787. In our busy trial court we have preserved the original cassette tapes since 1989. They are kept in a relatively constant room temperature environment in our modern courthouse. Should we take any further precautions to extend the life of these tapes, considering the possibility that they may need to be replayed one day, such as in the retrial of a death penalty case that is reversed a decade after trial? I've heard of the practice of unwinding and rewinding tapes for this purpose, but haven't attempted it yet. The time involved is daunting! What is your opinion? — JD, Bryan, TX
A magnetic recording tape is usually a Mylar ribbon, coated with a thin layer of plastic that's impregnated with tiny permanent magnets. As long as it's store away from heat and moisture, the Mylar film itself shouldn't age. However, the layer of permanent magnets can change slightly with time. When a tape is left tightly wound on its reel for a long time, the magnetic layers can begin to affect one another—the magnetic fields from one layer of tape can alter the magnetization of the layers above and below it. The result is that sounds from one layer of tape can gradually transfer themselves weakly to the adjacent layers, creating faint echo effects. The solution to this problem is to unwind and rewind the tape, so that the layers shift slightly relative to one another. But while these echoes may be annoying in a recording of classical music, they probably aren't important in a recording of a noisy courtroom. Unless I hear otherwise from someone reading this note, I wouldn't worry about unwinding and rewinding your tapes. The slight imperfections that will result from transfers between layers shouldn't affect their utility in later trials. Properly stored, I'd expect the tapes to outlive everyone involved with the trials, even without any unwinding and rewinding.