Tape bias is a high-frequency signal (generally from 40 to 150 kHz) added to the audio signal recorded on an analog tape recorder. Magnetic tape has a nonlinear response at low signal strengths (see coercivity); bias increases the signal quality of most audio recordings significantly compared to unbiased recordings by pushing the signal into the linear zone of the tape's transfer function. As the tape leaves the tape head, the bias partially demagnetizes the tape and the remaining net induction is essentially the difference between the positive and negative half-cycles of the previously recorded. This differencing operation further cancels some of the nonlinearity.
Early tape recorders simply applied the unadulterated input signal to the record head, resulting in mediocre recordings with poor low-frequency response. In 1940, J. von Braunmühl and Dr. W. Weber accidentally discovered that the addition of a high-frequency tone kept the recorded signal in the linear response zone, resulting in a striking quality improvement.
Different levels of bias are needed for different types of tape, so most recorders offer a bias setting switch on the front panel, or, in the case of the compact audio cassette, may switch automatically according to cutouts on the cassette shell.
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