Editing Dolby noise-reduction system (section)

==Dolby HX/HX-Pro{{anchor|Dolby HX|Dolby HX Pro}}==
{{ Further | Adaptive biasing }}
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[[File:Marantz SD-60 main audio PCB - NEC uPC1297 Dolby HX Pro IC.jpg|thumb|right|The Dolby HX circuitry driven by the industry-standard NEC uPC1297 integrated circuit. It modulates the incoming bias current and injects it into the two channels of the stereo recording head via two ferrite transformers.]]
Magnetic tape is inherently non-linear in nature due to [[hysteresis]] of the magnetic material. If an [[analog signal]] were recorded directly onto magnetic tape, its reproduction would be extremely distorted due to this [[non-linearity]]. To overcome this, a high-frequency signal, known as bias, is mixed in with the recorded signal, which "pushes" the envelope of the signal into the linear region. If the audio signal contains strong high-frequency content (in particular from percussion instruments such as [[hi-hat cymbals]]), this adds to the constant bias causing magnetic saturation on the tape. Dynamic, or adaptive, biasing automatically reduces the bias signal in the presence of strong high-frequency signals, making it possible to record at a higher signal level.

The original Dolby HX, where ''HX'' stands for ''Headroom eXtension'', was invented in 1979 by Kenneth Gundry of Dolby Laboratories, and was rejected by the industry for its inherent flaws. [[Bang & Olufsen]] continued work in the same direction, which resulted in a 1981 patent (EP 0046410) by Jørgen Selmer Jensen.<ref name="JSJensen"/> Bang & Olufsen immediately licensed HX-Pro to Dolby Laboratories, stipulating a priority period of several years for use in consumer products, to protect their own Beocord 9000 cassette tape deck.<ref name="Beocord9000"/><ref name="HX_1981"/> By the middle of the 1980s the Bang & Olufsen system, marketed through Dolby Laboratories, became an industry standard under the name of Dolby HX Pro.

HX-Pro only applies during the recording process. The improved signal-to-noise ratio is available no matter which tape deck the tape is played back on, and therefore HX-Pro is not a noise-reduction system in the same way as Dolby&nbsp;A, B, C, and S, although it does help to improve noise reduction encode/decode tracking accuracy by reducing tape non-linearity. Some record companies issued HX-Pro pre-recorded cassette tapes during the late 1980s and early 1990s.