A Dolby noise-reduction system (Dolby NR) is one of a series of noise reduction systems developed by Dolby Laboratories for use in analog audio tape recording.

Dolby B

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thumb|right|400px|Noise and frequency response analysis of a Nakamichi 580M high-performance cassette deck with Dolby B-type noise reduction system

thumb|400px|Three music cassettes encoded with Dolby&nbsp;B, showing 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 uses a Dolby logo with "B&nbsp;NR" (also with "HX&nbsp;PRO" to indicate the complementary use of that technology); Tour De Force uses a Dolby logo with "Dolby System"]]

Dolby&nbsp;B-type noise reduction was developed after Dolby&nbsp;A, and 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 cassette tapes. 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 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").

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  • Dolby Laboratories, Inc.
  • Some Not Well Known Aspects of Analog Tape -- concerning Dolby-HX/HX-Pro, AES 1990 March, Arndt Klingelnberg
  • Overview of Dolby and other noise reduction systems, Richard Hess