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, 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.^{[6][7][8][9][10]}

AC bias

The original patent for AC bias was filed by Wendell L. Carlson and Glenn L. Carpenter in 1921, eventually resulting in a patent in 1927.^[11] 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. The first rediscovery seems to have been by Dean Wooldridge at Bell Telephone Laboratories, around 1937, but their lawyers found the original patent, and Bell simply kept silent about their rediscovery of AC bias.^[12]

Teiji Igarashi, Makoto Ishikawa, and Kenzo Nagai of Japan published a paper on AC biasing in 1938 and received a Japanese patent in 1940.^[13] Marvin Camras (USA) also rediscovered high-frequency (AC) bias independently in 1941 and received a patent in 1944.^[14]

The reduction in distortion and noise provided by AC bias was rediscovered in 1940 by Walter Weber while working at the Reichs-Rundfunk-Gesellschaft (RRG).^[15]