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?