How to Digitize Analog Audio Recordings
Some formats, such as wax cylinders, lacquer discs, and wire recordings, are too fragile to be digitized by the average linguist, and should be handled by professionals. However, many field linguists have magnetic tapes that are candidates for digitization, both to make them more accessible, and to preserve them before they deteriorate. Caution: If the tape shows signs of decay (e.g., mold, or any visible damage to the tape layers), consult an audio preservation specialist first; any attempt to play a damaged tape may ruin it. But if the tape appears to be in good shape, you should be able to convert it yourself. Here is a general outline of the digitization process.
- An analog device to play back the tapes. It is easiest if this is the same kind of machine that was used to make the tapes originally. If not, be aware of the following concerns:
- Recording speed. Cassette tapes were usually recorded at 1 7/8-IPS (inches per second), but a small percentage were recorded at half of that speed. Reel-to-reel tapes may have been recorded at 3, 7, 1 7/8, or 15-IPS, but newer tape decks often can only play back at 7 and 15-IPS. If the playback speed is incorrect, you will be able to hear the difference, so be sure to listen to the recording before digitizing it.
- Monophonic vs. stereophonic. Older portable cassette tape recorders were often mono only; reel-to-reel decks came in both formats. Playing a mono tape on a stereo deck, or vice-versa, leads to loss of part of the signal, so be sure to use the right kind of playback device.
- An audio analog to digital (AD) converter. Internal sound cards introduce noise into the signal, and generally do not produce digital audio of sufficient quality for long-term preservation; therefore, a high-quality external converter is needed. If this is outside your budget, keep in mind that university libraries often have this sort of equipment available. The right sort of converter will have the following specifications:
- Both 44.1 (or 48) and 96 kHz sampling rates.
- Both 16 and 24-bit depths.
- A low noise level, displayed in dBs (decibels).
- An audio software program, installed on the computer you'll be using. Pro Tools, Adobe Audition, Audacity, and Sound Forge are examples of the sort of software you'll need. Again, these may be available through the library at your institution.
- Digital storage media. This could include CDs, DVDs, external hard drives, or network servers to which you have access.
- Connect the analog playback device to the converter, then connect the converter to your computer.
- Start the audio software application on your computer, and follow the application's instructions to capture the signal from the playback device.
- Keep in mind that the time needed to digitize a recording equals the time the recording lasts; therefore, if you have half an hour of audio on one side of a tape, it will take that long to capture the audio on your computer.
- If you have separate sessions recorded on a tape, for example, two separate narratives, then you will need to either stop the tape and the audio application between the two, or else save them as one file and divide them later. The software has no way of knowing when one segment ends and another begins.
- At a minimum, you should use a sampling rate of 44.1 (or 48) kHz, and a bit-depth of 16.
- When the recording is finished, stop the playback device and the application, and save the file.
- For long-term preservation, save the file in an uncompressed format, preferably WAV, Broadcast WAV (same as WAV. but with the capacity to save a small amount of metadata in the header), or AIFF.
- In general, the archival copy should not be edited in any way; that is, do not apply any form of noise reduction, equalization, or other processes that will affect the sound quality. If you like, you can apply these later when making presentation copies, but the master copy should not be enhanced.
- Give the file a name that will make sense to you, and hopefully others, later on. This could be a numeric ID, a date, some brief version of the topic or the consultant's name, or some other scheme. It's up to you, as long as you:
- use a consistent file-naming scheme for all of your audio files, and
- keep the metadata (e.g., name of consultant(s), recording date, recording location, subject, digitization date, software application used, sampling rate and bit-depth used, etc.) in a separate document.
- Make backup copies of the file as soon as possible, on CDs or DVDs or an external hard drive, or by uploading to a network. The more copies you make, the safer your work will be.
- Save your analog tapes; those are backup copies too. Furthermore, it's possible that future advances in technology will make it possible to create more accurate digital versions of the analog audio, so it's best to keep the tapes for as long as possible.
- After you've saved digital master copies of your recording, you can make presentation copies as needed (e.g., MP3 versions that can be downloaded quickly), but these should never replace the archival master copy.
See also Sound Directions: Best Practices for Audio Preservation.
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E-MELD School of Best Practice: How to Digitize Analog Audio Recordings
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