Technical Remarks on the Proposal for a Digital Remastering System
AD & DA Converters
Technical paper by James Hoagland and Gordon Kidd
This paper was written early in 1997 in connection with the Shambhala
Archives' upcoming Audio Recovery Project, during which we will undertake
the remastering of approximately 10,000 audio recordings in our care, using
the methods described below.
Introduction
The most critical link in the process of digital recording is the transition
from analog to digital. Unlike other digital processes, mistakes made in
the conversion cannot be "undone." Many aspects go into making a good converter,
not the least of which is an accurate "word clock" that ensures a stable
digital image without "jitter." Clocking error as little as a nanosecond
(teeny piece of a second) can cause digital jitter resulting in distortion
audible to the human ear. RF noise, current fluctuation, and ground loops
are a few of the other problems that can result in clocking errors and
inaccurate digital recording. Converters residing inside a computer are
especially vulnerable to the above mentioned problems.
There are a number of high quality stand-alone converters available
ranging in price from $1500 to $10,000. (Some sound studios have even been
known to use atomic clocks with their converters.) The Troisi Converters
are handmade by a small company outside of Boston, Massachusetts. Peter
Troisi has been making high end audio gear for over twenty years. His converters
are highly regarded by studio engineers who have worked with them for their
quality, attention to detail and unique modular construction, which allows
for future upgrades as new technology becomes available. Troisi Converters
have a crystal clock accurate to less than a nanosecond and are well shielded
from stray RF. They also have an external power supply, which helps to
isolate a potential noise source found in other designs.
The Digital Audio Workstation or D.A.W.
The D.A.W. is the backbone of the digital remastering process. It is here
that the digital number crunching takes place, allowing the sound engineer
to record, edit, and create new sound files required in the mastering process.
Some D.A.W.s also provide for equalization, or EQ., complex filtering,
noise reduction, crackle and click removal (mostly used in restoration
of old L.P. records) as well as other sound manipulation tools.
Not all D.A.W.s are created equal. Some are very inexpensive software
based systems used in desktop P.C.s for production of new sounds and music.
Others are very expensive stand-alone hardware units used in professional
broadcasting, while others are a combination of both hardware and software.
The three most widely used D.A.W.s are made by Digidesign, Sonic Solutions,
and SADie.
Digidesign
Digidesign's D.A.W.s enjoy a large professional user based and are used
by individual musicians on their home computers, as well as by some network
t.v. stations and film studios. Digidesign's products are designed primarily
to be used as production tools in the creation of new products (recordings),
not for remastering or sound restoration.
Sonic Solutions
Sonic Solutions and SADie's D.A.W.s are the tools of choice for many mastering
studios. Both products are recognized for their ability to manipulate sound
without creating artifacts. The SADie system uses noise reduction developed
for the restoration of old vinyl records and is an excellent D.A.W. for
restoring recordings done on cylinders or disks. (We have no recordings
on either of these media.) The Sonic Solutions NoNoise system of noise
reduction is considered by many to be the best for restoring analogue recordings
done on tape (our collection falls into this category). Sonic Solutions
NoNoise can even remove unwanted noise such as air conditioning or motor
noises recorded in the background on an original tape. Tape hiss can also
be removed without altering the rest of the content. Sonic Solutions has
been used in many important restoration projects and is the system employed
by the National Archives of both Canada and the U.S.. Sonic Solutions was
used in the remastering of the recently released Beatles CD.
The core of the Sonic Solutions hardware consists of one or more Sonic
Studio boards residing inside a Macintosh compatible computer (such as
the one we installed in 1996!) The most basic configuration consists of
a Sonic Studio-160 2/8 (see proposal) allowing for two channels of digital
audio recording and eight channels of playback in realtime, as well as
mono-processing (such as NoNoise) in nonreal-time. The addition of a Sonic
Studio-800 FX enables mono real-time de-noising as well as the ability
to record in the background while working on the same or different sound
files in the foreground. A second SS-800 could be acquired in the future
to provide increased processing power allowing for stereo real-time de-noising.
To perform basic sound restoration, additional software is necessary.
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Sonic Studio-201 Mixing Desk provides more realtime EQ, compression, and
utility filters.
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NoNoise-104 Broadband de-noising removes tape hiss and other unwanted artifacts
that interfere with the program content.
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NoNoise-105 Complex Filtering is a set of seventeen different filter types
required in cleaning up difficult problems like 60-cycle hum, which we
will encounter on our tapes.
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Other NoNoise software is available to handle special problems like click
removal, but need not be included in the basic system. These can be added
later as needed.
Media for Data Storage
When making digital audio recordings, one must consider the media, or the
means of storing the data. There are three types of storage that we propose:
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Immediate short term
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A one hour 24bit Sonic Solutions stereo sound file requires one gigabyte
of storage. A four gigabyte external hard drive is recommended as the minimal
configuration. There are many hard drives on the market but only a few
that can hold up to the stress of recording data nonstop for over one hour.
Dynatek configures hard drives that are among the highest rated for use
with digital audio workstation.
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Medium term
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Tape backup is still the most cost effective means of storing large amounts
of data. For a number of years now the standard tape backup format used
by audio mastering facilities has been the Exabyte Tape Drive. Exabyte
is a technology based on the 8mm video recorder that has been enhanced
for use in the storing of digital data. It is a very robust and reliable
design with a proven track record. Data tape is not considered to be suitable
for long term storage because it is subject to the same aging problems
that have been experienced with analog tape.
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Long term
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Although not a permanent solution, for nothing is permanent, optical media
like CDs or the new DVD (Digital Versatile Disc: a new two-sided CD soon
to be on the market) show great promise as long term storage media. The
Power Tower Pro 200 Multi-media computer that we have already installed
contains one CD-R recorder and one CD-R player for the production of long
term digital recordings.
It is the nature of digital data that it will play back just as it was
recorded until such time as it no longer plays at all, unlike analog recordings
that slowly degrade with time. The ability to make nearly identical copies
of digital recordings is one of the reasons for making digital masters.
One must remember though that when the digital data is no longer playable
the underlying information may be totally lost, and for this reason it
is highly recommended that new analog copies be made at the same time that
a program is digitally remastered. Even though information recorded on
analog media slowly degrades with time, in many cases, some if not all
of the information can be recovered. This is not the case with digital
loss.
James Hoagland
Gordon Kidd
February 14, 1997
Video Holdings
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