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The issue of archiving audio materials is a controversial one. Some argue for using analog open-reel tape because we have had more experience with it than any other high quality medium. However, there are are also strong arguments for archiving to optical digital media, particularly the recordable compact disc. Here are some of the pros and cons of archiving to CDR:
1. Open reel analog audio tape is, by definition, an impermanent medium. It was made to be erased and reused easily. While it retains signal reasonably well over time, some deterioration of the magnetic field does occur that is audible in the highest quality applications. In addition, the phenomenon of "print through" of the signal to adjacent tape layers (audible as a faint echo) also occurs during storage. Digital recordings are not subject to these problems.
2. The greatest problems with the archival performance of analog tape are caused by the deterioration of the physical medium itself. Such phenomenons as binder breakdown (the cause of sticky-shed syndrome), deformation of the tape backing due to improper storage, etc., are all too common.
3. Digital tape recordings suffer from the same physical deterioration as analog tape recordings, but the consequences are even more serious. A problem that would cause audible deterioration in the sound of an analog tape may render an equivalent digital tape completely unplayable. For this reason, digital tape (such as DAT) is not recommended for long-term storage of important recordings.
Optical disc recording technology offers a much more stable medium for digital data than tape. The recordable CD, in particular, has become very popular due to its convenience and low cost. It is a viable candidate for archival storage, but there are still some issues to be considered:
1. The organic dye used in the recording layer of CDR's is not perfectly stable, especially when exposed to light. Estimates of the expected archival life of a recorded CDR under normal storage conditions range from 25 to 100 years. Since the format has only been around for about a dozen years, we don't yet know how well this corresponds to reality. Of the various dyes used, phthalocyanine (as opposed to cyanine) seems to be more resistant to deterioration from exposure to light. CDR's made with that particular dye may be a better bet for archival use.
2. In any case, it is best to avoid cheap, no-name CDR discs for archival purposes. Marketers of such discs usually get them from whatever source will give them the lowest unit price at a particular time. So, while one batch might be entirely satisfactory, another batch might be from a different, less satisfactory source.
3. Under poor storage conditions (extreme heat, blazing sunlight on a window sill, killer mold compounds attacking the plastic, etc.), all bets are off. Nor is it a good idea to scratch or scuff the discs, so some control of who handles the discs is necessary.
(Typical archival practice is to make an accurate, unaltered copy of the program which is kept in storage and used only for making working copies, a working copy or two which may include enhancements to the sound [noise reduction, etc.], and circulating copies which may be lent out as needed. These last need not use as high quality media as the primary archival copies and can be replaced as necessary.)
4. Media incompatibility is also a factor. Some CD players do not like certain kinds of CDR's but play back other types fine. Theoretically, this should not be happening; all audio CD's are supposed to be burned to the same "Red Book" standards and should play on all properly functioning players. In the real world, I often burn copies on a couple of different brands of CDR blanks for my clients. That way, if their playback machine doesn't like a particular disc, the other brand of disc may play back OK. I do not know what the average incidence of this particular problem is, but I have encountered it often enough to be concerned about it. However...
5. If you have made a new disc and it plays back OK on a properly functioning machine, future media incompatibility problems can probably be overcome by simply trying a different playback machine from the one that malfunctions.
6. There is a good argument for burning archival CD-ROM's as a supplement to audio CD transfer: the CD-ROM format has additional error correction that the CD-audio format lacks. An audio CD that has uncorrectable errors may exhibit tics or pops or momentary dropouts, but may still be useable. However, a computer program stored on a CD-ROM may be unusable if the data is not recovered exactly, bit-for-bit. Thus, the extra error correction for that format.
7. A further archival advantage of the CD-ROM format over audio CD is the TOC (table of contents) structure of the respective discs. The audio CD TOC gives only an approximate description of the location of each track. The digital extraction software used in CD ripping must then work with the playback hardware to zero in on the exact location of the desired file when transferring it to a computer hard drive. This process is not always error free and sometimes produces glitches in the resulting files. In contrast, the CD-ROM TOC gives the precise location of each file on the disc, eliminating the necessity for this extra layer of processing (and its potential for errors) during file transfer.
8. From an archival point of view, items 6) and 7) imply that sound files stored as WAV or AIFF files on a CD-ROM may be recoverable error-free under conditions that would render an audio CDR unusable, or nearly so. Also, the audio CD standard is limited to 16 bit depth, 44.1 kHz sampling, but CD-ROM files can be encoded in virtually any bit depth and sample rate.
9. Whether we archive to audio CD or CD-ROM, we have the advantage of a huge base of installed players for each format. There are, of course, millions of consumer CD players in existence, and most modern computers include CD-ROM players, with software for playing audio files included with the operating system. (Advocates of analog tape archiving should note that production of professional analog open reel tape recorders has almost entirely ceased. While parts and support can still be gotten for many machines, that may not be so in the future.)
10. In any case, we should be archiving our work in a digital format. Digital formats have the great virtue of being independent of the carrier medium, up to a point. As long as you can recover the data, you can reconstruct the signal at its original quality level. If a particular CDR starts to show signs of deterioration, just make a digital copy of the contents on new media. Analog formats lose quality with every generation of copying. At some point, that quality loss will become unacceptable.
Copyright 2002, Robert Auld
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