Editing Tape bias (section)

== History ==
Magnetic recording was proposed as early as 1878 by [[Oberlin Smith]], who on 4 October 1878 filed, with the U.S. patent office, a caveat regarding the magnetic recording of sound and who published his ideas on the subject in the 8 September 1888 issue of ''The Electrical World'' as ''"Some possible forms of phonograph"''.<ref>Engel, Friedrich Karl, ed. (2006) "Oberlin Smith and the invention of magnetic sound recording:  An appreciation on the 150th anniversary of the inventor's birth".  Smith's caveat of 4 October 1878 regarding the recording of sound on magnetic media appears on pp. 14–16.  Available at:  [http://www.richardhess.com/tape/history/Engel--Oberlin_Smith_2006.pdf RichardHess.com]</ref><ref>Smith, Oberlin (1888 September 8) [https://books.google.com/books?id=zYVMAAAAYAAJ&pg=RA2-PA116 "Some possible forms of phonograph,"] ''The Electrical World'', '''12''' (10) :  116–117.</ref>  By 1898, [[Valdemar Poulsen]] had demonstrated a magnetic recorder and proposed magnetic tape.<ref>Poulsen, Valdemar, ''"Method of and apparatus for effecting the storing up of speech or signals by magnetically influencing magnetisable bodies"'', BP 8961, 1898, page 3, row 34.</ref> [[Fritz Pfleumer]] was granted a German patent for a non-magnetic ''"Sound recording carrier"'' with a magnetic coating, on 1 January 1928.<ref>{{patent|DE|500900}}</ref> Years earlier, Joseph O'Neil had created a similar recording medium, yet had not made a working machine that could record sound.<ref>{{cite web |last1=Kimizuka |first1=Masanori |title=Historical Development of Magnetic Recording and Tape Recorder |url=https://museumofmagneticsoundrecording.org/images/R2R/HistoricalDevelopmentTapeRecorderMasanoriKimizuka.pdf |website=Museum of Magnetic Sound Recording}}</ref>

=== 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.  However: the earlier [[wire recorder]]s were largely immune to the problem due to their high running speed and relatively large wire size. 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 [[Compact Cassette|cassette]] recorders.<ref>Steve Beeching. [https://books.google.com/books?id=0xXCNpEhFEMC&pg=PA5 "Video and Camcorder Servicing and Technology"]. 2001. section "Tape bias (audio)". p. 5.</ref><ref>Michael Talbot-Smith. [https://books.google.com/books?id=XOvf30iChsYC&pg=SA3-PA7 "Audio Engineer's Reference Book"]. 2013. section "Linearity improvement with bias". p. 3-7.</ref><ref>Charles G. Westcott. [https://books.google.com/books?id=tRBTAAAAMAAJ "Tape recorders, how they work"]. 1956. p. 83.</ref><ref name="magneto">Peter Hammar and Don Ososke.
[https://www.americanradiohistory.com/Archive-DB-Magazine/80s/DB-1985-09-10.pdf "The Birth of the German Magnetophon Tape Recorder 1928-1945"]. p. 34-35.</ref>

=== 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" />