Editing Dolby noise-reduction system (section)

== Dolby B{{anchor|Dolby NR}} ==
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[[File:Dolby B Noise Analysis.png|thumb|right|400px|Noise and frequency response analysis of a Nakamichi 580M high-performance cassette deck with Dolby B-type noise reduction system]]
[[File:Dolby B cassette albums.jpg|thumb|400px|Three different music cassettes encoded with Dolby&nbsp;B, showing the various ways in which the use of the system could be indicated. ''[[You Don't Bring Me Flowers (album)|You Don't Bring Me Flowers]]'' uses a simple Dolby logo; ''[[Apurimac (album)|Apurimac]]'' uses a Dolby logo with "B&nbsp;NR" (also with "HX&nbsp;PRO" to indicate the complementary use of that technology); ''[[Tour de Force (38 Special album)|Tour De Force]]'' uses a Dolby logo with "Dolby System."]]

Dolby&nbsp;B-type noise reduction was developed after Dolby&nbsp;A, and it was introduced in 1968. It consisted of a single sliding band system providing about 9&nbsp;dB of noise reduction ([[A-weighted]]), primarily for use with [[Compact Cassette|cassette tape]]s. It was much simpler than Dolby&nbsp;A and therefore much less expensive to implement in consumer products. Dolby&nbsp;B recordings are acceptable when played back on equipment that does not possess a Dolby&nbsp;B decoder, such as many inexpensive portable and car cassette players. Without the de-emphasis effect of the decoder, the sound will be perceived as brighter as high frequencies are emphasized, which can be used to offset "dull" high-frequency response in inexpensive equipment. However, Dolby&nbsp;B provides less effective noise reduction than Dolby&nbsp;A, generally by an amount of more than 3&nbsp;dB.

The Dolby&nbsp;B system is effective from approximately 1&nbsp;kHz upwards; the noise reduction that is provided is 3&nbsp;dB at 600&nbsp;Hz, 6&nbsp;dB at 1.2&nbsp;kHz, 8&nbsp;dB at 2.4&nbsp;kHz, and 10&nbsp;dB at 5&nbsp;kHz. The width of the noise reduction band is variable, as it is designed to be responsive to both the amplitude and the frequency distribution of the signal. It is thus possible to obtain significant amounts of noise reduction down to quite low frequencies without causing audible modulation of the noise by the signal ("breathing").<ref name="Dolby1971"/>

From the mid-1970s, Dolby&nbsp;B became standard on commercially pre-recorded music cassettes even though some low-end equipment lacked decoding circuitry, although it allows for acceptable playback on such equipment. Most pre-recorded [[compact audio cassette|cassette]]s use this variant. [[VHS]] video recorders used Dolby&nbsp;B on linear stereo audio tracks.

[[File:Advent tape recorder with Dolby noise-reduction system.jpg|thumb|[[Advent Corporation]] tape recorder with Dolby noise-reduction system (ca. 1972)]]
Prior to the introduction of later consumer variants (Dolby&nbsp;C being the first), cassette hardware supporting Dolby&nbsp;B and cassettes encoded with it would be labeled simply "Dolby System," "Dolby&nbsp;NR", or wordlessly with the Dolby symbol. This continued in some record labels and hardware manufacturers even after Dolby&nbsp;C had been introduced, during the period when the new standard was relatively little-known.

[[JVC]]'s {{ill|ANRS|ja|ANRS}}<ref name="Woram1976"/> system, used in place of Dolby&nbsp;B on earlier JVC cassette decks, is considered compatible with Dolby&nbsp;B. JVC eventually abandoned the ANRS standard in favor of official Dolby&nbsp;B support; some JVC decks exist whose noise-reduction toggles have a combined "ANRS&nbsp;/ Dolby&nbsp;B" setting.

===Dolby FM===
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In the early 1970s, some expected Dolby&nbsp;NR to become normal in [[FM radio]] broadcasts and some tuners and amplifiers were manufactured with decoding circuitry; there were also some tape recorders with a Dolby&nbsp;B "pass-through" mode. In 1971 [[WFMT]] started to transmit programs with Dolby&nbsp;NR,<ref name="Gilmore1971"/> and soon some 17 stations broadcast with noise reduction, but by 1974 it was already on the decline.<ref name="Free1974"/> Dolby&nbsp;FM was based on Dolby&nbsp;B,<ref name="Mielke1977"/> but used a modified 25&nbsp;μs pre-emphasis time constant<ref name="Reuber_1983"/> and a frequency-selective [[companding]] arrangement to reduce noise.

A similar system named [[High Com FM]] was evaluated in Germany between July 1979 and December 1981 by [[Institut für Rundfunktechnik|IRT]],<ref name="Schroeder"/> and field-trialed up to 1984. It was based on the [[Telefunken]] [[High Com]] broadband compander system, but never introduced commercially in FM broadcasting.<ref name="IRT1981"/> Another competing system was [[FMX (broadcasting)|FMX]], which was based on [[CX (audio)|CX]].

[[File:Sony TC-RX55 cassette deck.jpg|thumb|right|400px|A typical consumer cassette deck from the late 1980s, featuring automatic reverse, electronic transport controls, and Dolby&nbsp;B and C, among other features]]

===RMS===
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A fully Dolby&nbsp;B-compatible compander was developed and used on many [[tape recorder]]s in the former [[German Democratic Republic]] in the 1980s. It was called RMS (from {{lang|de|Rauschminderungssystem}}, English: "Noise reduction system").<ref name="SK900"/>

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