So in a tape recorder, the quantity of magnetic moment imparted to a unit area of magnetic tape is not a linear function of the applied magnetic flux; the transfer function looks kind of like a cumulative distribution function, if that makes sense. So if one just ran an audio signal directly through a magnetic tape head you'd get distortion on playback because of the non-linearity of the transfer function. The way to get rid of this distortion is to apply a bias current to the tape head so the audio signal stays in the linear region of the transfer function. However, on the schematics of various tape recorders I've seen the bias current isn't DC, but is usually an ultrasonic sine wave. Does anyone have any idea why this is so? I can't say I've ever seen a good explanation. Edit: I guess if one thinks of the tape head as a one half of a tiny transformer - maybe it has something to do with avoiding core saturation if one thinks of the tape itself as the secondary?