From: utzoo!decvax!harpo!eagle!karn
Newsgroups: net.audio
Title: Re: Why it might not be good to demagnetize
Article-I.D.: eagle.702
Posted: Mon Jan  3 11:29:34 1983
Received: Tue Jan  4 01:51:45 1983
References: rocheste.360

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