Dolby noise-reduction system
Product type | British invention |
---|---|
Owner | Dolby Laboratories |
Country | United Kingdom United States |
Introduced | 1965 |
Related brands | Dolby Stereo (1976-present) Dolby Digital (1986-present) |
Markets | Worldwide |
Previous owners | Ray Dolby |
Website | Dolby official website |
A Dolby noise-reduction system, or Dolby NR, is one of a series of
Usage
When recording a signal on magnetic tape, there is a low level of noise in the background which sounds like hissing. One solution to this issue is to use low-noise tape, which records more signal, and less noise. Other solutions are to run the tape at a higher speed or use a wider tape. Cassette tapes were originally designed to trade off fidelity for the convenience of recording voice by using a very narrow tape running at a very slow speed of 1+7⁄8 in/s (4.8 cm/s) housed in a simple plastic shell when 15 in/s (38 cm/s) or 7+1⁄2 in/s (19 cm/s) tape speeds were for high fidelity, and 3+3⁄4 in/s (9.5 cm/s) was of lower fidelity. As a result of their narrow tracks and slow speed, cassettes make tape hiss a very severe problem.
Dolby noise reduction is a form of dynamic
One cannot simply increase the volume of the recording to achieve this end; tapes have a maximum volume they can record, so already-loud sounds will become distorted. The idea is to increase the volume of the recording only when the original material is not already loud, and then reduce the volume by the same amount on playback so that the signal returns to the original volume levels. When the volume is reduced on playback, the noise level is reduced by the same amount. This basic concept, increasing the volume to overwhelm inherent noise, is known as pre-emphasis, and is found in a number of products.
On top of this basic concept, Dolby noise reduction systems add another improvement. This takes into account the fact that tape noise is largely heard at frequencies above 1,000 Hz. It is the lower-frequency sounds that are often loud, like drum beats, so by only applying the companding to certain frequencies, the total amount of distortion of the original signal can be reduced and focused only on the problematic frequencies. The differences in the various Dolby products are largely evident in the precise set of frequencies that they use and the amount of modification of the original signal volume that is applied to each of the frequency bands.
Within each band, the amount of pre-emphasis applied depends on the original signal volume. For instance, in Dolby B, a low-level signal will be boosted by 10 dB, while signals at the "Dolby Level", +3 VU, receive no signal modification at all. Between the two limits, a varying level of pre-emphasis is applied. On playback, the opposite process is applied (de-emphasis), based on the relative signal component above 1 kHz. Thus, as this portion of the signal decreases in amplitude, the higher frequencies are progressively increasingly attenuated, which also reduces in level the constant background noise on the tape when and where it would be most noticeable.
The two processes (pre- and de-emphasis) are intended to cancel each other out as far as the actual recorded program material is concerned. During playback, only de-emphasis is applied to the incoming off-tape signal and noise. After playback de-emphasis is complete, the apparent noise in the output signal is reduced, and this process should not produce any other effect noticeable to the listener other than reduced background noise. However, playback without noise reduction produces a noticeably brighter sound.
The correct calibration of the recording and playback circuitry is critical in order to ensure faithful reproduction of the original program content. The calibration can easily be upset by poor-quality tape, dirty or misaligned recording/playback heads, or using inappropriate
On some high-end consumer equipment, a Dolby calibration control is included. For recording, a reference tone at Dolby Level may be recorded for accurate playback level calibration on another transport. At playback, the same recorded tone should produce the identical output, as indicated by a Dolby logo marking at approximately +3 VU on the VU meter(s). In consumer equipment, Dolby Level is defined as 200 nWb/m, and calibration tapes were available to assist with the task of correct level setting. For accurate off-the-tape monitoring during recording on 3-head tape decks, both processes must be employed at once, and circuitry provided to accomplish this is marketed under the "Double Dolby" label.
Dolby A
Dolby A-type noise reduction was the Dolby company's first noise reduction system, presented in 1965.
As with the Dolby B-type system, correct matching of the compression and expansion processes is important. The calibration of the expansion (decoding) unit for magnetic tape uses a flux level of 185 nWb/m, which is the level used on industry calibration tapes such as those from Ampex; this is set to 0 VU on the tape recorder playback and to Dolby Level on the noise reduction unit. In the record (compression or encoding) mode, a characteristic tone (Dolby Tone) generated inside the noise reduction unit is set to 0 VU on the tape recorder and to 185 nWb/m on the tape.
The Dolby A-type system also saw some use as the method of noise reduction in optical sound for motion pictures.
Dolby B
Dolby B-type noise reduction was developed after Dolby A, and it was introduced in 1968. It consisted of a single sliding band system providing about 9 dB of noise reduction (
The Dolby B system is effective from approximately 1 kHz upwards; the noise reduction that is provided is 3 dB at 600 Hz, 6 dB at 1.2 kHz, 8 dB at 2.4 kHz, and 10 dB at 5 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").[7]
From the mid-1970s, Dolby 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
Prior to the introduction of later consumer variants (Dolby C being the first), cassette hardware supporting Dolby B and cassettes encoded with it would be labeled simply "Dolby System," "Dolby NR", or wordlessly with the Dolby symbol. This continued in some record labels and hardware manufacturers even after Dolby C had been introduced, during the period when the new standard was relatively little-known.
Dolby FM
In the early 1970s, some expected Dolby NR to become normal in
A similar system named
RMS
A fully Dolby B-compatible compander was developed and used on many
Dolby C
The Dolby C-type noise reduction system was developed in 1980.
As a result of the extra signal processing, Dolby C-type recordings will sound distorted when played back on equipment that does not have the required Dolby C decoding circuitry. Some of this harshness can be mitigated by using Dolby B on playback, which serves to reduce the strength of the high frequencies.
With Dolby C-type processing, noise reduction begins two octaves lower in frequency in an attempt to maintain a psychoacoustically-uniform noise floor. In the region above 8 kHz, where the ear is less sensitive to noise, special spectral-skewing and anti-saturation networks come into play. These circuits prevent cross modulation of low frequencies with high frequencies, suppress tape saturation when large signal transients are present, and increase the effective headroom of the cassette tape system. As a result, recordings are cleaner and crisper with a much improved high-frequency response that the cassette medium heretofore lacked. With a good quality tape, the Dolby C response could be flat to 20 kHz at the 0 dB recording level, a previously unattainable result. An A-weighted signal-to-noise ratio of 72 dB (re 3% THD at 400 Hz) with no unwanted "breathing" effects, even on difficult-to-record passages, was possible.
Dolby C first appeared on higher-end cassette decks in the 1980s. The first commercially available cassette deck with Dolby C was the
Dolby SR
The Dolby SR (Spectral Recording) system, introduced in 1986, was the company's second professional noise reduction system. It is a much more aggressive noise reduction approach than Dolby A. It attempts to maximize the recorded signal at all times using a complex series of filters that change according to the input signal. As a result, Dolby SR is much more expensive to implement than Dolby B or C, but Dolby SR is capable of providing up to 25 dB of noise reduction in the high-frequency range. It is only found on professional recording equipment.[1][19]
In the motion picture industry, as far as it concerns distribution prints of movies, the Dolby A and SR markings refer to
SR prints are fairly well backward compatible with old Dolby A equipment. The Dolby SR-D marking refers to both analog Dolby SR and digital Dolby Digital soundtracks on one print.
Dolby S
Dolby S was introduced in 1989. It was intended that Dolby S would become standard on commercial pre-recorded music cassettes in much the same way that Dolby B had in the 1970s, but it came to market when the
Dolby S is much more resistant to playback problems caused by noise from the tape transport mechanism than Dolby C. Likewise, Dolby S was also claimed to have playback compatibility with Dolby B in that a Dolby S recording could be played back on older Dolby B equipment with some benefit being realized. It is basically a cut-down version of Dolby SR and uses many of the same noise reduction techniques. Dolby S is capable of 10 dB of noise reduction at low frequencies and up to 24 dB of noise reduction at high frequencies.[21]
Dolby HX/HX-Pro
Magnetic tape is inherently non-linear in nature due to
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.[22] 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.[23][24] 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 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.
Technological trends
The widespread proliferation of digital audio in professional and consumer applications (e.g., compact discs, music download, music streaming) has made analog audio production less prevalent and therefore changed Dolby's focus on Dolby Vision, but Dolby's analog noise reduction systems are still widely used in niche analog production environments.
See also
- Dynamic Noise Reduction(DNR), a playback-only noise reduction system often confused with Dolby
- dbx (noise reduction), a competing system
- High Com, a competing system
References
- ^ a b White, Paul (January 1996). "Tape Noise Reduction". Sound On Sound. Archived from the original on 2016-06-04. Retrieved 2023-09-02.
- ^ "Dolby History: 50 Years of Innovation".
- ^ Dolby, Ray (October 1967). "An Audio Noise Reduction System". Journal of the Audio Engineering Society. 15 (4): 383–388. Retrieved 2023-09-02.
- ^ Dolby, Ray (October 1967). An Audio Noise Reduction System (PDF). 33rd Audio Engineering Society Convention, Paper Number 543. Los Angeles. Retrieved 2020-11-09.
- ^ Dolby, Ray M. (June 1968). "Audio Noise Reduction: Some Practical Aspects – Part 1: Operating principles and details of a professional noise-reduction system" (PDF). Audio: 19–22, 55. Archived (PDF) from the original on 2022-10-09. Retrieved 2020-11-09.
- ^ Dolby, Ray M. (July 1968). "Audio Noise Reduction: Part 2 (Conclusion)" (PDF). Audio: 26–30. Archived (PDF) from the original on 2022-10-09. Retrieved 2020-11-09.
- ^ Dolby, Ray (16–18 March 1971). A noise reduction system for consumer tape recording. Audio Engineering Society Convention. Cologne, Germany.
- ^ Woram, John (1976-12-04). "Update: Noise Reduction". Billboard. p. 30.
- ^ Gilmore, C. P. (September 1971). "Look and Listen". Popular Science: 38.
- ^ Free, John R. (1974) Look and Listen Popular Science, February 1974, p. 50
- ^ Mielke, E.-Jürgen (1977). "Einfluß des Dolby-B-Verfahrens auf die Übertragungsqualität im UKW-Hörrundfunk". Rundfunktechnische Mitteilungen (in German). 21. Institut für Rundfunktechnik (IRT): 222–228.
- (PDF) from the original on 2021-04-25. Retrieved 2021-04-25.
- ^ Schröder, Ernst F. "The Story of HIGH COM". Archived from the original on 2016-04-16.
- ^ "Prüfung eines modifizierten HIGH COM-Kompanders für den Einsatz bei der RF-Übertragung im UKW-Hörfunk" (in German). Institut für Rundfunktechnik (IRT). 1981-12-30. Technical Report 55/81.
{{cite journal}}
: Cite journal requires|journal=
(help) - ^ "SK 900" (in German). Archived from the original on 2021-05-09. Retrieved 2021-05-09.
- ^ Dolby, Ray (1981). A 20 dB Audio Noise Reduction System for Consumer Applications (PDF). 70th Audio Engineering Society Convention, Paper Number 1850. New York. Retrieved 2023-08-27.
- ^ Dolby, Ray (March 1983). "A 20 dB Audio Noise Reduction System for Consumer Applications". Journal of the Audio Engineering Society. 31 (3): 98–113. Retrieved 2023-08-27.
- ^ Hull, Joseph (May 1981). "Dolby C-Type Noise Reduction" (PDF). Audio: 20–25. Archived (PDF) from the original on 2022-10-09. Retrieved 2020-11-11.
- ^ Dolby Laboratories (1987). "Dolby SR: Dolby Spectral Recording" (PDF). Archived from the original (PDF) on 2012-10-02.
- ^ "Harman Kardon TD4800". Retrieved 2023-09-02.
- ^ Dolby Laboratories (2004). "Dolby B, C, and S Noise Reduction Systems: Making Cassettes Sound Better" (PDF). Archived from the original (PDF) on 2012-10-21.
- ^ "Jørgen Selmer Jensen" (in Danish). jollyjokers.dk. Archived from the original on 2013-09-14.
- ^ "B&O/Beocord 9000 - DDHFwiki" (in Danish). Datamuseum.dk. Archived from the original on 2021-08-04. Retrieved 2020-11-09.
- (PDF) from the original on 2021-07-10. Retrieved 2021-07-10. (3 pages)
External links
- 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