Tape bias

Tape bias is the term for two techniques, AC bias and DC bias, that improve the fidelity of analogue magnetic tape sound recordings. DC bias is the addition of a direct current to the audio signal that is being recorded. AC bias is the addition of an inaudible high-frequency signal (generally from 40 to 150 kHz) to the audio signal. Magnetic tape without AC bias has a nonlinear response at low signal strengths, as measured by its coercivity. Bias increases the signal quality of most audio recordings significantly by pushing the signal into the linear zone of the tape's transfer function.

Magnetic recording was proposed as early as 1888 by Oberlin Smith, who published 1888-09-08 in The Electrical World as "Some possible forms of phonograph". By 1898 Valdemar Poulsen had demonstrated a magnetic recorder and proposed magnetic tape.Fritz Pfleumer was granted a German patent for a non-magnetic "Sound recording carrier" with a magnetic coating, on 1928-01-31, but it was later overturned in favor of an earlier US patent by Joseph A. O'Neill.

The earliest magnetic recording systems simply applied the unadulterated (baseband) input signal to a recording head, resulting in recordings with poor low-frequency response and high distortion. Within short order, the addition of a suitable direct current to the signal, a DC bias, was found to reduce distortion by operating the tape substantially within its linear-response region. The principal disadvantage of DC bias was that it left the tape with a net magnetization, which generated significant noise on replay because of the grain of the tape particles. Some early DC-bias systems used a permanent magnet that was placed near the record head. It had to be swung out of the way for replay. DC bias was replaced by AC bias but was later re-adopted by some very low-cost cassette recorders.

Although the improvements are marked with such DC bias, even more dramatic improvement results if an alternating-current bias is used instead. While several people around the world rediscovered AC bias, it was the German developments that were widely used in practice and served as the model for future work.

...
Wikipedia