Audio Preservation
and Transfer

Preservation vs. Restoration | Past & Present | Digital Details | Today's Choices | Transfer Tips

Audio preservation is the transfer and storage of audio on a new medium in order to preserve the audio for the future. We can deliver on LTO-3 tape (up to 400GB capacity), RAID 5 NAS (up to 2TB), or hard drive(s).

As used here, "preservation" implies that the new medium is selected and managed on an ongoing basis, otherwise the transfer is simply "reformatting."

Conservation efforts are sometimes made to extend the life of the original source material, but the main purpose is to preserve the audio through an exacting transfer to a long term, managed digital storage medium.

Audio preservation doesn't have to be expensive - we recently reformatted the above project for an average of $32.22 per reel!

Traditional recording media is unstable over time. No matter what the audio medium - memobelt, cylinder, wire, magnetic tape, optical disc (i.e. audio CD), mechanical disk (i.e. phonograph record) - all recording media deteriorates over time.

In addition, there is no guarantee that either the skilled personnel or the equipment designed for playing back a particular format will be available in the future.

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Preservation vs. Restoration

Audio preservation and audio restoration have entirely different objectives. With audio restoration the goal is to improve the program material in order to increase its listenability for a specific purpose.

Audio restoration methods vary widely depending on the source material and its intended use.

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Past & Present

Until a few years ago, audio preservation transfers were done mostly by transferring audio to 1/4" open reel analog tape. Learn more.

Audio preservation today involves a digital transfer, usually to a broadcast wave file (BWF) format.

The Grammy Foundation often publishes their methodology for audio preservation, which is reportedly based on IASA standards. See http://www.grammy.com/GRAMMY_Foundation/Grants/ for currently available information.

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Digital Details

Ordinary Audio CDs have 16 bit resolution with a sample rate of 44.1kHz. This is the original specification set forth jointly by Sony and Philips in 1981; it's often referred to as the "Red Book" standard. To simplify greatly, the terms "bit resolution" and "sample rate" are technical terms used to roughly describe volume (in relation to noise) and frequency response capabilities.

For technical reasons, it is not possible to reproduce audio content at more than one-half (1/2) of the sample rate frequency. Above this point objectionable artifacts occur, so filtering is required to eliminate those artifacts. Even though human hearing does not exceed 20kHz, sample rates higher than 44.1kHz have come into favor, largely because of limitations of most current filter designs in eliminating these artifacts.

Under the premise that more is better, sample rates as high as 192kHz have been advocated for use in preservation transfers. While the selection of a single optimum sample rate is somewhat controversial, at least one study has shown that using a sample rate above 96kHz provides no benefit when used with well designed filtering, and in fact using 192 kHz results in reduced accuracy as well as unnecessary additional expense, both in storage and equipment requirements. See Dan Lavry's white paper on this topic.

24 bit resolution is the generally accepted choice for preservation transfers.

Although digital files can be duplicated perfectly (i.e., "cloned"), whenever a digital audio file is actually modified (whether by changing gain, equalization, balance, or any other reason) dither must be applied. Dither adds a small amount of random noise, but it is an acceptable trade off for reducing the inevitable distortion caused by changing the original file's structure.

Gratuitous changes in digital audio files should always be avoided because there is an unavoidable loss of quality with each modification.

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Today's Choices

In many people's minds today, audio preservation only means re-recording to audio CDs, but audio CDs do not have the high sample rates and 24 bit resolution recommended for audio preservation.

In addition to audio CDs, the MP3 file format is now in widespread use. However, the MP3 format uses lossy compression. This means that some information is permanently discarded to save file space. This makes the MP3 format unacceptable for use as a primary audio preservation destination format, but MP3s are highly convenient for casual, research use. Literally thousands of reels of audio recordings can be reduced to fit on a single iPod, albeit with some loss in sound quality.

MP3s are easily distributed over the Internet, over a local area network, or on DVD-ROM or CD-ROM. MP3s can be encoded in a wide variety of bit rates (and sample rates), and the selection of bit rates for MP3s is a compromise between bandwidth, storage and fidelity requirements. "Bit rate" is the number of bits transferred per unit of time; higher bit rates have less compression artifacts. There is no single standard for MP3 bit rates; instead, a particular bit rate is chosen for a specific intended use.

High resolution digital audio files should be the primary audio preservation format. For additional safety, these files can be copied, placed in different locations or servers, and stored as duplicate originals. Information about the files should be stored safely for provenance.

Although selecting any sample rate and bit resolution may be controversial, funding sources in the past have frequently specified 24 bit 96kHz, which has emerged as one standard.

For more information, see

http://www.archives.gov/preservation/conferences/papers-2003/fleischauer.html.

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Transfer Tips

1. The slower the tape speed, the more important the azimuth adjustment. This especially applies to audio cassettes because of their slow tape speed.
2. Use a 4-channel tape player to diagnose track formats on 1/4" tape. The use of special sprays on your tape is messy and will not provide a fast diagnosis; on amateur tapes, track formats can change without warning. Select a playback head to match track format. Stereo 1/4" tapes can be Euro butterfly, NAB or Ampex track widths, and the proper selection of playback head can reduce noise and eliminate the fringing effect.
3. Optimize stylus selection for disks, both in size and shape. Experiment freely in different areas of the disk, because a gouged groove can be rescued by using a stylus riding higher or lower in the groove, but may increase noise or distortion in unaffected areas.
4. Play back analog media at the proper speed during the initial transfer. Often a hum signature will provide a powerful clue, but not all power mains were regulated at precisely 60Hz in the United States until after WWII. After that time, most of the US metropolitan areas were synchronized. Without a hum signature, if you know the proper key the selection would have been performed in, you can sometimes adjust playback speed using a chromatic tuner. Speed correction in the digital domain is not trivial, and is usually not transparent - in spite of what the ads claim. Always document any speed adjustments you make and try to do it in the analog domain.
5. As a general rule, do not use an ordinary computer audio card to provide analog-to-digital conversion.
6. Always use the original digital transfer for preservation. If you must experiment or process later for restoration purposes, always work on a copy of the original, making detailed notes on what you have done - i.e., provide an audit trail for future use. Never compromise your original transfer.
7. Do not allow digital overloads during the initial transfer. You will not need the full dynamic range available with 24 bit resolution audio, so don't push levels.
8. To reduce hum, noise and distortion always connect your analog player directly to your a/d converter. Adjust signal levels properly.
   

Maintain a detailed audio preservation and transfer log, and label each preservation transfer clearly. The log should contain source and destination stop and start times, track formats, speed, disk diameter, etc. Together with your labeling, this log will provide complete provenance for your work product. Serialize each piece of source material using simple numbers rather than full, accession numbers because you will reduce errors in your log and be better able to cope with the initial transfer which requires your greatest attention and skills.

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