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At 11:00 AM 3/25/2003 -0500, Jonathan Wise wrote:
>Hello all,
>
>Can anyone tell me how tape heads and guides actually get magnetized? Is
>there only one way to demagnetize (using a demagnetizer), and finally, how
>frequently should the heads be demagnetized?

I had hoped that someone with expertise would reply. Failing that, I'll
offer what I recall.

Heads become magnetized by assymetry in the (AC) bias used in recording,
due to even harmonics in playback, and in the erase signal. Moving the tape
across such a head partially erases it, raising noise and decreasing
signal. Consequently degaussing is highly recommended. Properly used, a
degausser will do the job; I know of no alternative. As I recall, the
recommended frequency is every ten hours of use. I suspect that that is
overly conservative and that it may have arisen from people routinely
degaussing at the start or end of the workday. I know of no analysis and no
way that such an analysis could be conducted usefully. After all, no one
*intends* to use a distorted bias or erase signal, so the asymmetry will
vary from tape to tape in amplitude and direction. Thanks to random walk,
there will be a net effect - but how can any of that be quantified?

Note that if you are dealing with old, amateur tape, DC bias may have been
used. In that case, much more frequent degaussing is in order including a
thorough job when that set of tapes has been run.

One final note on bias. On good playback equipment, you can see the record
bias signal in quiet passages and determine the frequency used. That can be
useful in pitching the recording, though residual hum is more reliable. To
use bias, you must have first corrected one recording, then determined its
bias frequency. Another tape from the same machine at about the same time
may be assumed to have the same bias frequency, so the adjustment can be
computed, the sampling rate set correspondingly and an on-pitch transfer
effected easily.


Mike
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http://www.mrichter.com/