The first patent for AC bias was filed by W. L. Carlson and Glenn L. Carpenter in 1921, eventually resulting in . 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 .
The striking improvement in distortion and noise provided by AC bias was rediscovered in 1940 by Walter Weber (1907-1944), while working for Hans Joachim von Braunmühl at the Reichs-Rundfunk-Gesellschaft (RRG). The pair received several related patents, including for "high frequency treatment of the sound carrier".
The UK company Boosey & Hawkes produced a steel wire recorder under Government contract during the Second World War that was equipped with AC bias . It is unlikely that they were aware of the German developments (otherwise they would probably have copied the tape system in its entirety). Examples still surface from time to time, many having been disposed of as Government surplus stock. After the war, Boosey and Hawkes also produced a "Reporter" tape recorder in the early 1950s using magnetic tape, rather than wire, which was based on German wartime technology.
Note: that this is a very simplistic explanation of AC bias. In practice, the full modus operandi of AC bias is not adequately understood. There are several prevailing theories, though none of them fully explains the operation, and in particular, what happens if too little or too much bias is applied.
B&O invented and patented the so-called Dolby HX (Headroom eXtension) Pro principle for combining bias control with the Dolby system for better frequency response in cassette recorders. Akai invented the Crossfield system for tape recorders where during recording a separate head with the bias frequency modulating it was held very close to the recording head, producing a better bias, than by mixing the two signals in the recording head. But mechanical tolerances for Crossfield are tight, requiring frequent readjustment and this system was largely replaced.
Different amplitudes of bias field are optimal for different types of tape, so most recorders offer a bias setting switch on the control panel, or, in the case of the compact audio cassette, may switch automatically according to cutouts on the cassette shell. Ferric based tapes require the lowest bias field, with Chrome based tapes (including the pseudo chromes) requiring a higher level. Metal particle requires even more. Metal Evaporated tape accepts the highest level of bias, but it is mostly used for digital recording purposes. The same is valid for a combination cassette tape, the FeCr-variant, on which a thicker Ferric layer was covered by a thinner chrome layer. The idea behind this was that at lower frequencies and higher head currents the Fe layer would be deeper magnetised, while at higher frequencies only the top Cr-layer was active. In practice this proved disappointing and some claimed that this thin chrome layer was quickly polished off in heavy use.