Dolby Laboratories


  • Kevin J. Yeaman, President and CEO
  • Peter Gotcher, Chairman

Dolby Laboratories, Inc. is a company that has developed earlier compander noise reduction in analog audio range and is now specializing in digital multi-channel audio formats. It was founded in 1965 by Ray Dolby in England, in 1976 he moved it to the United States of America.

Analog Compressors ( noise reduction method )

In the analog Dolby procedures quiet sounds are raised in the inclusion in the volume and lowered when playing by the same amount again. Thus, the band noise is lowered with. This has nothing to do with the static pre-and de-emphasis.

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

The most common Dolby method with commercially available tape recorders called Dolby B. This is in higher quality devices that were formerly part of the standard scope of a stereo, sometimes complemented by the development of Dolby C and Dolby HX Pro. Dolby S, released at the beginning of the 1990s, should be in addition to B and C of the standard for higher-priced hi-fi tape decks in the home. However, by the advance of digital technology, this market segment largely broke away so that Dolby S was able to achieve any significant market share more.

Dolby A

Dolby A is the first developed by Dolby compression system for studio magnetic tape recorders from 1966. It consists of four independently operating compressors. 1 compressor operates below 80 Hz, compressor 2 from 80 Hz to 3 kHz and compressor 3 above 3 kHz. Compression is 10 dB. The fourth compressor is operating above 8 kHz in this frequency range and provides a further 5 dB.

From 1972 Dolby A has improved the surround sound ( optical sound ) is crucial in dynamic range and frequency response.

Dolby B, Dolby " NR"

The 1968 put on the market Dolby B acts of mid frequencies from about 1 kHz (with the JVC developed, Dolby -compatible from ANRS 500 Hz) up to the height range. While recording, the recorded signal is frequency - and level-dependent amplified loaded onto the tape and weakened when playing by the same amount. This also the tape noise is attenuated with during playback. The noise reduction system must be switched on when either recording as well as playback. Frequency and level-dependent compression and expansion means that the music signal is not processed even during recording and playback. The degree of processing is in the lower frequency range from 1 kHz low and at high frequencies more pronounced and begins from a level below 0 dB. Many cassette players the Dolby logo (double -D symbol ) is attached for guidance in the meter in this area. All levels that are lower than this mark will be processed by the Dolby system. The quieter they are, the stronger the processing. Therefore, it is important that the music signal loud enough auszusteuern, of course taking into account the recommended by the tape manufacturer maximum Aufsprechpegel. In older cassette equipment that are exclusively provided with Dolby B, the method commonly known as Dolby NR (NR for Noise Reduction) is called.

The use of only one stage of processing with Dolby B led by the quality increase of the other hi-fi components on the need for further development of this noise reduction method. Under some shooting conditions, such as a single drum pelvis or sound wood, that is with short, staccato sounds of then available circuits and processing of a relatively broad frequency spectrum by Dolby were heard B significant noise bursts after the end of the actual sound due to the inertia. This problem was considered in the development of the successor Dolby C.

A modification of the Dolby B process called Dolby FM with 25 microseconds pre-emphasis and adjusted compander frequency response was used in 1971 by a number of FM radio stations in the United States. In addition there was the receiver with corresponding expander circuits and tape recorders, whose Dolby B circuit could be made ​​for externally available in a " pass-through " mode. By 1974 the system was practically abandoned. A later between 1979 and 1981 in Germany tested by the IRT Telefunken High Com FM method has never been commercially introduced.

Dolby C

Dolby C is a the public in 1980 presented further development of Dolby B. It works with two cascaded compressors as well as an anti- saturation circuit. The first compressor is exactly the Dolby B compressor. The second operates at 20 dB lower levels with an insert point, which is a good two octaves lower (5- dB point: 200 Hz instead of 1 kHz). The antisaturation network improves Höhenaussteuerbarkeit at 10 kHz to about 4 dB and prevents briefly occurring high levels of super-saturation of the strip material by excessive modulation of the useful signal by the slightly reduced level.

Dolby S

Dolby S is a simpler version of the professional Dolby SR, Dolby A- successor, has three compressor stages and acts in addition to Dolby B and C in the bass. In addition, the three frequency bands are divided so that created with Dolby S tapes are played back on devices with Dolby B in acceptable quality. The first devices with Dolby S came out in 1990 on the market.

While the simple idea of the compression and expansion is the strength of Dolby noise reduction, it is also at the same time its weakness: a prerequisite for the proper functioning of the system namely, a precise calibration of the cassette unit to the belt material used. So it is not only important, precisely at right angles to align the head on the tape, the bias and particularly the Aufsprechpegel must be accurately adjusted. Thus, not only the coarse adjustment is normal on the type of tape used, meant chromium dioxide or metal, because even within these classes, there are subtle nuances. So it may happen that a recorded at 0 dB level due to the low sensitivity of the tape material during playback is less than 0 dB. This results in the signal processing of the Dolby system to inaccuracies that may manifest itself by a dull the treble or pump noise. Because of these problems, the Dolby Licensing Corporation has set strict license conditions for the introduction of Dolby S. Cassette players who want to use Dolby S must have a precisely aligned head and an externally accessible Vormagnetisierungseinstellung.

Dolby HX Pro

Dolby HX Pro ( HX stands for " Headroom eXtension " ) is not per se a noise reduction system such as Dolby A, B, C and S, but a device that extends the Höhenaussteuerbarkeit. Indirectly, this also reduces the noise level. It was developed and introduced in 1982 by Bang & Olufsen, a year later marketed as Dolby HX Pro. Function: A normal cassette deck has a selector switch for up to four tape places. This switch provides ( among other things) a the pre-specified bias of this band places. Well cause loud, high-pitched sounds in the recorded music signal also been a bias. This is added to the initial bias, and thus the tape is not optimally utilized is (too much bias limits the intake of high tones to low on the other hand increases the distortions ). HX -Pro - circuit now continuously measures the levels of the higher frequencies ( the signal from the recording amplifier ), which are supplied to the tape head. Increasing the proportion of high frequencies in the signal, the bias will be reduced accordingly, which ultimately leads to improved Hochtonaussteuerbarkeit or in other words, a uniform recording of the entire frequency spectrum is guaranteed. So HX increases per the dynamic range of cartridges, thus enabling clearer portraits (Source: The Professional Audio by Bang and Olufsen, 1982). The player on the decoder side, no special logic is needed.

Play Trim

Together with NAD Dolby developed the play- Trim control with the help of a faulty Hochtonfrequenzgang Dolby -encoded tapes once before ( important) the dynamics of expansion can be approximately corrected so that also sound such bands passable. A faulty Hochtonfrequenzgang may arise due to incorrect bias or frequency equalization, overdrive or non-use of HX Pro, aging tape or azimuth error between recording and playback and / or bad CC- housing.

Dolby SR

Dolby SR is a commonly used since 1987 noise suppression method for sound recording for analog optical sound on 35mm film and analog tapes. SR stands for spectral recording and is so named because it has adapted to the hearing, spectral compressor function.

The process is the most advanced audio noise reduction method and probably also marks the completion of the development of this method, since for digital systems no more noise reduction is needed. The recoverable with this technique dynamic range equates to approximately to a 16 -bit digital recording, in practice better results are due to the characteristics of analog recording technology even possible.

Multichannel analog audio formats

  • Dolby Stereo 6 Track: 6- channel magnetic at 70 -mm films ( no longer manufactured )
  • Dolby Stereo A: Professional Kinotonsystem with four matrixed channels and Dolby A-type noise reduction
  • Dolby Stereo SR: matrixing as Dolby stereo A, but with improved Dolby SR noise reduction
  • Dolby Surround: home users counterpart to Dolby Stereo
  • Dolby Pro Logic: How Dolby Surround, but improved encoding and decoding techniques (extensions: Pro Logic II, Pro Logic IIx and Pro Logic IIz )

Digital multi-channel sound formats

  • Dolby Digital: Digital 6-channel system (5.1), also known as AC-3 (extension: Dolby Digital EX )
  • Dolby Digital Plus
  • Dolby Virtual Speaker: System to emulate surround sound with only two speakers
  • Dolby E: Up to eight channels
  • Dolby TrueHD
  • Dolby Atmos


  • Dolby Contrast: A video format for enhanced contrast in LCD devices
  • Dolby 3D: A 3D projection method
  • Dolby Vision: An imaging technology, lifelike brightness as well as just such colors and contrasts supplies.