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Adaptive biasing in magnetic audio recording is the technology of continuously varying the bias current according to the level of high-frequency audio signal. Higher levels of high-frequency audio signal cause a proportionate decrease in bias current, using either feedforward or, preferably, negative feedback control system. Compared to traditional fixed bias technology, adaptive biasing provides higher maximum output level and higher dynamic range at treble and, to a lesser extent, midrange frequencies. The effect of adaptive biasing is most pronounced in compact cassette and low-speed reel-to-reel media. The first commercial implementation, the feedforward Dolby HX, was developed by Dolby Laboratories by 1979, and was rejected by the industry. The subsequent negative-feedback Dolby HX Pro, developed by Bang & Olufsen and marketed by Dolby, became the de facto standard of the consumer high fidelity industry in the middle of the 1980s.

Fixed and adaptive biasing

Tape bias is a strong high-frequency alternating current injected into the recording head along with the audio signal with the purpose of linearizing the inherently nonlinear response of the magnetic particles.^[3] The frequency of the bias signal in consumer cassette decks is usually fixed at 80–100 kHz. The quality of the bias signal is critical, because noise, hum and direct current in the bias severely degrade audio fidelity.^[4] The level of the bias signal defines the slope and shape of the resulting magnetization curve.^[5]

Optimal bias level for each tape formulation is a compromise between maximum output levels, noise, distortion and frequency response.^[6]^[7] Nominal bias, corresponding to maximum sensitivity and/or maximum output at 10 kHz, is less so desirable for midrange frequencies. Overbiasing improves handling of tape dropouts but reduces tape sensitivity at treble frequencies and degrades the signal-to-noise ratio.^[8]^[9]^[10] Underbiasing causes excessive distortion, modulation noise and worsens susceptibility to dropouts and is thus distinctly unwanted.^[4]^[10] In practice, tape is always mildly overbiased: the optimal bias current is set at 2-3 dB above the nominal value.^[8]^[9]^[11]^[10] This optimal setting improves linearity at midrange frequencies, but also reduces dynamic range and causes a droop in treble frequency response, which is compensated for with pre-emphasis in the recording chain.^[9]^[11]

Recording at very low wavelengths (tape speeds of 4.76 cm/s and 9.53 cm/s, or 1 7/8 in/s and 3 3/4 in/s) presents another challenge.^[12]^[4] Audible high-frequency components of the recorded signal themselves act as biasing currents. The resulting excessive overbiasing manifests itself in dynamic range compression and early onset of saturation at treble frequencies, especially when recording on low quality tapes with low saturation levels.^[4]^[10] Typical music of the 1970s, published on vinyl discs or transmitted via FM radio, did not contain much treble energy and usually could not drive the tape into saturation.^[11] However, the digitally mastered, direct-to-disc and disco recordings of the late 1970s and early 1980 often contained enough "hot" treble to trigger tape overload.^[11]

In the end of the 1970s the industry proposed three solutions to the problem.^[13] Metal particle tapes had very high maximum output levels and treble saturation levels, but were prohibitively expensive for most home users.^[13] The early metal tapes had high absolute level of hiss, and there were fears that metal tape would degrade quickly^[13] (this did not happen). The second solution, developed independently by Tandberg and Akai, relied on limiting recording levels.^[13] The patented Tandberg Dyneq and Akai ADRS circuits compressed the signal electronically before it could overload the tape.^[13] Finally, in 1979 Kenneth James Gundry of Dolby Laboratories proposed the third alternative: adapting the bias current to the treble content of the source signal.^[13]^[11] Increase in treble energy, which effectively overbiases the tape, should be compensated with a reciprocal decrease in the output of the bias generator.^[13]

The effect of such compensation is evident from the typical magnetization curves.^[2] By default, when the treble energy of the source signal is low, the recorder operates at a fixed optimal bias current I_b.opt. (blue curve).^[2] Initial overbiasing assures good linearity but low sensitivity and low saturation level.^[2] Reduced bias current value of I_b.red. allows operation at higher input and output levels, albeit with a higher sensitivity (red curve).^[2]^[10] A well-designed adaptively biased circuit must decrease bias current gradually, so that the increase in sensitivity compensates the saturation effects.^[2] The new, adaptive magnetization curve remains straight (green dotted line) all the way to the maximum recording current I_af.1.^[2] Owing to self-biasing effects, distortion at middle frequencies remains low, and intermodulation decreases.^[13]

The location of the breakpoint I_af.o on the control curve, and the slope of its high-level segment depend on the frequency of the input signal, and the various energy loss mechanisms in the tape and the recording head.^[2] A practical adaptive biasing system must employ heuristic weighing over all treble frequencies to attain best possible performance for a specific recorder.^[9] Fortunately, the effect of changes in tape formulations is insignificant, at least in the case of ferric tapes.^[9] Different tapes require different optimal bias settings, but the bias control curve can stay the same for all ferric tapes.^[9]

Dolby HX

The original Dolby HX (for 'Headroom eXtension'), designed by Gundry, operated as an add-on to the Dolby B noise reduction encoder. The Dolby B integrated circuit, by design, extracts the envelope of the middle and treble components of the source signals, and uses it to modulate the gain of its side channel.^[13] The HX circuit blends the left and right envelope signals together.^[13] The composite envelope modulates the output of a voltage source that powers the common erase/bias generator.^[13] Simultaneously, it adjust the depth of high-frequency pre-emphasis of the two recording channels.^[13] Controlling both stereo channels with a single bias modulator was deemed acceptable due to the high degree of correlation between the left and right stereo signals, and poor channel separation of the existing analog sources available to the consumer.^[14] Controlling erase and bias currents simultaneously could potentially cause instant drops in erase effectiveness, but this only happened during the loudest passages with rich treble content, which completely drowned any residual unerased signals.^[15]

Independent tests had shown that the HX could raise the saturation levels at 10–12 kHz by 10 dB.^[13] According to Dolby, the improvement was most pronounced with high quality, high coercivity tape formulations. "Bad tapes remained bad", with or without adaptive biasing.^[16]

Dolby Laboratories rolled out the HX at the Consumer Electronics Show in June 1979.^[17] The novelty was offered to all existing Dolby B licensees at no extra charge.^[17] In 1980–1981 Aiwa, Harman Kardon and TEAC^[18] integrated the HX in their cassette decks, but there were no further followers.^[19] Despite favorable reviews, the HX was a marketing flop and an engineering failure.^[19] It was tested and rejected by audio engineers all over the world.^[19] While most did not disclose their findings, Willi Studer spoke publicly against the adoption of the HX.^[19] The shortcomings of the HX, said Studer, greatly outweighed its intended benefits.^[19] The 1981 press release by Dolby for the German market indirectly blamed the failure on the conservatism of the industry. The HX, said the company, "intervenes very far into recorder development and cannot simply be added to the existing electronics. It requires a fundamental redevelopment of the recording amplifier".^[20] Nevertheless, the company still expressed hope that the HX will gain acceptance "because it enables high fidelity quality with the future microcassette recorders with a tape speed of 2.4 cm/s".^[21] The latter promise did not materialize, either.

From a purely engineering viewpoing, the main drawback of the HX was that, being a feedforward control, it monitored the signal at its source, but not the signal actually reaching the recording head.^[14]^[22] Any variation in the gain or the frequency response of the recording chain disrupted the bias control curve.^[14] Adjustable pre-emphasis subcircuit was unnecessarily complex and expensive for the consumer industry.^[14] The Dolby B envelope detector, which by design was fairly slow, could not reliably track fast transients.^[14] Bundling adaptive biasing with noise reduction at the hardware level, in itself, was the worst of all shortcomings.^[14] The user could not turn off Dolby B decoder and still use Dolby HX while recording.^[23] This discouraged the use of the far more effective dbx noise reduction.^[23] On the other hand, the 30 dB gain in signal-to-noise ratio provided by the dbx made the HX virtually unnecessary.^[23]

Dolby HX Pro

The alternative implementation of adaptive biasing was patented in 1980 by Jørgen Selmer Jensen of Bang & Olufsen (B&O).^[24] Unlike the feedforward Dolby HX, the B&O circuit was a feedback system.^[22] According to the patent, it monitored the high frequency voltage at the "hot" end of the recording head, extracting the combined envelope of bias and treble audio signals.^[25]^[22] An error amplifier continuously compared the envelope with the preset reference level, and adjusted the audio signal current to the recording head^[22] via a resistive opto-isolator^[25] (in practical applications the latter were replaced with variable-gain amplifiers, for example, built around the LM13700 transconductance amplifier).^[22] The monaural circuit was easily scalable for two-channel stereo or multitrack recording, and enabled easy adjustments of the normal bias level.^[25]

According to B&O, their system assured only 3-5 dB gain in treble saturation level, far less than the Dolby HX.^[14] However, it did not rely on the Dolby chip, and could be used with or without any noise reduction system.^[14] Negative feedback compensated any variations in gain and frequency response of the recording chain, thus eliminating the key shortcoming of the original Dolby HX.^[14] As a side benefit, the B&O system was also effective in reel-to-reel recorders.^[14]^[26]

Dolby Laboratories acquired the rights to the B&O patent, and became its sole worldwide distributor.^[22] The new system was named Dolby HX Professional, later shortened to Dolby HX Pro. B&O retained the rights to use it in their own products, and according to sources affiliated with Selmer, received a share in future licensing revenue.^[27] Exact terms of the contract were not disclosed, and are subject of much debate in amateur audio communities.

In the beginning, Dolby targeted the HX Pro to professional market.^[14] In August 1982 Electro Sound, manufacturer of industrial tape duplicators, introduced HX Pro in its cassette duplicator catalogue.^[28] Warner Records became the first major recording label to adopt the HX Pro for mass duplication.^[29] By February 1983, according to Dolby, the company managed to sign up two licensees in the home audio industry: Aiwa and Harman Kardon.^[30] The early adopters had to build the HX Pro circuitry with general-purpose operational amplifiers and transconductance amplifiers, until the introduction of a dedicated integrated circuit, the NEC μPC1297, in 1985.^[31] The new proposal was well received by the industry, and by 1986 it became a standard feature in the upper segment of consumer cassette decks.^[29] In the few following years the HX Pro migrated into the entry-level consumer segment, thus becoming the de facto standard equipment in consumer hi-fi, and into the professional reel-to-reel recorders.^[26]

Subsequent development

In 1983, the adaptive biasing principle gained popularity in the former Soviet Union. The earliest implementation, published by Nikolay Sukhov, was developed after the HX Pro. It blended the elements of both HX Pro (feedback control) and the original HX (varying the supply voltage to the common erase/bias generator), and added safeguards against transient overload, a common problem when recording from worn LP records.^[14] The revised design, using the new precision rectifier IC, was published in 1987. In line with the tastes of the home audio community, which still preferred reel-to-reel to cassette, the 1987 version was targeted at both cassette and reel-to-reel decks.^[9] Adaptive biasing cannot substantially improve the performance of the quarter-inch tape running at 19.5 cm/s (7.5 in/s) in standard reel-to-reel recorders. However, it permits a decrease in treble equalization from the standard 50 μs to 10 μs.^[9] A fivefold reduction of the time constant corresponds to a fivefold decrease in apparent noise floor at middle and treble frequencies, and, according to the inventor, enables practical signal-to-noise ratio of more than 80 dB, without any noise reduction.^[9] Sukhov's designs were the subject of five patents issued between 1984 and 1989, all referenced to the earlier Selmer patent as prior art.^[32]

Notes

^ Sukhov 1987, p. 40, fig. 1-2.
^ ^a ^b ^c ^d ^e ^f ^g ^h Sukhov 1987, pp. 39–40.
^ Watkinson 1998, p. 310.
^ ^a ^b ^c ^d Watkinson 1998, p. 312.
^ Watkinson 1998, pp. 310–312.
^ Watkinson 1998, pp. 312–313.
^ Dolby 1981, p. 270.
^ ^a ^b Watkinson 1998, p. 313.
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ Sukhov 1987, p. 40.
^ ^a ^b ^c ^d ^e Dolby 1981, p. 269.
^ ^a ^b ^c ^d ^e Stark, Craig (1982). "The Dolby HX System" (PDF) (Tape Recording and Buying Guide): 18. {{cite journal}}: Cite journal requires |journal= (help)
^ Sukhov 1983, p. 36.
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m Sukhov 1983, p. 37.
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l Sukhov 1983, p. 38.
^ Sukhov 1983, p. 39.
^ Dolby 1981, p. 271: "Schlechte Bänder bleiben schlecht".
^ ^a ^b "Engineers sound off to high frequencies". New Scientist (21 Jun): 999. 1979.
^ "Teac decks and test tapes" (PDF). Modern Recording and Music (9): 31. 1980.
^ ^a ^b ^c ^d ^e Sukhov 1983, p. 40.
^ Dolby 1981, p. 269: "Eine technische Begründung für die spärliche Verbreitung des HX-Systems ist darin zu sehen, das es sehr weit in die Recorderentwicklung eingreift, und sich nicht einfach zu der vorhandenen Elektronik hinzufügen läßt. Dolby-HX erfordert eine grundlegende Neuentwicklung der Aufnahmeverstärker".
^ Dolby 1981, p. 269: "Dennoch stehen die Chancen für das HX-System nicht schlecht, weil es bei den kommenden (Mikro)-Cassettenrecordern mit 2,4 cm/s Bandgeschwindigkeit HiFi-Qualität ermöglicht.".
^ ^a ^b ^c ^d ^e ^f Self 2020, p. 56.
^ ^a ^b ^c Burstein, H. (1983). "Tape guide: Doubling in NR" (PDF). Audio (USA) (February): 26.
^ "G11B 5/47 0046410: Bias control method and apparatus for magnetic recording" (PDF). Europen Patent Bulletin (24.02): 40. 1982. ISSN 0170-9305.
^ ^a ^b ^c EP0046410A1, Jørgen Selmer Jensen, "Bias control method and apparatus for magnetic recording", published 1982-02-24, issued 1984-10-31, assigned to Bang & Olufsen
^ ^a ^b Hood 1999, p. 45.
^ "Jørgen Selmer Jensen". 2013.
^ "New Products / Dolby HX". Billboard: 31. 1982-08-28.
^ ^a ^b Shea, T. (1986). "Rx for tapes: HX Pro". Popular Mechanics (August): 34–35.
^ Dolby Laboratories (1983). "Dolby HX Professional" (PDF). Audio (USA) (February): 7.
^ "NEC IC Expands Bias in Audio Tape Heads". Journal of Electronic Engineering. 22: 19. 1985.
^ Patents SU 1448356 A1, SU 1448357 A1), SU 1539830 A1, SU 1531134 A1, SU 1610487 A1

References

In English

Watkinson, John (1998). The Art of Sound Reproduction. Taylor & Francis. ISBN 9780240515120.
Hood, John Linslay (1999). Audio Electronics. Newnes. p. 45. ISBN 9780750643320.
Self, Douglas (2020). Small Signal Audio Design, 3rd edition. CRC Press. ISBN 9781000050448.

In German

Dolby (1981). "Bessere Höhenaufzeichnung mit dem Dolby-HX-System" [Better treble recording with the Dolby HX System] (PDF). Funktechnik (in German) (August): 269–271.

In Russian

Sukhov, N. (1983). "Динамическое подмагничивание" [Dynamic biasing]. Радио (5): 36–40.
Sukhov, N. (1987). "СДП-2" [SDP-2]. Радио (1): 39–42.

[FOOTNOTESukhov198740,_fig._1-2-1] Sukhov 1987, p. 40, fig. 1-2.

[FOOTNOTESukhov198739–40-2] ^ ^a ^b ^c ^d ^e ^f ^g ^h Sukhov 1987, pp. 39–40.

[FOOTNOTEWatkinson1998310-3] Watkinson 1998, p. 310.

[FOOTNOTEWatkinson1998312-4] Watkinson 1998, p. 312.

[FOOTNOTEWatkinson1998310–312-5] Watkinson 1998, pp. 310–312.

[FOOTNOTEWatkinson1998312–313-6] Watkinson 1998, pp. 312–313.

[FOOTNOTEDolby1981270-7] Dolby 1981, p. 270.

[FOOTNOTEWatkinson1998313-8] Watkinson 1998, p. 313.

[FOOTNOTESukhov198740-9] ^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ Sukhov 1987, p. 40.

[FOOTNOTEDolby1981269-10] Dolby 1981, p. 269.

[CS-11] Stark, Craig (1982). "The Dolby HX System" (PDF) (Tape Recording and Buying Guide): 18. {{cite journal}}: Cite journal requires |journal= (help)

[FOOTNOTESukhov198336-12] Sukhov 1983, p. 36.

[FOOTNOTESukhov198337-13] ^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m Sukhov 1983, p. 37.

[FOOTNOTESukhov198338-14] ^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l Sukhov 1983, p. 38.

[FOOTNOTESukhov198339-15] Sukhov 1983, p. 39.

[FOOTNOTEDolby1981271:_"Schlechte_Bänder_bleiben_schlecht"-16] Dolby 1981, p. 271: "Schlechte Bänder bleiben schlecht".

[NS79-17] "Engineers sound off to high frequencies". New Scientist (21 Jun): 999. 1979.

[18] "Teac decks and test tapes" (PDF). Modern Recording and Music (9): 31. 1980.

[FOOTNOTESukhov198340-19] Sukhov 1983, p. 40.

[FOOTNOTEDolby1981269:_"Eine_technische_Begründung_für_die_spärliche_Verbreitung_des_HX-Systems_ist_darin_zu_sehen,_das_es_sehr_weit_in_die_Recorderentwicklung_eingreift,_und_sich_nicht_einfach_zu_der_vorhandenen_Elektronik_hinzufügen_läßt._Dolby-HX_erfordert_eine_grundlegende_Neuentwicklung_der_Aufnahmeverstärker"-20] Dolby 1981, p. 269: "Eine technische Begründung für die spärliche Verbreitung des HX-Systems ist darin zu sehen, das es sehr weit in die Recorderentwicklung eingreift, und sich nicht einfach zu der vorhandenen Elektronik hinzufügen läßt. Dolby-HX erfordert eine grundlegende Neuentwicklung der Aufnahmeverstärker".

[FOOTNOTEDolby1981269:_"Dennoch_stehen_die_Chancen_für_das_HX-System_nicht_schlecht,_weil_es_bei_den_kommenden_(Mikro)-Cassettenrecordern_mit_2,4_cm/s_Bandgeschwindigkeit_HiFi-Qualität_ermöglicht."-21] Dolby 1981, p. 269: "Dennoch stehen die Chancen für das HX-System nicht schlecht, weil es bei den kommenden (Mikro)-Cassettenrecordern mit 2,4 cm/s Bandgeschwindigkeit HiFi-Qualität ermöglicht.".

[FOOTNOTESelf202056-22] ^ ^a ^b ^c ^d ^e ^f Self 2020, p. 56.

[BUR-23] Burstein, H. (1983). "Tape guide: Doubling in NR" (PDF). Audio (USA) (February): 26.

[24] "G11B 5/47 0046410: Bias control method and apparatus for magnetic recording" (PDF). Europen Patent Bulletin (24.02): 40. 1982. ISSN 0170-9305.

[PAT01-25] EP0046410A1, Jørgen Selmer Jensen, "Bias control method and apparatus for magnetic recording", published 1982-02-24, issued 1984-10-31, assigned to Bang & Olufsen

[FOOTNOTEHood199945-26] Hood 1999, p. 45.

[SE-27] "Jørgen Selmer Jensen". 2013.

[28] "New Products / Dolby HX". Billboard: 31. 1982-08-28.

[PM86-29] Shea, T. (1986). "Rx for tapes: HX Pro". Popular Mechanics (August): 34–35.

[30] Dolby Laboratories (1983). "Dolby HX Professional" (PDF). Audio (USA) (February): 7.

[31] "NEC IC Expands Bias in Audio Tape Heads". Journal of Electronic Engineering. 22: 19. 1985.

[32] Patents SU 1448356 A1, SU 1448357 A1), SU 1539830 A1, SU 1531134 A1, SU 1610487 A1

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@@ Line 1: / Line 1: @@
 {{Copy edit|date=January 2022}}
-'''Adaptive biasing''' in [[Tape recorder|magnetic audio recording]] is the technology of continuously varying the [[Tape bias|bias current]] according to the level of high-frequency audio signal. Higher levels of high-frequency audio signal cause a proportionate decrease in bias current, using either [[Feed forward (control)|feedforward]] or, preferably, [[negative feedback]] [[control system]]. Compared to traditional fixed bias technology, adaptive biasing provides higher maximum output level and higher dynamic range at treble and, to a lesser extent, midrange frequencies. The effect of adaptive biasing is most pronounced in [[compact cassette]] and low-speed [[Reel-to-reel audio tape recording|reel-to-reel]] media. The first commercial implementation, the feedforward '''Dolby HX''', was developed by [[Dolby Laboratories]] by 1979, and was rejected by the industry. The subsequent negative-feedback '''Dolby HX Pro''', developed by [[Bang & Olufsen]] and marketed by Dolby, became the de-facto standard of the consumer [[high fidelity]] industry in the middle of the 1980s.
+'''Adaptive biasing''' in [[Tape recorder|magnetic audio recording]] is the technology of continuously varying the [[Tape bias|bias current]] according to the level of high-frequency audio signal. Higher levels of high-frequency audio signal cause a proportionate decrease in bias current, using either [[Feed forward (control)|feedforward]] or, preferably, [[negative feedback]] [[control system]]. Compared to traditional fixed bias technology, adaptive biasing provides higher maximum output level and higher dynamic range at treble and, to a lesser extent, midrange frequencies. The effect of adaptive biasing is most pronounced in [[compact cassette]] and low-speed [[Reel-to-reel audio tape recording|reel-to-reel]] media. The first commercial implementation, the feedforward '''Dolby HX''', was developed by [[Dolby Laboratories]] by 1979, and was rejected by the industry. The subsequent negative-feedback '''Dolby HX Pro''', developed by [[Bang & Olufsen]] and marketed by Dolby, became the de facto standard of the consumer [[high fidelity]] industry in the middle of the 1980s.
 == Fixed and adaptive biasing ==
 [[File:Magnetization curve - adaptive biasing principle 03.svg|thumb|right|300px|Simplified graphic explanation of the adaptive biasing principle.{{sfn|Sukhov|1987|p=40, fig. 1-2 }} The shown magnetization curves (top) and bias control curve (bottom) are valid only at treble frequencies. Exact values of the breakpoints and slopes vary with frequency.{{sfn|Sukhov|1987|pp=39-40}}]]
 [[Tape bias]] is a strong high-frequency [[alternating current]] injected into the [[Tape head|recording head]] along with the audio signal with the purpose of linearizing the [[Magnetic hysteresis|inherently nonlinear]] response of the magnetic particles.{{sfn|Watkinson|1998|p=310}} The frequency of the bias signal in consumer cassette decks is usually fixed at 80{{endash}}100 [[Hertz|kHz]]. The quality of the bias signal is critical, because [[Noise (electronics)|noise]], [[Mains hum|hum]] and [[direct current]] in the bias severely degrade audio fidelity.{{sfn|Watkinson|1998|p=312}} The level of the bias signal defines the slope and shape of the resulting magnetization curve.{{sfn|Watkinson|1998|pp=310-312}}
-Optimal bias level for each tape formulation is a compromise between maximum output levels, noise, distortion and frequency response.{{sfn|Watkinson|1998|pp=312-313}}{{sfn|Dolby|1981|p=270}} Nominal bias, corresponding to maximum sensitivity and/or maximum output at 10 kHz, is less so desirable for midrange frequencies. Overbiasing improves handling of tape dropouts but reduces tape sensitivity at treble frequencies and degrades the [[signal-to-noise ratio]].{{sfn|Watkinson|1998|p=313}}{{sfn|Sukhov|1987|p=40}}{{sfn|Dolby|1981|p=269}} Underbiasing causes excessive distortion, [[modulation noise]] and worsens susceptibility to dropouts and is thus distinctly unwanted.{{sfn|Watkinson|1998|p=312}}{{sfn|Dolby|1981|p=269}} In practice, tape is always mildly overbiased: the optimal bias current is set at 2-3 [[decibel|dB]] above the nominal value.{{sfn|Watkinson|1998|p=313}}{{sfn|Sukhov|1987|p=40}}<ref name=CS/>{{sfn|Dolby|1981|p=269}} This optimal setting improves linearity at midrange frequencies, but also reduces dynamic range and causes a droop in treble frequency response, which is compensated for with [[Emphasis (telecommunications)|pre-emphasis]] in the recording chain.{{sfn|Sukhov|1987|p=40}}<ref name=CS/>
+Optimal bias level for each tape formulation is a compromise between maximum output levels, noise, distortion and frequency response.{{sfn|Watkinson|1998|pp=312-313}}{{sfn|Dolby|1981|p=270}} Nominal bias, corresponding to maximum sensitivity and/or maximum output at 10&nbsp;kHz, is less so desirable for midrange frequencies. Overbiasing improves handling of tape dropouts but reduces tape sensitivity at treble frequencies and degrades the [[signal-to-noise ratio]].{{sfn|Watkinson|1998|p=313}}{{sfn|Sukhov|1987|p=40}}{{sfn|Dolby|1981|p=269}} Underbiasing causes excessive distortion, [[modulation noise]] and worsens susceptibility to dropouts and is thus distinctly unwanted.{{sfn|Watkinson|1998|p=312}}{{sfn|Dolby|1981|p=269}} In practice, tape is always mildly overbiased: the optimal bias current is set at 2-3 [[decibel|dB]] above the nominal value.{{sfn|Watkinson|1998|p=313}}{{sfn|Sukhov|1987|p=40}}<ref name=CS/>{{sfn|Dolby|1981|p=269}} This optimal setting improves linearity at midrange frequencies, but also reduces dynamic range and causes a droop in treble frequency response, which is compensated for with [[Emphasis (telecommunications)|pre-emphasis]] in the recording chain.{{sfn|Sukhov|1987|p=40}}<ref name=CS/>
-Recording at very low wavelengths ([[Audio_tape_specifications#Tape_speeds|tape speeds]] of 4.76 cm/s and 9.53 cm/s, or 1 7/8 [[Inch per second|in/s]] and 3 3/4 in/s) presents another challenge.{{sfn|Sukhov|1983|p=36}}{{sfn|Watkinson|1998|p=312}} Audible high-frequency components of the recorded signal themselves act as biasing currents. The resulting excessive overbiasing manifests itself in [[dynamic range compression]] and early onset of saturation at treble frequencies, especially when recording on low quality tapes with low saturation levels.{{sfn|Watkinson|1998|p=312}}{{sfn|Dolby|1981|p=269}} Typical music of the 1970s, published on vinyl discs or transmitted via FM radio, did not contain much treble energy and usually could not drive the tape into saturation.<ref name=CS/> However, the [[Mastering_(audio)#Digital_technology|digitally mastered]], [[Direct-to-disc recording|direct-to-disc]] and [[disco]] recordings of the late 1970s and early 1980 often contained enough "hot" treble to trigger tape overload.<ref name=CS>{{ cite journal | url=https://worldradiohistory.com/Archive-All-Audio/Archive-HiFI-Stereo/SPECIALS/HiFi-Stereo-Review-1982-Tape-Recording-Guide.pdf | last=Stark | first=Craig | year=1982 | issue=Tape Recording and Buying Guide | title=The Dolby HX System | pages=18 }}</ref>
+Recording at very low wavelengths ([[Audio tape specifications#Tape speeds|tape speeds]] of 4.76&nbsp;cm/s and 9.53&nbsp;cm/s, or 1 7/8 [[Inch per second|in/s]] and 3 3/4 in/s) presents another challenge.{{sfn|Sukhov|1983|p=36}}{{sfn|Watkinson|1998|p=312}} Audible high-frequency components of the recorded signal themselves act as biasing currents. The resulting excessive overbiasing manifests itself in [[dynamic range compression]] and early onset of saturation at treble frequencies, especially when recording on low quality tapes with low saturation levels.{{sfn|Watkinson|1998|p=312}}{{sfn|Dolby|1981|p=269}} Typical music of the 1970s, published on vinyl discs or transmitted via FM radio, did not contain much treble energy and usually could not drive the tape into saturation.<ref name=CS/> However, the [[Mastering (audio)#Digital technology|digitally mastered]], [[Direct-to-disc recording|direct-to-disc]] and [[disco]] recordings of the late 1970s and early 1980 often contained enough "hot" treble to trigger tape overload.<ref name=CS>{{ cite journal | url=https://worldradiohistory.com/Archive-All-Audio/Archive-HiFI-Stereo/SPECIALS/HiFi-Stereo-Review-1982-Tape-Recording-Guide.pdf | last=Stark | first=Craig | year=1982 | issue=Tape Recording and Buying Guide | title=The Dolby HX System | pages=18 }}</ref>
-In the end of the 1970s the industry proposed three solutions to the problem.{{sfn|Sukhov|1983|p=37}} [[Compact_Cassette_tape_types_and_formulations#Metal_particle_Type_IV_tapes|Metal particle tapes]] had very high maximum output levels and treble saturation levels, but were prohibitively expensive for most home users.{{sfn|Sukhov|1983|p=37}} The early metal tapes had high absolute level of hiss, and there were fears that metal tape would degrade quickly{{sfn|Sukhov|1983|p=37}} (this did not happen). The second solution, developed independently by [[Tandberg]] and [[Akai]], relied on limiting recording levels.{{sfn|Sukhov|1983|p=37}} The patented Tandberg Dyneq and Akai ADRS circuits compressed the signal electronically before it could overload the tape.{{sfn|Sukhov|1983|p=37}} Finally, in 1979 [[Kenneth James Gundry]] of [[Dolby Laboratories]] proposed the third alternative: adapting the bias current to the treble content of the source signal.{{sfn|Sukhov|1983|p=37}}<ref name=CS/> Increase in treble energy, which effectively overbiases the tape, should be compensated with a reciprocal decrease in the output of the bias generator.{{sfn|Sukhov|1983|p=37}}
+In the end of the 1970s the industry proposed three solutions to the problem.{{sfn|Sukhov|1983|p=37}} [[Compact Cassette tape types and formulations#Metal particle Type IV tapes|Metal particle tapes]] had very high maximum output levels and treble saturation levels, but were prohibitively expensive for most home users.{{sfn|Sukhov|1983|p=37}} The early metal tapes had high absolute level of hiss, and there were fears that metal tape would degrade quickly{{sfn|Sukhov|1983|p=37}} (this did not happen). The second solution, developed independently by [[Tandberg]] and [[Akai]], relied on limiting recording levels.{{sfn|Sukhov|1983|p=37}} The patented Tandberg Dyneq and Akai ADRS circuits compressed the signal electronically before it could overload the tape.{{sfn|Sukhov|1983|p=37}} Finally, in 1979 [[Kenneth James Gundry]] of [[Dolby Laboratories]] proposed the third alternative: adapting the bias current to the treble content of the source signal.{{sfn|Sukhov|1983|p=37}}<ref name=CS/> Increase in treble energy, which effectively overbiases the tape, should be compensated with a reciprocal decrease in the output of the bias generator.{{sfn|Sukhov|1983|p=37}}
 The effect of such compensation is evident from the typical magnetization curves.{{sfn|Sukhov|1987|pp=39-40}} By default, when the treble energy of the source signal is low, the recorder operates at a fixed optimal bias current I<sub>b.opt.</sub> (blue curve).{{sfn|Sukhov|1987|pp=39-40}} Initial overbiasing assures good linearity but low sensitivity and low saturation level.{{sfn|Sukhov|1987|pp=39-40}} Reduced bias current value of I<sub>b.red.</sub> allows operation at higher input and output levels, albeit with a higher sensitivity (red curve).{{sfn|Sukhov|1987|pp=39-40}}{{sfn|Dolby|1981|p=269}} A well-designed adaptively biased circuit must decrease bias current gradually, so that the increase in sensitivity compensates the saturation effects.{{sfn|Sukhov|1987|pp=39-40}} The new, adaptive magnetization curve remains straight (green dotted line) all the way to the maximum recording current I<sub>af.1</sub>.{{sfn|Sukhov|1987|pp=39-40}} Owing to self-biasing effects, distortion at middle frequencies remains low, and [[intermodulation]] decreases.{{sfn|Sukhov|1983|p=37}}
-The location of the breakpoint I<sub>af.o</sub> on the control curve, and the slope of its high-level segment depend on the frequency of the input signal, and the various energy loss mechanisms in the tape and the recording head.{{sfn|Sukhov|1987|pp=39-40}} A practical adaptive biasing system must employ [[heuristic]] [[Weight function|weighing]] over all treble frequencies to attain best possible performance for a specific recorder.{{sfn|Sukhov|1987|p=40}} Fortunately, the effect of changes in tape formulations is insignificant, at least in the case of [[Compact_Cassette_tape_types_and_formulations#Type_I_tapes|ferric tapes]].{{sfn|Sukhov|1987|p=40}} Different tapes require different optimal bias settings, but the bias control curve can stay the same for all ferric tapes.{{sfn|Sukhov|1987|p=40}}
+The location of the breakpoint I<sub>af.o</sub> on the control curve, and the slope of its high-level segment depend on the frequency of the input signal, and the various energy loss mechanisms in the tape and the recording head.{{sfn|Sukhov|1987|pp=39-40}} A practical adaptive biasing system must employ [[heuristic]] [[Weight function|weighing]] over all treble frequencies to attain best possible performance for a specific recorder.{{sfn|Sukhov|1987|p=40}} Fortunately, the effect of changes in tape formulations is insignificant, at least in the case of [[Compact Cassette tape types and formulations#Type I tapes|ferric tapes]].{{sfn|Sukhov|1987|p=40}} Different tapes require different optimal bias settings, but the bias control curve can stay the same for all ferric tapes.{{sfn|Sukhov|1987|p=40}}
 == Dolby HX ==
@@ Line 20: / Line 20: @@
 The original Dolby HX (for 'Headroom eXtension'), designed by Gundry, operated as an add-on to the [[Dolby B]] noise reduction encoder. The Dolby B [[integrated circuit]], by design, extracts the [[Envelope (waves)|envelope]] of the middle and treble components of the source signals, and uses it to modulate the gain of its side channel.{{sfn|Sukhov|1983|p=37}} The HX circuit blends the left and right envelope signals together.{{sfn|Sukhov|1983|p=37}} The composite envelope modulates the output of a [[voltage source]] that powers the common erase/bias generator.{{sfn|Sukhov|1983|p=37}} Simultaneously, it adjust the depth of high-frequency [[Emphasis (telecommunications)|pre-emphasis]] of the two recording channels.{{sfn|Sukhov|1983|p=37}} Controlling both stereo channels with a single bias modulator was deemed acceptable due to the high degree of [[correlation]] between the left and right stereo signals, and poor channel separation of the existing analog sources available to the consumer.{{sfn|Sukhov|1983|p=38}} Controlling erase and bias currents simultaneously could potentially cause instant drops in erase effectiveness, but this only happened during the loudest passages with rich treble content, which completely drowned any residual unerased signals.{{sfn|Sukhov|1983|p=39}}
-Independent tests had shown that the HX could raise the saturation levels at 10-12 kHz by 10 [[decibel|dB]].{{sfn|Sukhov|1983|p=37}} According to Dolby, the improvement was most pronounced with high quality, [[Compact_Cassette_tape_types_and_formulations#Magnetic_properties|high coercivity]] tape formulations. "Bad tapes remained bad", with or without adaptive biasing.{{sfn|Dolby|1981|p=271: "Schlechte Bänder bleiben schlecht"}}
+Independent tests had shown that the HX could raise the saturation levels at 10–12&nbsp;kHz by 10 [[decibel|dB]].{{sfn|Sukhov|1983|p=37}} According to Dolby, the improvement was most pronounced with high quality, [[Compact Cassette tape types and formulations#Magnetic properties|high coercivity]] tape formulations. "Bad tapes remained bad", with or without adaptive biasing.{{sfn|Dolby|1981|p=271: "Schlechte Bänder bleiben schlecht"}}
-[[Dolby Laboratories]] rolled out the HX at the [[Consumer Electronics Show]] in June 1979.<ref name=NS79/> The novelty was offered to all existing Dolby B licensees at no extra charge.<ref name=NS79>{{ cite journal | url=https://books.google.ru/books?id=2WgG5GxHkI0C | title=Engineers sound off to high frequencies | journal=New Scientist | date=1979 | issue=21 Jun | pages=999 }}</ref> In 1980{{endash}}1981 [[Aiwa]], [[Harman Kardon]] and [[TEAC]]<ref>{{ cite journal | journal=Modern Recording and Music | date=1980 | issue=9 | url=https://worldradiohistory.com/Archive-All-Audio/Archive-Modern-Recording/80s/Modern-Recording-1980-09.pdf | title=Teac decks and test tapes | p=31 }}</ref> integrated the HX in their cassete decks, but there  were no further followers.{{sfn|Sukhov|1983|p=40}} Despite favorable reviews, the HX was a marketing flop and an engineering failure.{{sfn|Sukhov|1983|p=40}} It was tested and rejected by audio engineers all over the world.{{sfn|Sukhov|1983|p=40}} While most did not disclose their findings, [[Studer|Willi Studer]] spoke publicly against the adoption of the HX.{{sfn|Sukhov|1983|p=40}} The shortcomings of the HX, said Studer, greatly outweighed its intended benefits.{{sfn|Sukhov|1983|p=40}} The 1981 press release by Dolby for the German market indirectly blamed the failure on the conservatism of the industry. The HX, said the company, "intervenes very far into recorder development and cannot simply be added to the existing electronics. It requires a fundamental redevelopment of the recording amplifier".{{sfn|Dolby|1981|p=269: "Eine technische Begründung für die spärliche Verbreitung des HX-Systems ist darin zu sehen, das es sehr weit in die Recorderentwicklung eingreift, und sich nicht einfach zu der vorhandenen Elektronik hinzufügen läßt. Dolby-HX erfordert eine grundlegende Neuentwicklung der Aufnahmeverstärker"}} Nevertheless, the company still expressed hope that the HX will gain acceptance "because it enables [[high fidelity]] quality with the future [[microcassette]] recorders with a tape speed of 2.4 cm/s".{{sfn|Dolby|1981|p=269: "Dennoch stehen die Chancen für das HX-System nicht schlecht, weil es bei den kommenden (Mikro)-Cassettenrecordern mit 2,4 cm/s Bandgeschwindigkeit HiFi-Qualität ermöglicht."}} The latter promise did not materialize, either.
+[[Dolby Laboratories]] rolled out the HX at the [[Consumer Electronics Show]] in June 1979.<ref name=NS79/> The novelty was offered to all existing Dolby B licensees at no extra charge.<ref name=NS79>{{ cite journal | url=https://books.google.ru/books?id=2WgG5GxHkI0C | title=Engineers sound off to high frequencies | journal=New Scientist | year=1979 | issue=21 Jun | pages=999 }}</ref> In 1980{{endash}}1981 [[Aiwa]], [[Harman Kardon]] and [[TEAC]]<ref>{{ cite journal | journal=Modern Recording and Music | year=1980 | issue=9 | url=https://worldradiohistory.com/Archive-All-Audio/Archive-Modern-Recording/80s/Modern-Recording-1980-09.pdf | title=Teac decks and test tapes | p=31 }}</ref> integrated the HX in their cassette decks, but there  were no further followers.{{sfn|Sukhov|1983|p=40}} Despite favorable reviews, the HX was a marketing flop and an engineering failure.{{sfn|Sukhov|1983|p=40}} It was tested and rejected by audio engineers all over the world.{{sfn|Sukhov|1983|p=40}} While most did not disclose their findings, [[Studer|Willi Studer]] spoke publicly against the adoption of the HX.{{sfn|Sukhov|1983|p=40}} The shortcomings of the HX, said Studer, greatly outweighed its intended benefits.{{sfn|Sukhov|1983|p=40}} The 1981 press release by Dolby for the German market indirectly blamed the failure on the conservatism of the industry. The HX, said the company, "intervenes very far into recorder development and cannot simply be added to the existing electronics. It requires a fundamental redevelopment of the recording amplifier".{{sfn|Dolby|1981|p=269: "Eine technische Begründung für die spärliche Verbreitung des HX-Systems ist darin zu sehen, das es sehr weit in die Recorderentwicklung eingreift, und sich nicht einfach zu der vorhandenen Elektronik hinzufügen läßt. Dolby-HX erfordert eine grundlegende Neuentwicklung der Aufnahmeverstärker"}} Nevertheless, the company still expressed hope that the HX will gain acceptance "because it enables [[high fidelity]] quality with the future [[microcassette]] recorders with a tape speed of 2.4 cm/s".{{sfn|Dolby|1981|p=269: "Dennoch stehen die Chancen für das HX-System nicht schlecht, weil es bei den kommenden (Mikro)-Cassettenrecordern mit 2,4 cm/s Bandgeschwindigkeit HiFi-Qualität ermöglicht."}} The latter promise did not materialize, either.
-From a purely engineering viewpoing, the main drawback of the HX was that, being a [[Feed forward (control)|feedforward control]], it monitored the signal at its source, but not the signal actually reaching the recording head.{{sfn|Sukhov|1983|p=38}}{{sfn|Self|2020|p=56}} Any variation in the gain or the frequency response of the recording chain disrupted the bias control curve.{{sfn|Sukhov|1983|p=38}} Adjustable pre-emphasis subcircuit was unnecessarily complex and expensive for the consumer industry.{{sfn|Sukhov|1983|p=38}} The Dolby B envelope detector, which by design was [[Rise time|fairly slow]], could not reliably track fast [[Transient (acoustics)|transients]].{{sfn|Sukhov|1983|p=38}} Bundling adaptive biasing with noise reduction at the hardware level, in itself, was the worst of all shortcomings.{{sfn|Sukhov|1983|p=38}} The user could not turn off Dolby B decoder and still use Dolby HX while recording.<ref name=BUR/> This discouraged the use of the far more effective [[dbx (noise reduction)|dbx noise reduction]].<ref name=BUR/> On the other hand, the 30 dB gain in signal-to-noise ratio provided by the dbx made the HX virtually unnecessary.<ref name=BUR>{{ cite journal | last=Burstein | first=H. | title=Tape guide: Doubling in NR | url=https://worldradiohistory.com/Archive-All-Audio/Archive-Audio/80s/Audio-1983-02.pdf | journal=Audio (USA) | date=1983 | issue=February | pages=26 }}</ref>
+From a purely engineering viewpoing, the main drawback of the HX was that, being a [[Feed forward (control)|feedforward control]], it monitored the signal at its source, but not the signal actually reaching the recording head.{{sfn|Sukhov|1983|p=38}}{{sfn|Self|2020|p=56}} Any variation in the gain or the frequency response of the recording chain disrupted the bias control curve.{{sfn|Sukhov|1983|p=38}} Adjustable pre-emphasis subcircuit was unnecessarily complex and expensive for the consumer industry.{{sfn|Sukhov|1983|p=38}} The Dolby B envelope detector, which by design was [[Rise time|fairly slow]], could not reliably track fast [[Transient (acoustics)|transients]].{{sfn|Sukhov|1983|p=38}} Bundling adaptive biasing with noise reduction at the hardware level, in itself, was the worst of all shortcomings.{{sfn|Sukhov|1983|p=38}} The user could not turn off Dolby B decoder and still use Dolby HX while recording.<ref name=BUR/> This discouraged the use of the far more effective [[dbx (noise reduction)|dbx noise reduction]].<ref name=BUR/> On the other hand, the 30&nbsp;dB gain in signal-to-noise ratio provided by the dbx made the HX virtually unnecessary.<ref name=BUR>{{ cite journal | last=Burstein | first=H. | title=Tape guide: Doubling in NR | url=https://worldradiohistory.com/Archive-All-Audio/Archive-Audio/80s/Audio-1983-02.pdf | journal=Audio (USA) | year=1983 | issue=February | pages=26 }}</ref>
 == Dolby HX Pro ==
 [[File:Marantz SD-60 main audio PCB - NEC uPC1297 Dolby HX Pro IC.jpg|thumb|300px|right|The complete Dolby HX circuitry built around the NEC μPC1297, made in 1989. The shown implementation is a rare example of a defeatable HX Pro, controlled with a user-accessible on/off switch.]]
-The alternative implementation of adaptive biasing was patented in 1980 by [[Jørgen Selmer Jensen]] of [[Bang & Olufsen]] (B&O).<ref>{{ cite journal | url=https://www.epo.org/archive/epo/pubs/bulletin/1982/bulletin8208.pdf | journal=Europen Patent Bulletin | date=1982 | issue=24.02 | issn=0170-9305 | page=40 | title=G11B 5/47 0046410: Bias control method and apparatus for magnetic recording }}</ref> Unlike the feedforward Dolby HX, the B&O circuit was a [[feedback]] system.{{sfn|Self|2020|p=56}} According to the patent, it monitored the high frequency voltage at the "hot" end of the recording head, extracting the combined envelope of bias and treble audio signals.<ref name=PAT01/>{{sfn|Self|2020|p=56}} An [[error amplifier]] continuously compared the envelope with the preset reference level, and adjusted the audio signal current to the recording head{{sfn|Self|2020|p=56}} via a [[resistive opto-isolator]]<ref name=PAT01/> (in practical applications the latter were replaced with [[variable-gain amplifier]]s, for example, built around the [[LM13700]] [[Operational transconductance amplifier|transconductance amplifier]]).{{sfn|Self|2020|p=56}} The monaural circuit was easily scalable for [[Stereophonic sound|two-channel stereo]] or [[multitrack recording]], and enabled easy adjustments of the normal bias level.<ref name=PAT01>
+The alternative implementation of adaptive biasing was patented in 1980 by [[Jørgen Selmer Jensen]] of [[Bang & Olufsen]] (B&O).<ref>{{ cite journal | url=https://www.epo.org/archive/epo/pubs/bulletin/1982/bulletin8208.pdf | journal=Europen Patent Bulletin | year=1982 | issue=24.02 | issn=0170-9305 | page=40 | title=G11B 5/47 0046410: Bias control method and apparatus for magnetic recording }}</ref> Unlike the feedforward Dolby HX, the B&O circuit was a [[feedback]] system.{{sfn|Self|2020|p=56}} According to the patent, it monitored the high frequency voltage at the "hot" end of the recording head, extracting the combined envelope of bias and treble audio signals.<ref name=PAT01/>{{sfn|Self|2020|p=56}} An [[error amplifier]] continuously compared the envelope with the preset reference level, and adjusted the audio signal current to the recording head{{sfn|Self|2020|p=56}} via a [[resistive opto-isolator]]<ref name=PAT01/> (in practical applications the latter were replaced with [[variable-gain amplifier]]s, for example, built around the [[LM13700]] [[Operational transconductance amplifier|transconductance amplifier]]).{{sfn|Self|2020|p=56}} The monaural circuit was easily scalable for [[Stereophonic sound|two-channel stereo]] or [[multitrack recording]], and enabled easy adjustments of the normal bias level.<ref name=PAT01>
 {{ cite patent
 | country	=
@@ Line 45: / Line 45: @@
 | class		=
 | url		= https://patents.google.com/patent/EP0046410A1/en?
 }}</ref>
-According to B&O, their system assured only 3-5 dB gain in treble saturation level, far less than the Dolby HX.{{sfn|Sukhov|1983|p=38}} However, it did not rely on the Dolby chip, and could be used with or without any noise reduction system.{{sfn|Sukhov|1983|p=38}} Negative feedback compensated any variations in gain and frequency response of the recording chain, thus eliminating the key shortcoming of the original Dolby HX.{{sfn|Sukhov|1983|p=38}} As a side benefit, the B&O system was also effective in reel-to-reel recorders.{{sfn|Sukhov|1983|p=38}}{{sfn|Hood|1999|p=45}}
+According to B&O, their system assured only 3-5&nbsp;dB gain in treble saturation level, far less than the Dolby HX.{{sfn|Sukhov|1983|p=38}} However, it did not rely on the Dolby chip, and could be used with or without any noise reduction system.{{sfn|Sukhov|1983|p=38}} Negative feedback compensated any variations in gain and frequency response of the recording chain, thus eliminating the key shortcoming of the original Dolby HX.{{sfn|Sukhov|1983|p=38}} As a side benefit, the B&O system was also effective in reel-to-reel recorders.{{sfn|Sukhov|1983|p=38}}{{sfn|Hood|1999|p=45}}
-Dolby Laboratories acquired the rights to the B&O patent, and became its sole worldwide distributor.{{sfn|Self|2020|p=56}} The new system was named Dolby HX Professional, later shortened to Dolby HX Pro. B&O retained the rights to use it in their own products, and according to sources affiliated with Selmer, received a share in future licensing revenue.<ref name=SE>{{ cite web | url=https://archive.ph/20130914071524/http://jollyjokers.dk/index.php | title=Jørgen Selmer Jensen | date=2013 }}</ref> Exact terms of the contract were not disclosed, and are subject of much debate in amateur audio communities.
+Dolby Laboratories acquired the rights to the B&O patent, and became its sole worldwide distributor.{{sfn|Self|2020|p=56}} The new system was named Dolby HX Professional, later shortened to Dolby HX Pro. B&O retained the rights to use it in their own products, and according to sources affiliated with Selmer, received a share in future licensing revenue.<ref name=SE>{{ cite web | url=https://archive.ph/20130914071524/http://jollyjokers.dk/index.php | title=Jørgen Selmer Jensen | year=2013 }}</ref> Exact terms of the contract were not disclosed, and are subject of much debate in amateur audio communities.
-In the beginning, Dolby targeted the HX Pro to professional market{{sfn|Sukhov|1983|p=38}}. In August 1982 [[Electro Sound]], manufacturer of industrial tape duplicators, introduced HX Pro in its cassette duplicator catalogue.<ref>{{ cite journal | url=https://books.google.com/books?id=8SQEAAAAMBAJ&pg=PT52 | journal=Billboard | title=New Products / Dolby HX | page=31 | date=1982-08-28 }}</ref> [[Warner Records]] became the first [[Record_label#Major_labels|major recording label]] to adopt the HX Pro for mass duplication.<ref name=PM86>{{ cite journal | url=https://books.google.com/books?id=0OMDAAAAMBAJ&pg=PA34 | journal=Popular Mechanics | year=1986 | issue=August | pages=34-35 | last=Shea | first=T. | title=Rx for tapes: HX Pro }}</ref> By February 1983, according to Dolby, the company managed to sign up two licensees in the home audio industry: [[Aiwa]] and [[Harman Kardon]].<ref>{{ cite journal | url=https://worldradiohistory.com/Archive-All-Audio/Archive-Audio/80s/Audio-1983-02.pdf | journal=Audio (USA) | date=1983 | issue=February | pages=7 | last=Dolby Laboratories | title=Dolby HX Professional }}</ref> The early adopters had to build the HX Pro circuitry with general-purpose [[operational amplifier]]s and [[Operational transconductance amplifier|transconductance amplifier]]s, until the introduction of a dedicated [[integrated circuit]], the [[NEC]] μPC1297, in 1985.<ref>{{ cite journal | journal=Journal of Electronic Engineering | title=NEC IC Expands Bias in Audio Tape Heads | pages=19 | date=1985 | volume=22 }}</ref> The new proposal was well received by the industry, and by 1986 it became a standard feature in the upper segment of consumer cassette decks<ref name=PM86/>. In the few following years the HX Pro migrated into the entry-level consumer segment, thus becoming the de-facto standard equipment in consumer hi-fi, and into the professional reel-to-reel recorders.{{sfn|Hood|1999|p=45}}
+In the beginning, Dolby targeted the HX Pro to professional market.{{sfn|Sukhov|1983|p=38}} In August 1982 [[Electro Sound]], manufacturer of industrial tape duplicators, introduced HX Pro in its cassette duplicator catalogue.<ref>{{ cite journal | url=https://books.google.com/books?id=8SQEAAAAMBAJ&pg=PT52 | journal=Billboard | title=New Products / Dolby HX | page=31 | date=1982-08-28 }}</ref> [[Warner Records]] became the first [[Record label#Major labels|major recording label]] to adopt the HX Pro for mass duplication.<ref name=PM86>{{ cite journal | url=https://books.google.com/books?id=0OMDAAAAMBAJ&pg=PA34 | journal=Popular Mechanics | year=1986 | issue=August | pages=34–35 | last=Shea | first=T. | title=Rx for tapes: HX Pro }}</ref> By February 1983, according to Dolby, the company managed to sign up two licensees in the home audio industry: [[Aiwa]] and [[Harman Kardon]].<ref>{{ cite journal | url=https://worldradiohistory.com/Archive-All-Audio/Archive-Audio/80s/Audio-1983-02.pdf | journal=Audio (USA) | year=1983 | issue=February | pages=7 | last=Dolby Laboratories | title=Dolby HX Professional }}</ref> The early adopters had to build the HX Pro circuitry with general-purpose [[operational amplifier]]s and [[Operational transconductance amplifier|transconductance amplifier]]s, until the introduction of a dedicated [[integrated circuit]], the [[NEC]] μPC1297, in 1985.<ref>{{ cite journal | journal=Journal of Electronic Engineering | title=NEC IC Expands Bias in Audio Tape Heads | pages=19 | year=1985 | volume=22 }}</ref> The new proposal was well received by the industry, and by 1986 it became a standard feature in the upper segment of consumer cassette decks.<ref name=PM86/> In the few following years the HX Pro migrated into the entry-level consumer segment, thus becoming the de facto standard equipment in consumer hi-fi, and into the professional reel-to-reel recorders.{{sfn|Hood|1999|p=45}}
 == Subsequent development ==
-In 1983, the adaptive biasing principle gained popularity in the former [[Soviet Union]]. The earliest implementation, published by Nikolay Sukhov, was developed after the HX Pro. It blended the elements of both HX Pro (feedback control) and the original HX (varying the supply voltage to the common erase/bias generator), and added safeguards against transient overload, a common problem when recording from worn [[LP record]]s.{{sfn|Sukhov|1983|p=38}}. The revised design, using the new [[precision rectifier]] IC, was published in 1987. In line with the tastes of the home audio community, which still preferred reel-to-reel to cassette, the 1987 version was targeted at both cassette and reel-to-reel decks.{{sfn|Sukhov|1987|p=40}}. Adaptive biasing cannot substantially improve the performance of the quarter-inch tape running at 19.5 cm/s (7.5 in/s) in ''standard'' reel-to-reel recorders. However, it permits a decrease in treble [[Equalization (audio)|equalization]] from the standard 50 [[microsecond|μs]] to 10 μs.{{sfn|Sukhov|1987|p=40}} A fivefold reduction of the [[time constant]] corresponds to a fivefold decrease in apparent noise floor at middle and treble frequencies, and, according to the inventor, enables practical signal-to-noise ratio of more than 80 dB, ''without'' any noise reduction.{{sfn|Sukhov|1987|p=40}} Sukhov's designs were the subject of five patents issued between 1984 and 1989, all referenced to the earlier Selmer patent as [[prior art]].<ref>Patents SU 1448356 A1, SU 1448357 A1), SU 1539830 A1, SU 1531134 A1, SU 1610487 A1</ref>
+In 1983, the adaptive biasing principle gained popularity in the former [[Soviet Union]]. The earliest implementation, published by Nikolay Sukhov, was developed after the HX Pro. It blended the elements of both HX Pro (feedback control) and the original HX (varying the supply voltage to the common erase/bias generator), and added safeguards against transient overload, a common problem when recording from worn [[LP record]]s.{{sfn|Sukhov|1983|p=38}} The revised design, using the new [[precision rectifier]] IC, was published in 1987. In line with the tastes of the home audio community, which still preferred reel-to-reel to cassette, the 1987 version was targeted at both cassette and reel-to-reel decks.{{sfn|Sukhov|1987|p=40}} Adaptive biasing cannot substantially improve the performance of the quarter-inch tape running at 19.5&nbsp;cm/s (7.5 in/s) in ''standard'' reel-to-reel recorders. However, it permits a decrease in treble [[Equalization (audio)|equalization]] from the standard 50 [[microsecond|μs]] to 10 μs.{{sfn|Sukhov|1987|p=40}} A fivefold reduction of the [[time constant]] corresponds to a fivefold decrease in apparent noise floor at middle and treble frequencies, and, according to the inventor, enables practical signal-to-noise ratio of more than 80&nbsp;dB, ''without'' any noise reduction.{{sfn|Sukhov|1987|p=40}} Sukhov's designs were the subject of five patents issued between 1984 and 1989, all referenced to the earlier Selmer patent as [[prior art]].<ref>Patents SU 1448356 A1, SU 1448357 A1), SU 1539830 A1, SU 1531134 A1, SU 1610487 A1</ref>
 == Notes ==
@@ Line 62: / Line 62: @@
 == References ==
 === In English ===
-* {{ cite book | last=Watkinson | first=John | date=1998 | title=The Art of Sound Reproduction | publisher=Taylor & Francis | isbn=9780240515120 | url=https://books.google.com/books?id=BikEpOF3FHgC }}
+* {{ cite book | last=Watkinson | first=John | year=1998 | title=The Art of Sound Reproduction | publisher=Taylor & Francis | isbn=9780240515120 | url=https://books.google.com/books?id=BikEpOF3FHgC }}
-* {{ cite book | url=https://books.google.com/books?id=EOE1tx3CJ98C | last=Hood | first=John Linslay | title=Audio Electronics | date=1999 | page=45 | isbn=9780750643320 | publisher=Newnes | author-link=John Linsley Hood }}
+* {{ cite book | url=https://books.google.com/books?id=EOE1tx3CJ98C | last=Hood | first=John Linslay | title=Audio Electronics | year=1999 | page=45 | isbn=9780750643320 | publisher=Newnes | author-link=John Linsley Hood }}
-* {{ cite book | last=Self | first=Douglas | author-link=Douglas Self | title=Small Signal Audio Design, 3rd edition | date=2020 | isbn=9781000050448 | publisher=CRC Press | url=https://books.google.com/books?id=L2PdDwAAQBAJ }}
+* {{ cite book | last=Self | first=Douglas | author-link=Douglas Self | title=Small Signal Audio Design, 3rd edition | year=2020 | isbn=9781000050448 | publisher=CRC Press | url=https://books.google.com/books?id=L2PdDwAAQBAJ }}
 === In German ===
-* {{ cite journal | last=Dolby | date=1981 | pages=269{{endash}}271 | title=Bessere Höhenaufzeichnung mit dem Dolby-HX-System | trans-title=Better treble recording with the Dolby HX System | journal=Funktechnik | issue=August | language=de | url=https://nvhrbiblio.nl/biblio/tijdschrift/Funktechnik/1981/FT_1981_Heft_08_OCR.pdf }}
+* {{ cite journal | last=Dolby | year=1981 | pages=269{{endash}}271 | title=Bessere Höhenaufzeichnung mit dem Dolby-HX-System | trans-title=Better treble recording with the Dolby HX System | journal=Funktechnik | issue=August | language=de | url=https://nvhrbiblio.nl/biblio/tijdschrift/Funktechnik/1981/FT_1981_Heft_08_OCR.pdf }}
 === In Russian ===
-* {{ cite journal | last=Sukhov | first=N. | title=Динамическое подмагничивание | trans-title=Dynamic biasing | journal=Радио | date=1983 | issue=5 | pages=36{{endash}}40 }}
+* {{ cite journal | last=Sukhov | first=N. | title=Динамическое подмагничивание | trans-title=Dynamic biasing | journal=Радио | year=1983 | issue=5 | pages=36{{endash}}40 }}
-* {{ cite journal | last=Sukhov | first=N. | title=СДП-2 | trans-title=SDP-2 | journal=Радио | date=1987 | issue=1 | pages=39{{endash}}42 }}
+* {{ cite journal | last=Sukhov | first=N. | title=СДП-2 | trans-title=SDP-2 | journal=Радио | year=1987 | issue=1 | pages=39{{endash}}42 }}
 [[Category:Tape recording]]