio in the digital domain. This publication includes detailed recommendations for
signal extraction from analog sources, equipment in the digital preservation chain, sample
rate and bit depth, characteristics of Preservation Master Files, target preservation file format,
guidelines for storage, and others.
In effect, best practices developed during Phase 1 of Sound Directions put into action both
TC 03 and TC 04 principles, using them to produce detailed practices and procedures as
reported in this document.
1.3.3 Capturing Analog Sound for Digital Preservation: Report of a Roundtable
Discussion of Best Practices for Transferring Analog Discs and Tapes. NRPB,
CLIR, LC
11
This report summarizes discussions and recommendations from a meeting of audio preservation
engineers that was organized by the Council on Library and Information Resources and the
Library of Congress under the auspices of the National Recording Preservation Board. The
heart of this document is its detailed discussion of issues relating to the analog playback
of both discs and tapes. David Ackerman of Harvard University and three members of the
Sound Directions Advisory Board—Chris Lacinak, George Massenburg, and Peter Alyea—
were invited to participate in this meeting.
1.3.4 Broadcast Wave Format (BWF or BWAV)
12
The Broadcast Wave Format, based on the Microsoft WAVE audio file format, was introduced
by the EBU in 1996 to allow files to be exchanged between the increasing number of digital
audio workstations used in radio and television production. Broadcast Wave is a special type
of WAVE file that may contain basic metadata (residing with the file itself) about its audio
content, and carries a sample-accurate time stamp that can be used to place related files in the
proper sequence. BWF is not a destination for the extensive metadata that must be collected
10 IASA-TC 04 is available through the website of the International Association of Sound and Audiovisual Archives
at http://www.iasa-web.org/ or in the US through Nauck’s Vintage Records: http://78rpm.com/. International
Association of Sound and Audiovisual Archives, Technical Committee, IASA-TC 03 The Safeguarding of the Audio
Heritage: Ethics, Principles and Preservation Strategy, ver. 3 ([Budapest]: International Association of Sound and
Audiovisual Archives, Technical Committee, December 2005). Also available online: http://www.iasa-web.org/
IASA_TC03/IASA_TC03.pdf.
11 Council on Library and Information Resources and Library of Congress, Capturing Analog Sound for Digital
Preservation: Report of a Roundtable Discussion of Best Practices for Transferring Analog Discs and Tapes, CLIR
publication no. 137 (Washington, DC: Council on Library and Information Resources and Library of Congress,
2006). Also available online: http://www.clir.org/PUBS/reports/pub137/pub137.pdf.
12 European Broadcasting Union, “BWF – A Format for Audio Data Files in Broadcasting,” ver. 1, Tech 3285
(Geneva: Switzerland: European Broadcasting Union, July 2001),
http://www.ebu.ch/CMSimages/en/tec_doc_t3285_tcm6-10544.pdf.
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during digital preservation projects. The Broadcast Wave Format itself has become a de facto
standard in the audio world. In addition to its widespread use in Europe and Australia, it is
specifically recommended by IASA, AES, and the National Academy of Recording Arts and
Sciences as the target format for audio preservation.
13
1.3.5 AES31-3-1999
14
Published by the Audio Engineering Society in 1999, AES31 is an international standard
designed to enable simple interchange of audio files and projects between workstations. Part
3 includes a format for the communication of edit decision lists, called Audio Decision Lists
(ADLs) in the standard, using ASCII text that is human-readable but also may be parsed by
software.
AES31-3 is used in archival work to model the relationship between the source recording
and resulting digital files. It provides a standard way to link the various files that are created,
sometimes through multiple stops and starts during transfer of a deteriorating source, thereby
reconstructing the source recording. Without it, future researchers are left with one engineer’s
interpretation of the edit points. This standard may also be used for the collection of marker
information, or cue points, based on the start and stop times of performances in a digital file.
As of this writing, this is not officially supported by the standard, but the data may reside in
an ADL in a proprietary section depending on a manufacturer’s implementation. AES31-3
is under revision to include this marker information as an official part of the standard, with
public release expected soon along with eventual adoption by software manufacturers.
1.3.6 AES SC-03-06 Working Group on Digital Library and Archive Systems,
Task Group SC-03-06-A Metadata Harmonization
This emerging standard, developed in consultation with the Library of Congress by the AES
in a working group chaired by Harvard’s David Ackerman, guides the collection of technical
metadata for audio objects, including the source recording and file derivatives, as well as the
digitizing process. Mike Casey from Indiana University and Sound Directions board member
Chris Lacinak are active participants in this working group. The standard was implemented
during Phase 1 of Sound Directions for the first time in a real world project. Both Indiana
University and Harvard University have developed software for the collection of technical
metadata using this standard.
1.3.7 Open Archival Information System (OAIS)
15
The Open Archival Information System (OAIS) Reference Model, ISO standard 14721:2003, is
a conceptual framework for an archival system dedicated to preserving and maintaining access
to digital information over the long term. It describes the environment in which an archive
resides, the functional components of the archive itself, and the information infrastructure
13 See IASA, Technical Committee, IASA-TC 04, 7. See also The Recording Academy, Producers &
Engineers Wing and Audio Engineering Society, Technical Committee on Studio Practices and Production,
“Recommendation for Delivery of Recorded Music Projects,” AES Technical Council Document AESTD
1002.1.03-10; 030930 rev 33 (New York: Audio Engineering Society, 2003),
http://www.aes.org/technical/documents/AESTD1002.1.03-10_1.pdf.
14 Available through: Audio Engineering Society, Standards Committee, “Standards in Print,”
http://www.aes.org/publications/standards/.
15 Consultative Committee for Space Data Systems, Reference Model for an Open Archival Information System
(OAIS), CCSDS 650.0-B-1 Blue Book January 2002 (Washington, DC: CCSDS Secretariat, 2002). Also available
online: http://public.ccsds.org/publications/archive/650x0b1.pdf.
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supporting the archive’s processes. Due in part to endorsement by OCLC and RLG, the OAIS
Reference Model is used by many libraries, archives, and other cultural heritage institutions
as a means of defining their own digital preservation infrastructure. Indiana and Harvard are
using OAIS concepts in implementing their digital library object repository systems.
1.3.8 Metadata Encoding and Transmission Standard (METS)
16
The Metadata Encoding and Transmission Standard (METS), developed as an initiative of
the Digital Library Federation and maintained by the Library of Congress, specifies an XML
document format for packaging metadata necessary for both management of digital library
objects within a repository and exchange of such objects between repositories, or between
repositories and their users. A METS document is capable of wrapping together all of the
descriptive, administrative, and structural metadata for a digital object in many versions, plus
references to the object’s data files, or optionally, inclusion of the data files themselves. METS
is frequently used as the wrapper format for OAIS Submission Information Packages (SIPs),
Archival Information Packages (AIPs), or Dissemination Information Packages (DIPs).
1.4 Overview of this Publication
Our purpose in writing this publication is to present the results of research and development
carried out by the Sound Directions project with funding from the National Endowment for
the Humanities in the U.S. Our work has naturally led to some conclusions that are detailed
and highly technical along with others that are more general. Both are presented here, in
separate sections of each chapter as discussed in the note to readers above.
The work undertaken by the Sound Directions project focused largely on what happens after
analog-to-digital conversion. We report on our experience with pre-conversion parts of the
preservation chain, and even offer a few recommended technical practices, but have not
attempted to be exhaustive in these areas. The heart of our work begins with the creation
of digital files and continues to long-term preservation storage. This fills a sizeable gap in
the audio preservation field as there are no best practices documents that address this part
of the preservation pathway in detail. Our aim was to use our real world project to add
specificity to the best practices that do exist, as well as to develop best practices in areas
where they have not yet been established. These are presented by topic at the beginning of
the recommended technical practices section in each chapter and as a group in Chapter 8.
Sound Directions best practices are based on general principles either widely recognized
by the audio preservation community or, in a few cases, newly proposed by our project.
While tools, formats, and practices will change over time as our field evolves, these basic
principles should remain constant. In some areas that are either out of our scope (analog
playback, management of preservation repositories, for example) or are necessarily specific
to individual institutions (workflow) we have not developed detailed best practices but report
on our own operations, which can be used as a starting point for institutions developing
audio preservation systems.
We invite you to continue this conversation on audio preservation issues. Questions,
comments, and suggestions may be emailed to the Sound Directions project at
soundir@indiana.edu.
16 Library of Congress, “METS: Metadata Encoding & Transmission Standard” (12 July 2007),
http://www.loc.gov/standards/mets/.
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2 Personnel and Equipment for Preservation Transfer
2.1 Preservation Overview
I
f the primary goal of preservation transfer work is the creation of a surrogate that is an
accurate, authentic, and very high quality representation of the original, then both the
equipment in the preservation system and the personnel operating it are of key importance.
Best practices documents provide guidance for a preservation studio’s signal chain and
personnel, analog playback, and analog-to-digital conversion, as discussed below. In some
cases this guidance is specific, while in others it is necessary to apply the knowledge of an
audio engineer to derive particular practices from general statements.
Personnel for Preservation Transfer
Familiarity with obsolete media, its historically accepted qualities and characteristics, its
production techniques, playback equipment calibration and equipment maintenance is
essential for solid preservation transfer. Such familiarity is in decline. The sophisticated
technical equipment used in preservation studios must be operated by appropriately trained
personnel. IASA-TC 03 and TC 04, in addition to stating that equipment must be optimally
adjusted and maintained, suggest that playback “requires knowledge of the historic audio
technologies and a technical awareness of the advances in replay technology.”
17
Fragile audio carriers are damaged by the stress of repeated and inexpert playback attempts and
lack of timely intervention in the face of playback problems. The CLIR/LC report, “Capturing
Analog Sound,” addresses this directly, suggesting that “there are many areas in which a
trained ear and years of experience are by far the most important tools….in some archives,
frag ile audio recordings are being handled, played, and transferred for digital preservation
by staff who have limited experience working with audio recordings or little knowledge
about the sonic character istics and weaknesses of various audio formats.” This report strongly
recommends, “audio preservation transfers be done by trained and experienced audio
engineers.”
18
Professional audio experience, musical knowledge, and the ability to verify or confute their
human perceptions with precise measurement, make audio engineers and technicians, rather
than automated systems or untrained students, the best candidates for recognizing playback
problems and intervening during archival transfers. In addition, engineers and technicians
are equipped with the necessary critical listening skills to ensure that not only playback, but
also the performance of the studio signal chain itself, is optimal.
Ideally, an audio preservation workflow would also involve the services of a specialized
programmer. Software that automates the mechanistic aspects of the work (such as metadata
entry) cuts costs, saves time and reduces the opportunity for human error.
Preservation Studios
Best practices documents contain few specific recommendations for the signal chain in a
preservation studio. IASA-TC 04 stipulates: “The combination of reproduction equipment,
signal cables, mixers and other audio processing equipment should have specifications
17 IASA, Technical Committee, IASA-TC 03, 6; IASA, Technical Committee, IASA-TC 04, 3.
18 CLIR, “Capturing Analog Sound,” 4 and 15.
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that equal or exceed that of digital audio at the specified sampling rate and bit depth. The
quality of the replay equipment, audio path, target format and standards must exceed that
of the original carrier.”
19
The CLIR/LC report discusses the need for accurate monitoring
systems to evaluate quality as well as test equipment to evaluate potential problems.
20
In
addition, the characteristics of the room used for preservation transfer work must be carefully
considered.
From recommendations such as these, basic audio engineering principles, and experience,
we deduce the following:
The room in which we monitor transfers can be thought of as an unavoidable lens
through which the audio content is experienced. Preservation transfer work is best
undertaken in a studio designed as a critical listening space. A critical listening space
should have an ambient noise level well below that of the quietest sound we wish
to audition when listening at a safe, comfortable, non-fatiguing playback level.
21
The
room should not distort the frequency spectrum of interest, the accuracy of the sonic
images, the sense of space, or the timing of the audio content
If a critical listening space is not possible, then the studio must at least be free from
ambient noise, it must be removed from other work areas and traffic, and its acoustic
weaknesses should be well understood. Knowing the acoustic weaknesses of the room
informs one of the aspects of the sound that can be reliably analyzed by ear and those
aspects that cannot. This is vital for the engineer who must be able to make accurate
judgments during transfer and when selecting and aligning equipment
All signal chain components must be professional-quality
The most direct and clean signal path must be used from source to destination. Signal
chain components that are not used for preservation transfer work should be removed
to prevent additional noise from entering the system
The studio should include test/calibration equipment to test and monitor the transfer
chain itself for noise as well as to test individual components for performance. During
transfer, the test/calibration equipment shall not be inserted between the playback
machine and the recorder
The studio should include a monitoring chain that enables the engineer to monitor
the signal directly from both the playback machine and after the analog-to-digital
converter to verify the quality of the converted signal
In the digital age, preservation studio signal chain components feed the audio signal
into a computer where the audio content is recorded and further processed in the digital
domain. The computer-based audio workstation, called a digital audio workstation or DAW,
historically required dedicated hardware to efficiently process the audio signal. Integrated,
turnkey systems with proprietary hardware and software specifically designed for digital audio
processes were commonly used. In recent years, as the processing power on the average
desktop computer has increased, these systems have declined considerably in popularity.
A standard desktop computer can now handle multiple channels of audio, at least in terms
of processing power and memory, although dedicated systems may have advantages for
applications that require significant signal processing. It is no longer necessary to invest in
19 IASA, Technical Committee, IASA-TC 04, 8.
20 CLIR, “Capturing Analog Sound,” 25.
21 An example of one room characteristic that must be addressed is noise level. Richard Warren’s storage document
published in the ARSC Journal recommends a Noise Criteria-level of 20-25 dB for critical listening areas. More
generally, he also calls for consideration of the “proper acoustical conditions to prevent the room from distorting
the sounds to be studied.” Richard Warren, Jr., “Storage of Sound Recordings,” ARSC Journal 24, no. 2 (1993),
137. Readers are also directed to the publications of the Audio Engineering Society for detailed information on the
characteristics of audio studios.
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expensive proprietary, dedicated systems for audio preservation transfer work. If this work
is relatively simple, emphasizing signal capture without much downstream manipulation,
using a carefully designed desktop audio methodology with what is sometimes called a
host-based or native audio processing system is a valid approach. Both native and dedicated
hardware/software systems are viable options for audio preservation.
22
Analog Playback
Although all points along the preservation chain are important, audio preservation engineers
generally agree that playback of analog source recordings is a particularly key juncture at
which, if not performed optimally, the quality of the end product will be lessened. According
to IASA-TC 04 “any transfer should attempt to extract the optimal signal from the original
[as] the original carrier may deteriorate, and future replay may not achieve the same quality,
or may in fact become impossible, and secondly, signal extraction is such a time consuming
effort that financial considerations call for optimization at the first attempt.”
23
No amount of
effort or expense in the remainder of the signal chain can recover information that was not
retrieved from the analog original at the moment of playback. TC 04, as well as the CLIR/LC
report “Capturing Analog Sound,” provides detailed best practices for the playback of analog
recordings.
Both the abilities of staff and the equipment used greatly impact the success of the analog
playback stage. The engineer must understand how field recordings carried on obsolete,
deteriorating historic formats may be optimally reproduced despite degradation, taking into
account specific characteristics of both the individual recording and the format itself. The
engineer must also align, calibrate, and verify the performance of the playback machine,
which itself must be able to reproduce the recording at the highest fidelity possible.
The analog playback stage must utilize the highest quality copy of the content that is available.
For recordings made in the field this is usually, although not always, the original recording.
In some cases the original may have deteriorated to the point that a first copy is the highest
quality carrier of the content. Locating and identifying the best copy in existence, even if it
resides in another archive, will enable the judicious use of preservation resources, prevent
duplication of effort, and result in carrying the highest quality version forward into the future.
In order to enable future re-consultation for the purpose of assessing past work, analyzing
secondary information such as notes on a container, or other reasons, all original recordings
should be retained.
24
Conversion
If analog playback is one exceptionally key juncture in a preservation system, then analog-
to-digital (A/D) conversion is the other. Choices made in both of these areas can dramatically
and permanently affect the fidelity of the audio signal that is carried in the digital domain
into the future. Ken Pohlmann, in a paper published by CLIR states that “errors introduced by
the A/D converter will accompany the audio signal throughout digital
22 This discussion is largely from Francis Rumsey, Desktop Audio Technology: Digital Audio and MIDI Principles
(Oxford; England, Burlington, MA: Focal Press, 2004), 2 and 156.
23 IASA, Technical Committee, IASA-TC 04, 11.
24 See IASA, Technical Committee, IASA-TC 03, 7; IASA, Technical Committee, IASA-TC 04, 11; and International
Association of Sound and Audiovisual Archives, Editorial Group, Task Force to Establish Selection Criteria of
Analogue and Digital Audio Contents for Transfer to Data Formats for Preservation Purposes (Hungary, October
2003), 5. Also available online: http://www.iasa-web.org/taskforce/taskforce.pdf.
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processing and storage and, ultimately, back into its analog state.”
25
The technical quality
of the audio signal can never be made better once converted. IASA-TC 04 suggests that the
converter “is the most critical component in the digital preservation pathway.”
26
Best practices documents all recommend a professional-quality stand-alone (external) A/D
converter rather than one incorporated into the computer’s sound card with its low cost
circuitry subject to the electrical noise inside the computer.
27
IASA-TC 04 includes minimum
specifications for such a converter, although it is often difficult to match specifications to the
information provided by any given manufacturer. Pohlmann suggests both measurement and
listening tests before purchase but also counsels seeking expert advice, perhaps recognizing
that most archives are not capable of conducting scientifically valid tests in either of these
areas. It may be necessary to engage the audio engineering community as there does appear
to exist an informal, short list of converters that engineers believe are of high-enough quality
for preservation transfer work. These tend to range in price from around $1,000 to $10,000
and more.
Both the IASA and Pohlmann documents assert that audio transparency—neither adding
to nor subtracting from the audio signal present on the analog original—is the most
important characteristic for a converter used for preservation transfer. Most converters are
not transparent, only the best approach transparency, and the differences are apparently
audible to some audio engineers, although they may be subtle.
28
However, some feel there
may be diminishing returns in analyzing perceivable improvement in quality versus increase
in price, especially with professional-quality devices. The performance of A/D converters,
many of which use the same brand of converter chip, often relies on other factors such as
how well the analog input stage is implemented and the design of the circuitry supporting
the chip.
29
The characteristics of digital conversion are established at the A/D converter with the choice
of sampling rate and word length or bit depth. The audio CD was established with a sampling
rate of 44.1 kHz at a bit depth of 16. This combination is now almost universally considered
inadequate for audio preservation of analog recordings. There is currently wide agreement
on bit depth for preservation transfer of analog sources with 24 bits recommended. A well-
designed converter operating at 24 bits will provide a noise floor at the limits of audibility
and capture enough dynamic range to make level setting less critical. It will also provide a
longer word length to allow for some types of downstream processing stages (of derivative
files) that may decrease useful word length.
30
There is less agreement on sampling rate and this topic remains somewhat controversial.
IASA-TC 04, the CLIR/LC document, the Pohlmann article on converters, along with other
25 Ken C. Pohlmann, “Measurement and Evaluation of Analog-to-Digital Converters Used in the Long Term
Preservation of Audio Recordings” (roundtable discussion, “Issues in Digital Audio Preservation Planning and
Management,” Washington, DC, March 10-11, 2006). Also available online:
http://www.clir.org/activities/details/AD-Converters-Pohlmann.pdf.
26 IASA, Technical Committee, IASA-TC 04, 6.
27 An external converter is recommended by IASA, Technical Committee, IASA-TC 04, 6 and Pohlmann,
“Measurement and Evaluation of Analog-to-Digital Converters,” 8 and 12. It is also recommended in Rumsey,
“Desktop Audio Technology,” 13.
28 See Pohlmann, “Measurement and Evaluation of Analog-to-Digital Converters,” 2 and IASA, Technical
Committee, IASA-TC 04, 6.
29 See Pohlmann, “Measurement and Evaluation of Analog-to-Digital Converters,” 6 for example.
30 See Pohlmann, “Measurement and Evaluation of Analog-to-Digital Converters,” 3-4 for reasons to digitize at
24 bit.
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