Tape bias

[3] Fritz Pfleumer was granted a German patent for a non-magnetic "Sound recording carrier" with a magnetic coating, on 1 January 1928.

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.

[10] The value of AC bias was somewhat masked by the fact that wire recording gained little benefit from the technique and Carlson and Carpenter's achievement was largely ignored.

[11] Teiji Igarashi, Makoto Ishikawa, and Kenzo Nagai of Japan published a paper on AC biasing in 1938 and received a Japanese patent in 1940.

The AC biased Magnetophon machines reduced the harmonic distortion to well under 3 percent; extended the dynamic range to 65 dB and the frequency response was now from 40 Hz to 15 kHz at the same tape speed.