[net.audio] Why it might not be good to demagnetize

stevans (01/03/83)

No, I do demagnetize my tape recorder heads, about once a month.

The exact reason I think it may not be necessary or desirable
is as follows:

   A completely demagnetized head is not demagnetized relative to the
   tapes recorded and played in the deck.  A truly demagnetized head
   is at "zero" magnetic level, while the average magnetic level of
   the recorded tapes is the nonzero bias.  As you play and record
   tapes, the magnetic field of a head will asymptotically assume
   that of the bias level.  The truly demagnetized head will thus erase
   more of the recorded information than the one which has adjusted itself
   to the bias level, since the average difference between the demagnetized
   head and the tape is greater than the average difference between
   the bias level head and the tape.

   In time, the head will always adjust itself to the bias level of the
   tapes, erasing them less, but producing more noise on playback.  I am
   not sure of how much more noise, but I don't believe it is more than
   a few dB.  Though I don't like noise, I would rather hear it more
   and erase my tapes less.

Can anyone explain why I might be wrong in these theories?

          Mark Stevans, U of Rochester, NY
          (seismo or allegra or brl-bmd)!rochester!stevans

karn (01/03/83)

I don't understand the discussion of "bias" recorded on tapes that could
affect playback heads.

A long long time ago, before "hi fi", cheap recorders did indeed use DC
bias on record heads, in place of the high frequency oscillator now
standard.  This had the tendency to leave residual magnetism in
the record head, and since the record head was usually also the playback
head, it would cause all the nasty symptoms attributable to head
magnetization.

However, modern recorders use high frequency bias oscillators which are
carefully designed to generate NO dc component that could magnetize a
head; the average head current is zero.  Furthermore, the bias signal
appears ONLY during the recording process. Its wavelength on the
tape, even at the highest tape speed, is considerably smaller than the
head gap, so it cannot be recorded.  Its sole function is to remove the
severe cross-over distortion that would otherwise result from overcoming
the hysteresis threshold inherent in magnetic materials.

Phil Karn

jwb (01/04/83)

I tried a mail response but could not make it.  The bias is AC, therefore
its average value is zero.  Therefore if the head assymptotically assumes
this value then its residual magnetism will be zero.  Is this wrong?