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