Dolby noise reduction system
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Dolby NR is a noise reduction system developed by Dolby Laboratories for use in analogue magnetic tape recording. It works by companding, i.e. reducing the dynamic range of the sound during recording and expanding it during playback. It is not the only system that works in this way, but it is the most widely used. It was one of the most important innovations that made high fidelity practical on cassette tapes which normally have high noise because of the slow speed and narrow tape format created initially for compact voice recorders, and is common on stereo tape players and recorders to the present day.
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[edit] History
Several types of Dolby NR were developed, including A (1966), B (1968), C (1980), S, and SR. Most widely used in consumer products is the B type, which allows for acceptable playback on devices without noise reduction. Most pre-recorded cassettes use this variant. In the mid-1970s, some expected Dolby NR to become normal in FM radio broadcasts and some tuners and amplifiers were manufactured with decoding circuitry.
[edit] Dolby noise reduction systems user groups
- Dolby A and Dolby SR were developed for professional use.
- Dolby B, C, and S were designed for the consumer market.
[edit] How Dolby noise reduction systems work
Dolby B (and C which is similar) is a form of dynamic preemphasis, used to improve the signal-to-noise ratio, especially in the range of hearing in which the background hiss of a tape (white noise) is most noticeable, in higher frequencies (above 1 kHz).
The Dolby preemphasis boosts the strength of the audio signal at higher frequencies during recording, compressing the dynamic range of the signal so that quiet sounds receive a proportionally greater boost. When the tape is recorded, the amplitude of the signal above 1 kHz is used to determine how much pre-emphasis to apply - a low level signal is boosted by 10 dB (Dolby B) or 20 dB (Dolby C). As the signal rises in amplitude, less and less pre-emphasis is applied until at the "Dolby level" (+3 VU), no signal modification is performed.
The sound is thus recorded at a higher strength on the tape relative to the tape's noise level. On playback, the opposite process is applied (deemphasis), based on the signal level. Thus as the signal level drops, the higher frequencies are progressively more strongly filtered, which also filters the constant background noise level. This is called psychoacoustic masking.
The two (pre and de-emphasis) processes cancel each other out as far as the signal is concerned. Only one process during playback is used. Then the de-emphasis curve is applied to the incoming signal and noise. After playback de-emphasis is complete, noise in the output signal is reduced.
- The calibration of the recording and playback circuitry is important for faithful cancellation of the complementary processes, and is easily upset by poor quality tapes, dirty playback heads or using incorrect bias levels. This usually manifests itself as muffled-sounding playback, or "breathing" of the noise level as the signal varies (in some high end equipment a Dolby playback calibration control is included).
[edit] Practical use of Dolby Noise Reduction systems
Inexpensive tape players generally lack the Dolby noise reduction feature. Yet many listeners still prefer the extended highs of unprocessed Dolby tapes as these are the most difficult to reproduce on inexpensive decks and small speakers or headphones.
- High fidelity decks usually have a Dolby front-panel switch.
- Cassettes are coded on the shell to detect equalization settings, but cannot detect noise reduction. Auto decks which have this feature will have a button with the Dolby "B" symbol on it.
- If you use a cassette adapter to play your iPod or MP3 player, you should make sure it is switched off, or you may be missing your high frequencies.
[edit] Dolby A, B and C (FAQ)
- Dolby A operates in four bands providing 10 dB noise reduction across the full frequency spectrum. It was designed first for professional users.
- Dolby B was developed after Dolby A as a single band system providing up to 10 dB noise reduction on frequencies above 1 kHz.
- Dolby B was much simpler than Dolby A and therefore much less expensive to implement in consumer products. Dolby B recordings are acceptable when played back on equipment that does not possess a Dolby B decoder.
- Dolby B provides far less effective noise reduction than Dolby A, generally by a factor of more than 3 db.
- Dolby SR system is a much more aggressive noise reduction approach than Dolby A. It attempts to maximise 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 noise reduction in the high frequency range.
- Dolby SR is only found on professional recording equipment. [1] [2]
[edit] Dolby B & C in the marketplace
From the mid 1970s, Dolby B became standard on commercially prerecorded music cassettes in spite of the fact that some low-end equipment lacked decoding circuitry.
- Dolby C provides up to 20 dB noise reduction in the high frequency range, but the resulting recordings sound much worse when played back on equipment that does not have Dolby C noise reduction. Some of this harshness can be mitigated by using Dolby B on playback. It utilises anti-saturation and spectral skewing techniques.
- 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 played back on older Dolby B equipment with some benefit being realised.
[edit] Dolby S
The Dolby S system 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.[3]
Dolby S is found on some Hi-Fi and semi-professional recording equipment. It was intended that Dolby S would become standard on commercial prerecorded musicassettes in much the same way that Dolby B had in the 1970s, but this never happened, mainly for the following reasons.
- Dolby S came to market at a time when the Compact Cassette was being replaced by the compact disc as the dominant mass market music format.
- Dolby Labs claimed that most members of the general public couldn't differentiate between the sound of a CD and a Dolby S encoded cassette.
- Dolby S only ever appeared as a feature on high-end audio equipment. Ten years after the introduction of Dolby S many cassette recorders only featured Dolby C, with the majority of tape players having no Dolby noise reduction at all.
[edit] Dolby HX
Dolby developed another system in 1982 called Dolby HX, which works by modifying the ultrasonic bias signal, used by all analogue tape decks, to increase the headroom for high-frequency audio signals. HX stands for "headroom extension". This system was modified by Bang & Olufsen and marketed by Dolby as Dolby HX Pro. (Reference.) While not a classic noise reduction system per se, Dolby HX Pro provides a cleaner original recording.
[edit] Dolby HX Pro
HX or "Headroom eXtension" is a method for further increasing the dynamic range of a cassette tape by dynamically adjusting the level of bias. Because tape is magnetic, it is inherently non-linear in nature, due to the hysteresis of the magnetic particles. If an analogue signal were recorded directly onto magnetic tape, its reproduction would be extremely distorted, due to this non-linearity.
To overcome this, a high frequency signal is mixed in with the recorded signal, which "pushes" the envelope of the signal into the linear region.
- With fixed frequency and amplitude high frequency strong signals the amount of bias signal needed is reduced. Due to group and phase delay the audio signal itself creates a variable amount of self-bias.
- If the added bias remains constant, these high frequency signals become overbiased. This overbias creates distortion as the tape becomes saturated.
- Dolby HX Pro automatically reduces the bias signal in the presence of strong high frequency signals. This optimises the amount of self bias, reducing distortion caused from saturation of the magetic tape.
- By adjusting the bias with respect to group and phase delay the overall distortion of high frequency signals is also greatly reduced. This kind of bias adjustment increases the high frequency dynamic range available. The net effect for the listener is a crisper sounding high frequency reproduction.
- Because this noise reduction system optimises how the signal is laid down on the tape, it does not require special decoding and will be present regardless of how cheap or expensive the playback unit is.
[edit] Technological obsolesence
Dolby's analogue noise reduction systems, though still used in some professional applications, as well in the large installed base of consumer tape decks, are becoming increasingly obsolete due to the widespread adoption of digital audio (in the form of compact discs, MP3s, MiniDiscs, and to a lesser extent DAT) in the home for entertainment and professional studios for recording.
[edit] See also
Technology
- Dynamic Noise Reduction (DNR), a noise reduction system often confused with Dolby
- dbx (noise reduction), a rival system
[edit] External links
- More information available at Dolby Laboratories, Inc.
- Commercializing the Dolby Noise Reduction System
