Tape bias

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Tape bias is the term for two phenomena, DC bias and AC 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 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 1878 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.[1] Fritz Pfleumer was granted DE 500900  for a "Sound recording carrier" on 1928-01-31, but it was later overturned in favour of the earlier US 1653467  by Joseph A. O'Neill.

DC bias

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 was found to reduce distortion by operating the head substantially within its linear response region. The principal disadvantage of DC bias was that it left the tape with a net magnetisation which, because of the grain of the tape particles, generated significant noise on replay. 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 re-adopted by some very low cost cassette recorders.

AC bias

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 discovered AC bias, it was the German developments that were widely used in practice and served as the model for future work.

The first patent for AC bias was filed by W. L. Carlson and Glenn L. Carpenter in 1921, eventually resulting in US 1640881  . The value of AC bias was somewhat masked by the primitive state of other aspects of magnetic recording, however, and Carlson and Carpenter's achievement was largely ignored. Teiji Igarishi, Mokoto Ishikawa, and Kenzo Nagai of Japan published a paper on AC biasing in 1938 and received a Japanese patent in 1940. Marvin Camras (USA) also discovered High Frequency (AC) Bias independently in 1941 and received US 2351004  .

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