Editing Tape bias (section)

=== 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.<ref>Carlson, Wendell L. and Carpenter, Glenn W., "Radio telegraph system" {{US patent|1640881}} (filed:  26 March 1921 ; issued:  30 April 1927).</ref> 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.  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.<ref>{{Cite web|url = http://www.aes.org/aeshc/pdf/mcknight_ac-bias-at-btl-1936-1939.pdf|title = AC Bias at Bell Telephone Laboratories, 1936...1939|last = McKnight|first = Jay}}</ref>

Teiji Igarashi, Makoto Ishikawa, and Kenzo Nagai of Japan published a paper on AC biasing in 1938 and received a Japanese patent in 1940.<ref>{{citation |author1=Jay McKnight |author2=Jeffrey McKnight |date=2012 |url=http://www.aes.org/aeshc/pdf/mcknight_some-popular-misconceptions.pdf |title=Some Popular Misconceptions About Magnetic Recording History and Theory |publisher=[[Audio Engineering Society]] |access-date=2018-12-18}}</ref> [[Marvin Camras]] (USA) also rediscovered high-frequency (AC) bias independently in 1941 and received a patent in 1944.<ref>Camras, Marvin, "Method and means of magnetic recording" {{US patent|2351004}} (filed:  22 December 1941; issued: 13 June 1944).</ref>

The reduction in distortion and noise provided by AC bias was accidentally rediscovered in 1940 by [[Walter Weber (engineer)|Walter Weber]]  while working at the [[Reichs-Rundfunk-Gesellschaft]] (RRG) when a DC-biased [[Magnetophon]] that he had been working on developed an 'unwanted' oscillation in its record circuitry.<ref name="engel-weber">{{cite web |url=http://www.richardhess.com/tape/history/Engel--Walter_Weber_2006.pdf |title=Walter Weber's Technical Innovation at the Reichs-Rundfunk-Gesellschaft |last=Engel |first=Friedrich Karl |date=August 2006 |access-date=18 June 2010}}</ref> 

The last production DC biased Magnetophon machines had harmonic distortion in excess of 10 percent; a dynamic range of 40&nbsp;dB and a frequency response of just 50&nbsp;Hz to 6&nbsp;kHz at a tape speed slightly in excess of 30 inches per second (76.8&nbsp;cm/sec). The AC biased Magnetophon machines reduced the harmonic distortion to well under 3 percent; extended the dynamic range to 65&nbsp;dB and the frequency response was now from 40&nbsp;Hz to 15&nbsp;kHz at the same tape speed. These AC biased magnetophons provided a fidelity of recording that outperformed any other recording system of the time.<ref name="magneto" />