What does Dolby B NR from


Dolby Laboratories, Inc.
legal formCorporation
founding 1965
SeatSan Francisco, United States


  • Kevin J. Yeaman, President and CEO
  • Peter Gotcher, chairman
Employee 1.867[1]
sales $ 970.64 million[1]
Branch Audio technology
Status: September 25, 2015 Template: Infobox Company / Maintenance / Status 2015
Dolby Laboratories is headquartered in San Francisco

Dolby Laboratories, Inc. is a company that previously developed compander systems for noise reduction in the analog audio sector and is now specialized in digital multi-channel audio formats. It was founded in England in 1965 by Ray Dolby, and in 1976 he relocated it to the United States of America.

On February 24, 2014 Doremi Labs, a manufacturer of video editing software.[2]

Analog compressors (noise reduction method)

Double cassette deck from JVC with Dolby B, C and HX Pro

With the analog Dolby method, the volume of soft sounds is increased during recording and decreased again by the same amount during playback. This also reduces the tape noise. This has nothing to do with static pre-emphasis and de-emphasis.

Dolby A and Dolby SR are mainly used in the professional sector, for example in the analog optical sound process.

The most common Dolby method in commercially available cassette recorders is called Dolby B. This is in higher quality devices, which used to be part of the standard scope of a stereo system, through further development Dolby C and sometimes Dolby HX Pro added. Dolby S, published in the early 1990s, should be used as a supplement to B. and C. become the standard for higher-priced hi-fi tape decks in the home. Due to the triumph of digital technology, however, this market segment largely broke away, so that Dolby S could no longer achieve any noteworthy market shares.

Dolby A

Dolby A is the first compression system developed by Dolby for studio magnetic tape recorders from 1966. It consists of four independent compressors. Compressor 1 works below 80 Hz, compressor 2 between 80 Hz and 3 kHz and compressor 3 above 3 kHz. Compression is 10 dB. The fourth compressor works above 8 kHz and delivers an additional 5 dB in this frequency range.

From 1972 onwards Dolby A the cinema sound (light sound) decisively improves dynamic range and frequency response.

Dolby B, Dolby "NR"

The one launched in 1968 Dolby B acts in the upper frequency range from about 1 kHz (with the Dolby-compatible ANRS developed by JVC from 500 Hz) up to the high range. During the recording, the signal is amplified on the tape depending on the frequency and level and is weakened by the same amount as exactly as possible in mirror image during playback. During playback, this also reduces the annoying tape noise that has been added. The noise reduction system must be turned on during both recording and playback. Frequency and level-dependent compression and expansion means that the degree of processing is lower in the frequency range directly above 1 kHz and more pronounced in the upper treble range. These changes only take place at medium to low levels. With Dolby B, it is particularly important that the frequency limit above which a reduction takes place shifts dynamically (sliding band). This significantly improves the lowering of the audible noise while at the same time reducing the interference effects. Other systems only change the strength of the influence in a constant frequency range. This in turn generates pumps and / or noise plumes more easily, or (in order to avoid this) necessarily requires complex processing in several frequency ranges.

Working in mirror image requires calibration. Many cassette devices have a Dolby symbol (double-D symbol) on the level indicator for calibration.

Dolby insists that the system cannot simply be called Dolby but as Dolby NO (NR for Noise reduction) referred to as.

Nowadays means DOLBY no more DOLBY Bas it did until the 1990s, but rather relates to Dolby Digital (TV sound) or 5.1 surround (DVD).

Since compact cassettes did not achieve the quality of good reel devices even under Dolby-B-NR, and the introduction of the CD later raised the hi-fi quality requirements again, the demand for further development of this noise reduction process increased. Even if the sliding-band principle largely prevented noise flags in Dolby B, the overall noise reduction was not sufficient. This is particularly the case when the level of control was only moderate due to the high frequency problems with the compact cassette. The first successor was Dolby C. However, none of the successors achieved such global usage as Dolby-B-NR. Probably also because music cassettes almost always used Dolby B.

A special use of the Dolby B process took place in FM-Broadcast. Dolby FM with 25 µs pre-emphasis and adapted compander frequency response was used from 1971 by a number of VHF radio stations in the USA.[3] There were also receivers with Dolby B expanders and cassette recorders whose Dolby B circuit could be used externally in a “pass-through” operating mode. The system was practically abandoned as early as 1974.

A Telefunken that was later tested by the IRT in Germany between 1979 and 1981 High Com FM- The process was never introduced commercially.[4][5][6]

Dolby C

Dolby C is a further development of Dolby B presented to the public in 1980. It works with two cascaded compressors and an anti-saturation circuit. The first compressor corresponds exactly to the Dolby B compressor. The second works at 20 dB lower levels with a starting point that is well two octaves lower (5 dB point: 200 Hz instead of 1 kHz). The anti-saturation network improves the treble control at 10 kHz by around 4 dB and prevents over-saturation of the tape material due to excessive control of the useful signal by slightly reducing the level if the levels are too high for a short time.

Dolby S

Dolby S is a simpler version of the professional Dolby SR, the Dolby A successor. It has three compressor levels and, compared to Dolby B and C, also works in the bass range. In addition, the three frequency bands are divided in such a way that recordings made with Dolby S can also be played back on devices with Dolby B in acceptable quality. The first devices with Dolby S appeared on the market in 1990.

Dolby level, Dolby calibration

The simple idea of ​​compression and expansion is the strength of Dolby noise reduction and, at the same time, its weakness: The prerequisite for the system to function properly is the exact mirror image mode of operation during recording and playback. For this it is necessary that all compressors and expanders rate the same level. In addition, an exact calibration to the tape material used is necessary. Furthermore, it is important to align the tape head exactly at right angles to the tape run (azimuth), the premagnetization (frequency response) and especially the recording level must be set precisely. This is not just a rough adjustment to the type of tape used Normal, chromium dioxide or metal meant, because even within these classes there are manufacturer-related differences.

For example, a level recorded with 0 dB can be weaker or stronger than 0 dB during playback, depending on the sensitivity of the tape material used. This leads to inaccuracies in the signal processing of the Dolby system, which can usually be expressed in the form of a dull and, in rare cases, too bright reproduction of the highs and slight pumping noises.

Because of this problem, the Dolby Licensing Corporation specified particularly strict licensing conditions when Dolby S was introduced. Cassette recorders that want to use Dolby S must have a precisely aligned tape head and an externally accessible pre-magnetization setting.

To ensure that the compact cassette can be replaced, all devices must be calibrated to the same level. For this purpose, Dolby has defined a test tone with a specified tape magnetization.

Dolby HX Pro

Dolby HX Pro (HX stands for “Headroom eXtension”) per se is not a noise reduction system like Dolby A, B., C. and S., but a device that extends the height adjustment. This also indirectly reduces the audibility of the noise. It was developed and presented in 1982 by Bang & Olufsen, and a year later it was marketed by Dolby as the HX Pro (Dolby had already developed a simpler circuit: Dolby HX).

Function: A normal cassette deck has a selector switch for up to four types of tape. This switch sets (among other things) the pre-magnetization in rough steps suitable for these types of tape. Loud tones in the music signal to be recorded with a strong high-frequency component also have the effect of a premagnetization. This is added to the original premagnetization. The tape is not used optimally (too much pre-magnetization significantly worsens the recording quality in the high-frequency range, on the other hand, too little increases the distortion in the lower and middle frequency range). An HX circuit now constantly measures the high frequency level in the signal from the recording head. If the proportion of high frequencies in the signal increases, the premagnetization is adjusted (Dolby HX). With Dolby HX Pro, the entire effective premagnetization is continuously regulated to a setpoint, which leads to significantly more reliable results than with the simple HX.

The dynamic pre-magnetization improves the high frequency range enormously. Improvements are possible in the entire frequency spectrum.[7] The HX or HX PRO is particularly important at low tape speeds (small wavelengths) such as compact cassettes. HX Pro increases the dynamic range of cassettes and enables clearer recordings.[8] No additional circuit is necessary in the playback device, the advantage is effective with every playback.

Play Trim

Together with NAD, Dolby developed the play trim control, with the help of which a faulty high-frequency response of Dolby-coded cassette tapes is detected in front (important!) the dynamic expansion can be corrected approximately, so that such bands sound passable. A faulty high frequency response can result from incorrect premagnetization or frequency response equalization, overloading or failure to use the HX Pro, tape aging or azimuth errors between recording and playback and / or a poor compact cassette.

Dolby SR

Dolby SR plug-in card for multi-channel sound from 1986

Dolby SR is a noise reduction method in use since 1987 for sound recording for analog optical sound on 35 mm film and on analog tapes. SR stands for spectral recording and so called because it has a spectral compressor function adapted to the hearing.

The method is the most advanced audio noise reduction method and probably marks the end of the development of this method, since noise reduction is no longer required for digital systems. The dynamic range that can be achieved with this technology is mathematically roughly the same as that of a 16-bit digital recording, in practice even better values ​​are possible due to the properties of analog recording technology.

Analog multi-channel sound formats

  • Dolby Stereo 6-Track: 6-channel magnetic sound for 70 mm films (no longer in production)
  • Dolby Stereo A: Professional cinema sound system with four matrixed channels and Dolby A noise reduction
  • Dolby Stereo SR: Matrix like Dolby Stereo A, but with improved Dolby SR noise reduction
  • Dolby Surround: Home use counterpart to Dolby Stereo
  • Dolby Pro Logic: Like Dolby Surround, but with improved encoding and decoding techniques (extensions: Pro Logic II, Pro Logic IIx and Pro Logic IIz)

Digital multi-channel sound formats

Dolby E encoders and decoders in the form of 19 "devices, as they are used in the processing of television companies



  • Gustav Büscher, Alfred Wiegelmann: Little ABC of electroacoustics. (= Radio-Praktiker-Library. Vol. 29 / 30a). 6th, completely revised and expanded edition, Franzis, Munich 1972, ISBN 3-7723-0296-3.
  • Roland Enders: The home recording manual. The way to optimal recordings. 3rd, revised edition, revised by Andreas Schulz, Carstensen, Munich 2003, ISBN 3-910098-25-8.
  • Hubert Henle: The recording studio manual. Practical introduction to professional recording technology. 5th, completely revised edition, Carstensen, Munich 2001, ISBN 3-910098-19-3.

Web links

Individual evidence

  1. 1 2 Dolby Laboratories 2015 Form 10-K Report, accessed November 19, 2016
  2. ↑ latimes.com: Dolby acquires Doremi Labs
  3. ↑ C. P. Gilmore: Look and lists. In Popular Science. Vol. 199, No. 3, September 1971, p. 38.
  4. ↑ IRT (December 30, 1981). IRT Technical Report 55/81. Testing of a modified HIGHCOM compander for use in RF transmission in VHF radio.
  5. ↑ Ernst F. Schröder: The story of HIGHCOM.
  6. ↑ E.-J. Mielke: Influence of the Dolby-B method on the transmission quality in VHF radio broadcasting. In: Broadcast communications. Vol. 21, 1977, ISSN 0035-9890, pp. 222-228.
  7. Some Not Well Known Aspects of Analog Tape. - regarding Dolby HX / HX Pro, AES Montreux 1990 March, Arndt Klingelnberg
  8. ^ The Audio Professional by Bang and Olufsen, 1982
  9. Dolby Debuts New Video Technologies at International CES 2008.Dolby press release. Retrieved March 28, 2008.
  10. Dolby Vision - more color and contrast even with 4k or UHD.Ben Mueller. Retrieved February 17, 2014.