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228:) 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
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176:. Without bias, this response results in poor performance, especially at low signal levels. A recording signal that generates a magnetic field strength less than the tape's coercivity cannot magnetise the tape and produces little playback signal. Bias increases the signal quality of most audio recordings significantly by pushing the signal into more
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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. These AC biased magnetophons provided a fidelity of recording that outperformed any other recording system of the time.
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The last production DC biased
Magnetophon machines had harmonic distortion in excess of 10 percent; a dynamic range of 40 dB and a frequency response of just 50 Hz to 6 kHz at a tape speed slightly in excess of 30 inches per second (76.8 cm/sec). The AC biased Magnetophon machines
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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
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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. 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
314:
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:
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196:, 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
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with a magnetic coating, on 1 January 1928, Years earlier, Joseph O'Neil had created a similar recording medium, yet had not made a working machine that could record sound.
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Visualization of the magnetic field on a stereo cassette containing a 1 kHz audio tone. Individual high-frequency magnetic domains are visible.
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Teiji
Igarashi, Makoto Ishikawa, and Kenzo Nagai of Japan published a paper on AC biasing in 1938 and received a Japanese patent in 1940.
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250:, around 1937, but their lawyers found the original patent, and Bell simply kept silent about their rediscovery of AC bias.
349:"Method of and apparatus for effecting the storing up of speech or signals by magnetically influencing magnetisable bodies"
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153:. DC bias is the addition of direct current to the audio signal that is being recorded. AC bias is the addition of an
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that he had been working on developed an 'unwanted' oscillation in its record circuitry.
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largely ignored. The first rediscovery seems to have been by Dean
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The earliest magnetic recording systems simply applied the unadulterated (
602:"Some Popular Misconceptions About Magnetic Recording History and Theory"
536:"Walter Weber's Technical Innovation at the Reichs-Rundfunk-Gesellschaft"
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Some
Popular Misconceptions About Magnetic Recording History and Theory
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Carlson, Wendell L. and
Carpenter, Glenn W., "Radio telegraph system"
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161:) to the audio signal. Most contemporary tape recorders use AC bias.
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had demonstrated a magnetic recorder and proposed magnetic tape.
381:"Historical Development of Magnetic Recording and Tape Recorder"
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Technique that improves the fidelity of analogue tape recorders
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444:"The Birth of the German Magnetophon Tape Recorder 1928-1945"
417:. 2013. section "Linearity improvement with bias". p. 3-7.
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Camras, Marvin, "Method and means of magnetic recording"
461:(filed: 26 March 1921 ; issued: 30 April 1927).
473:"AC Bias at Bell Telephone Laboratories, 1936...1939"
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Magnetic recording was proposed as early as 1878 by
149:and DC bias, that improve the fidelity of analogue
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524:(filed: 22 December 1941; issued: 13 June 1944).
212:was granted a German patent for a non-magnetic
402:"Video and Camcorder Servicing and Technology"
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565:"Some Not Well Known Aspects of Analog Tape"
404:. 2001. section "Tape bias (audio)". p. 5.
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120:Learn how and when to remove this message
600:Jay McKnight; Jeffrey McKnight (2012).
489:Jay McKnight; Jeffrey McKnight (2012),
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534:Engel, Friedrich Karl (August 2006).
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563:Klingelnberg, Arndt (1 March 1990).
329:"Some possible forms of phonograph,"
58:adding citations to reliable sources
202:"Some possible forms of phonograph"
388:Museum of Magnetic Sound Recording
327:Smith, Oberlin (1888 September 8)
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415:"Audio Engineer's Reference Book"
351:, BP 8961, 1898, page 3, row 34.
145:is the term for two techniques,
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428:"Tape recorders, how they work"
157:(generally from 40 to 150
155:inaudible high-frequency signal
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172:response as determined by its
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442:Peter Hammar and Don Ososke.
180:zones of the tape's magnetic
571:. Audio Engineering Society.
266:Reichs-Rundfunk-Gesellschaft
248:Bell Telephone Laboratories
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651:Biasing in Tape Recording
609:Audio Engineering Society
500:Audio Engineering Society
214:"Sound recording carrier"
585:The Inventor's Notebook
268:(RRG) when a DC-biased
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413:Michael Talbot-Smith.
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521:U.S. patent 2,351,004
458:U.S. patent 1,640,881
426:Charles G. Westcott.
338:(10) : 116–117.
264:while working at the
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379:Kimizuka, Masanori.
332:The Electrical World
198:The Electrical World
54:improve this article
347:Poulsen, Valdemar,
576:O'Kelly, Terence.
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52:Please help
47:verification
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505:18 December
446:. p. 34-35.
270:Magnetophon
237:recorders.
69:"Tape bias"
660:Categories
302:References
296:Hysteresis
174:coercivity
80:newspapers
587:(3). BASF
362:DE 500900
170:nonlinear
143:Tape bias
110:July 2022
614:9 August
280:See also
235:cassette
226:baseband
591:8 April
544:18 June
241:AC bias
220:DC bias
188:History
147:AC bias
94:scholar
18:AC bias
578:"Bias"
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291:Dither
178:linear
168:has a
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101:JSTOR
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507:2018
73:news
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