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sources all recommend higher sampling rates than 44.1 kHz for the following reasons:
31
It is important to accurately capture noise, such as clicks and pops on a disc, and other
inaudible, high frequency information so that improved signal processing algorithms
in the future that are able to take advantage of higher frequency information will have
enough data to work as effectively as possible. Some of this noise resides in frequency
ranges higher than can be captured at 44.1 kHz
Higher sampling frequencies enable manufacturers to build better anti-aliasing low-
pass filters that operate more efficiently, thereby improving performance within the
range of human hearing
32
Many musical instruments are capable of producing information in higher frequency
ranges—including inaudible higher frequency harmonic content that also impacts our
perception of sounds
Higher sampling frequencies provide improved temporal response, or the timing of the
arrival of sounds, that in turn improves spatial imaging (the locations of sounds from
within a stereo or surround sound-field)
The limit of human hearing acuity is not yet known, therefore the point of transparency
of a recording system cannot be known
33
IASA-TC 04 recommends encoding to linear pulse-code modulation (PCM) with a minimum
sample rate of 48 kHz, and for many purposes suggests transferring at 24 bit with a 96
kHz sampling rate. In fact, 24/96 has become the standard choice for audio preservation
reformatting. For the reasons listed above, and for the format’s wide support and sustainability,
both Sound Directions institutions have selected 24 bit, 96 kHz linear PCM encoding. For
the evaluation of other potential encoding schemes such as 1-bit sigma-delta, the cautious
preservationist is well served by the work of Caroline R. Arms and Carl Fleischhauer at the
Library of Congress on the sustainability of digital formats.
34
This document explores a number
of sustainability factors for any digital format including disclosure, adoption, transparency,
self-documentation, and impact of patents.
Creating these “high resolution” digital audio files is analogous to practices employed in the
still image preservation world, where the term “rich” is sometimes applied to high quality
preservation masters. Although there are obvious differences between the media that are the
targets of preservation, there are some overlapping issues. According to one imaging tutorial,
creating a rich digital master
provides enough information to eliminate the user’s need to use the original;
satisfies all research, legal, and fiscal requirements related to use;
supports creation of higher quality derivatives especially if they are processed;
accommodates future applications as user expectations are likely to be more demanding
over time.
35
31 IASA, Technical Committee, IASA-TC 04, 6; Pohlmann, “Measurement and Evaluation of Analog-to-Digital
Converters,” 2-4; and CLIR, “Capturing Analog Sound,” 11.
32 A 44.1 kHz sampling rate requires the converter to have a steep anti-aliasing filter that smears high frequency
energy over a wide frequency band within the range of human hearing.
33 This and the previous two points are from Pohlmann, “Measurement and Evaluation of Analog-to-Digital
Converters,” 3.
34 Library of Congress, National Digital Information Infrastructure and Preservation Program, “Sustainability of
Digital Formats Planning for Library of Congress Collections” (21 May 2007),
http://www.digitalpreservation.gov/formats/.
35 Paraphrased from Cornell University Library, Research Department, “Moving Theory into Practice: Digital
Imaging Tutorial,” Cornell University Library,
http://www.library.cornell.edu/preservation/tutorial/conversion/conversion-03.html. This document also
suggests, for still images, matching the conversion process to the informational content of the original and to
scan at that level and no more or less. Note that informational content is more easily defined for still images
than for audio recordings, which are time-based media and generally more complex.
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2.2 Recommended Technical Practices
2.2.1 Personnel for Preservation Transfer Work
2.2.1.1 Best Practices
Best Practice 1: Use audio engineers and technicians with solid technical skills and well-
developed critical listening abilities at points in the preservation transfer workflow where
their skill is required.
2.2.1.2 Rationale
Even though this best practice echoes the IASA-TC 04 and CLIR/LC documents discussed
above, we feel that it bears restating based on the experience of both institutions hiring and
training audio engineers for preservation work.
2.2.1.3 Staff at Indiana and Harvard
Both Sound Directions institutions have found that there is no substitute for experience
with audio formats and equipment. Nor is there a substitute for the critical listening skills
that are informed by such experience. Therefore, both institutions use professional audio
engineers with knowledge of historical formats for preservation transfer work. Engineers with
excellent technical and critical listening skills who lack experience with some historical
formats receive additional training from either in-house sources or outside consultants. Both
institutions have utilized programmers as part of the audio preservation staff, but only with
the help of external grant funding. Unfortunately, with rapidly changing technology, one-
time programming has a short shelf life. Programming staff would ideally be available on an
as-needed basis to support the creation of tools as audio transfer workflows evolve.
Harvard has committed to using only audio engineers or technicians for archival transfers.
The distinction between an audio engineer and a technician is based upon experience.
An audio engineer has a broad range of audio production experience in general, and has
experience with historical formats for preservation in particular—along with knowledge of the
entire preservation workflow. A technician has some general audio production knowledge,
and is skilled in specific tasks of the preservation workflow. Roy Pritts, Past Chairman of
the Audio Engineering Society Education Committee, considers a technician to be “…one
who takes the prescription or design of an engineer and implements it.”
36
In practice, the
lines between engineer and technician become less distinct as technicians broaden their
experience and education. At that point, one of the challenges might be developing the
necessary unit structure in order to support these increasingly valuable individuals in their
current role, or expanding their responsibilities, and possibly expanding the unit. Another
challenge as we learn and grow is to retain hard-earned knowledge so that it is not lost as
personnel eventually change. At the HCL-APS we have instituted a staff knowledge-base
wiki in order to document our learning and our procedural changes, and to aid in training
new staff in our procedures. We also plan training sessions with subject matter experts when
adopting new technologies and methods. At this time, Harvard has committed to assigning
a single audio engineer or technician to monitor an entire single transfer only. We do not
36 Roy Pritts, “The Educated Audio Engineer: A Life-Long Learner,” Audio Engineering Society, Education
Committee (2007), http://www.aes.org/education/pritts.html.
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yet see sufficient benefit to dividing one individual’s attention among multiple, simultaneous
monitoring tasks, but we will continually re-evaluate the benefits and risks.
Our two preservation studios are operated through a first shift by audio engineers and second
shift by audio technicians. What would be the third shift time slot is reserved for automated
data backup. Rounding-out the staff of HCL-APS is our programmer for the Sound Directions
project, who together with our lead engineer has developed a suite of scriptable software
tools that has streamlined our preservation workflow, thereby allowing the engineers and
technicians to focus their talents more effectively on the critical archival transfers.
The Indiana University Archives of Traditional Music has specific experience informing its
decision to use audio engineers for preservation transfers. Over many years both talented
graduate students and audio engineers have undertaken transfer work and, in fact, we still
actively use graduate students for access-only digitizing to fill orders for CDs of our holdings.
We have learned through this experience that trained audio engineers with highly-developed
critical listening skills and deep technical knowledge are necessary for preservation-quality
transfer work for most analog sources. Throughout the Sound Directions project we have
observed the innumerable decisions that must be made—aligning tape machines and
verifying their performance, repairing a deteriorating tape or disc, setting levels, analyzing
completed files, monitoring converter performance and studio noise floor, selecting styli,
determining track configuration, adjusting azimuth, to name a few— which require technical
skill, judgment, and effective critical listening and directly impact the quality of the final
product. The ATM does, however, see a role for students with technical aptitude. We have
plans to use students from the IU Department of Recording Arts for preservation transfer
of non-problematic sources to increase throughput into our preservation system. These
students, who have formal audio training, may be considered apprentices and will be trained
and supervised by the Sound Directions engineer. The ATM produces written procedural
documents that guide the work of both audio engineers and audio student workers and serve
to document and maintain institutional knowledge.
Both institutions also realize that not every audio engineer is able to understand the principles
of archival preservation transfer work which is not only different from, but sometimes at odds
with, the commercial sector. It is critical to use engineers who are interested in learning an
archival adaptation of their craft and who can commit to fulfilling the preservation mission.
These engineers do not always have formal academic training and many have learned
their profession through the time-honored apprenticeship tradition. In order to maintain
the flexibility to hire the most qualified candidate it is important when advertising for
engineering positions to specify technical skills and knowledge that are desirable, including
a good sense for music and performance, but not necessarily academic qualifications.
Finding personnel who are equally capable in both the playback of obsolete formats and
operating modern digital audio workstations is a significant challenge, and it is reasonable
to expect that additional training will be necessary to fill gaps in an engineer’s knowledge.
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2.2.2 Studio Signal Chain for Archival Preservation
2.2.2.1 Best Practices
Best Practice 2: Perform preservation transfers in an appropriately designed, critical listening
environment. If such a space is not available, choose a room that is quiet and is removed
from other work areas and traffic, and be acutely aware of its sonic deficiencies.
Best Practice 3: Route the signal from the playback machine to the analog-to-digital converter
using the cleanest, most direct signal path possible.
Best Practice 4: Design the monitoring chain to allow instant comparison of the signal from
the playback machine to the signal that has passed through the analog-to-digital converter.
Best Practice 5: Preservation studios must 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.
2.2.2.2 Rationale
The best practices cited above evolved from both our work and the documents discussed
in section 2.1. The project’s initial objective in this section was to provide a case study of
two audio preservation studios. However, our experience was particularly compelling in
several areas and these critical best practices emerged. Note that we have not attempted to
be comprehensive—the reader must engage the audio engineering community for further
specifics on audio preservation studios that we have not covered.
Here is the reasoning behind the above best practices:
Working in a critical listening environment enables the engineer to accurately evaluate
the audio when monitoring the preservation transfer
Keeping the signal path in the transfer chain as direct and clean as possible helps
in meeting the basic preservation objective of obtaining the most accurate, highest-
quality transfer of the original possible
The ability to monitor the signal from both the playback machine and post-A/D
converter enables verification of the A/D conversion and allows easier diagnosis of
potential problems heard during transfer
Test/calibration equipment is essential for verifying such things as the studio noise floor
and the performance of the signal chain including playback devices
Below are some basic principles that we have found useful in designing our audio preservation
studios:
Design the preservation studio as a critical listening environment and know its limitations
All signal chain components must be tested so that they are known to be of
professional-quality, that they are reliable, and that they do not alter the level or quality
of the audio signal at unity
The most direct and clean signal path from source to destination must be used at all times.
There may be no unused devices in the signal path. If there are multiple destination formats
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for the transfer, then the signal must be routed in parallel without any daisy-chaining of
devices
Signals shall be split or distributed using only calibrated, high-quality distribution
amplifiers, routers, or properly designed and wired balanced cables and patchbays
that demonstrably do not degrade the signal
Use the highest quality signal format present on the source equipment and throughout
the chain. For instance, use a balanced signal source rather than an unbalanced signal
source
2.2.2.3 Preservation Studio at the IU Archives of Traditional Music
The audio preservation studio at Indiana is a purpose-built facility designed to do one
thing—preservation transfer work—simply and well. It was configured to incorporate just
enough flexibility to handle this one function. Projects that require flexible routing, usually
access-related, are handled in other ATM studios. The preservation studio was intentionally
developed within the bounds of a budget typical of a large preservation grant. Audio is handled
natively on a standard Dell PC using an excellent external analog-to-digital converter and
WaveLab audio recording/editing software. Our goal was to produce preservation-worthy
digital objects equivalent in quality to those produced on all but the highest-end systems,
and we believe we have been successful, although engineering skill is notably critical to this
success. Below is a generic diagram of the signal chain in the ATM Preservation Studio.
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19
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Digital-to-
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Monitor
Control
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Audio
Monitors
Headphone
Amplifier
Headphones
Laptop
Software:
SpectraFoo
Digital
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Audio Interface
For Laptop
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Figure 1: Indiana University ATM preservation studio signal chain
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2.2.2.3.1 Indiana University Transfer Chain
Characteristics of the transfer chain:
The playback machine is hardwired directly to the analog-to-digital converter for tape
transfers using one cable that is physically moved as the playback machine changes.
Moving this cable is only necessary when moving to a new collection or format
The signal is split after the playback machine with one side routed to the converter
and the other to the monitoring chain. The split is done by a cable built by hand by
the engineer taking into account impedance matching issues in our relatively simple
signal chain
We do not object to including a patchbay in the chain but haven’t as yet felt it
necessary
If we need additional gain for a tape transfer we can insert a line-level amp into the
chain.
For disc transfers the signal from the turntable is sent to a preamp which is then routed
directly to the converter
The audio recording/editing software used at the ATM for preservation work is WaveLab
6.0. We chose WaveLab for its support of both the Broadcast Wave Format and the AES31-3
standard. We also prefer many of its features, such as the audio analysis tools which we use
for quality control.
Below are two tables with the specifics of the transfer chain—first, for tape and second, for
disc transfers.
Device Type
Device
Channel/Connector
Comments
Playback
machines
Studer A810 or
Tascam 122 MKII
Analog/XLR
Analog-
to-digital
converter
Benchmark
ADC1
Digital AES/EBU
output/XLR
Sound card
Lynx AES16
Digital AES/EBU input/
DB 25 connector
PCI card
Computer
Dell Optiplex
GX620
Pentium 4 processor,
3.8 GHz, 2.0 GB RAM
Slow speed
playback
machine and
line amp
Revox B77 and
Gaines Balanced
Line Interface
Analog RCA outputs
from Revox
For tapes recorded at
1.875 and 0.938 ips.
The Gaines device is
inserted between the
Revox and converter
Table 1: Devices in the transfer chain for tape transfers
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Device Type
Device
Channel/
Connector
Comments
Playback
machine
Technics SP-15
turntable
Analog RCA outputs
from the cartridge
Includes SME 3012
tonearm, Stanton 500
cartridge, various styli
Preamp 1
KAB Souvenir
EQS MK12 using
flat setting
Analog RCA in/
Analog balanced
TRS out
Preamp 2
Owl 1 using
a playback eq
curve
Analog unbalanced
RCA in and out
Used only when necessary
for playback curve. Both
preamps are used together
to generate flat and
equalized files at the same
time
Same
converter,
card, and
computer as
above
Table 2: Devices in the transfer chain for disc transfers
2.2.2.3.2 Indiana University Monitoring Chain
Table 3, below, details the monitoring chain in the ATM Preservation Studio.
Device Type
Device
Channel/
Connector
Comments
Digital-to-
analog converter
Benchmark DAC1
Digital AES/EBU
input/Analog XLR
output
Monitor
controller
StudioComm Model 55/
Model 56
Balanced TRS input
Accepts inputs from
playback machine
and D/A converter.
Routes to speakers
and provides
switches to choose
listening source
Audio monitors
Genelec 8040A
Headphone amp
StudioComm Model 35
Headphones
Ultrasone Proline 650
Table 3: Devices in the monitoring chain
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2.2.2.3.3 Indiana University Test/Calibration Chain
The test/calibration chain in the ATM preservation studio is a parallel sidechain to the transfer
chain, and features a laptop running Metric Halo’s test/calibration software, SpectraFoo.
37
Because SpectraFoo is only available for the Mac, we use Metric Halo’s Mobile I/O as an
audio interface for the Mac laptop. SpectraFoo is used for the following:
Regularly checking the overall noise floor in the studio using a spectrum analyzer
Reading the results of azimuth adjustments for tape machines during alignment using
an x-y display
Calibrating levels throughout the studio using a level meter
Checking for consistency in the overall frequency response at the beginning and ends
of tapes during transfer using a spectrum analyzer
Assessing the frequency response of tape machines using a precise level meter
Below is a table that provides details of our test/calibration chain.
Device Type
Device
Channel/Connector
Comments
Audio interface
Mobile I/O 2882
Digital AES/EBU input/
Firewire output
Serves as a high
quality interface
between the
sound card and
the laptop
Computer
Apple Powerbook G4
laptop
SpectraFoo is
available only
for the Mac
Test software
Metric Halo’s
SpectraFoo 4.0
Table 4: Devices in the test/calibration chain
We quickly learned through practical experience that test equipment and software are
essential for preservation work. While assessing the preservation studio shortly after
installation, SpectraFoo revealed a noise floor that appeared high at certain frequencies.
Later in the day while taking notes on this problem, the engineer noticed that this spike had
decreased dramatically without anything else changing. We finally traced the problem to a
dimmer controlling a bank of lights in a nearby room. Dimmers are notorious for creating
noise problems in audio systems. Due to the sensitivity of the SpectraFoo tools, we were
able to uncover this problem immediately and correct it. It otherwise might have remained
undiscovered for some time, putting inappropriate noise into our digital files. SpectraFoo
also revealed other less dramatic, though still unwanted, problems with our studio noise
floor that we attribute to electromagnetic interference (EMI).
38
37 Metric Halo. http://www.mhlabs.com/metric_halo/.
38 We tested nearly every location within the ATM for null points (lower interference) by loading the laptop
with SpectraFoo and the Mobile I/O on top of a Studer open reel tape machine and rolling it throughout the
facility. Some amount of EMI is inevitable in most buildings but care should be taken to minimize it by finding the
location with the lowest level. During the process of assessing both of these problems we also engaged university
electricians to check all outlets for proper wiring and grounding. As part of this work we were able to match
outlets to circuits and to place all preservation studio equipment on a dedicated circuit.
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2.2.2.4 Preservation Studios at the Harvard College Library’s Audio Preservation
Services (HCL-APS)
At this time, HCL-APS is a two-room facility with accompanying machine room, office
space, and shelving and supply space. All of the computers and DAWs are accessed from
either of two keyboard and monitor positions in either room using a keyboard and video
monitor (KVM) switch. Each room is equipped with Genelec S-30DVR M magnetically-
shielded studio monitors. Normally, each room has two open reel machines including an
Otari MX-5050B MK III for 1 7/8 ips transfers and either an Ampex ATR-102 or Studer 807.
Each room is otherwise similarly equipped except that Studio ‘A’ is designated as the archival
disc transfer room, since it has a Technics SP-15 turntable for coarse-groove discs and a Leica
MZ75 microscope which swings over the turntable for examining and measuring grooves,
an Owl 1 phono pre-amp and a Stanton 310B phono pre-amp. For LP and 45 rpm vinyl
discs in studio ‘A’ there is a Technics SL1200 turntable. Both studio ‘A’ and studio ‘B’ were
constructed to provide a similarly accurate monitoring environment.
2.2.2.4.1 HCL-APS Preservation Signal Chain
Harvard’s preservation studios use an analog signal path that flows from the playback device
to a patchbay. The patch points in the bay are either “normalled”
39
to a mixing console for
monitoring purposes only, or normalled to the inputs of the next logical device such as
from the turntable to the phono pre-amplifier or from the phono pre-amplifier to a level-
matching device like our Gaines Audio Balanced Line Interface. The patchbay allows us the
convenience and flexibility to easily monitor or test the signal at multiple stages in the signal
chain.
During the transfer of analog media, the analog signal is connected through the patchbay to
the inputs of the Prism AD-2 Analog-to-Digital Converter for digitization. The AD-2 is set for
internal synchronization. The AD-2’s AES/EBU balanced digital audio outputs are connected
directly to the AES/EBU inputs of our Merging Technologies Pyramix Digital Audio Workstation
for recording onto our Storage Area Network appliance, or SAN. The Pyramix DAW’s AES/
EBU output is connected to a Prism DA-2 Digital-to-Analog Converter. The DA-2’s analog
outputs are normalled in the patchbay to, and monitored through, the mixing console. One
of the DA-2’s multiple digital outputs are connected to a Metric Halo, Mobile I/O Firewire
Interface which is the digital audio interface to the Macintosh G4 or G5 computer running
SpectraFoo audio analyzing software. In this configuration we are analyzing the results of the
entire digitization signal chain. During transfer we watch SpectraFoo’s analysis of the signal
for anomalies. This procedure greatly benefited the quality of our transfers. We can see as
well as hear potential problems.
40
Alternatively, we can analyze the digital audio output of
the AD-2 with SpectraFoo by connecting one of the AD-2’s outputs to the Mobile I/O. In
addition, we use an Audio Precision Cascade System Two, Dual Domain Audio Analyzer
at specific nodes in the signal chain when necessary. Since we have one Audio Precision
system at our two-room facility, this very powerful tool is normally used to address specific
problems rather than for general analysis of the transfer. It is worth noting that during transfer
the analysis is done from a separate output of the AD-2 so that the signal does not pass
through any analyzing equipment or analyzing software on its way to being recorded.
39 “Normalled” means that the devices in question are normally connected unless a patch cord is inserted into
the jack; which would break the normal signal flow.
40 Similarly to IU’s experience, using SpectraFoo, HCL-APS engineers quickly discovered that our Studer A807
open reel tape player was susceptible to an EMI field in a particular orientation, with the quarter-track head stack
installed. The problem was solved by modifying the head stack to accept a hum shield and reorienting the Studer
until the offending interference was minimized.
Documents you may be interested
Documents you may be interested
