Dolby noise reduction system

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Dolby NR is the name given to a series of noise reduction systems developed by Dolby Laboratories for use in analogue magnetic tape recording. The first was Dolby A professional noise reduction for recording studios in 1966, but the best-known is Dolby B (introduced 1968), a simplified version of Dolby A for the home market, which helped make high fidelity practical on cassette tapes, and is common on stereo tape players and recorders to the present day. Of the noise reduction systems, Dolby A and Dolby SR were developed for professional use. Dolby B, C, and, S were designed for the consumer market. All the Dolby variants work by companding, or compressing the dynamic range of the sound during recording and expanding it during playback.

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[edit] How Dolby noise reduction works

Dolby noise reduction is a form of dynamic preemphasis, used to improve the signal-to-noise ratio. While Dolby A works across the whole spectrum, the other systems specifically emphasize 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.

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] Dolby A

Dolby A was the company's first noise reduction system, intended for use in professional recording studios, where it became commonplace, gaining widespread acceptance at the same time that multitrack recording became standard. The input signal is split into four frequency bands with 12 dB/oct slopes: lowpass @ 80Hz; bandpass 80 Hz - 3 kHz; highpass @ 3 kHz; highpass @ 9 kHz. The compansion circuit has a threshold of -40dB, with a ratio of 2:1 for a compression / expansion of 10dB, except for the 9 kHz highpass which has only 5dB of gain change. This provides about 10 dB of broadband noise reduction, which increases to a possible 15dB at 15 kHz according to articles written by Ray Dolby in JAES (Oct 1967) and Audio (June / July 1968).

[edit] Dolby B

Dolby B was developed after Dolby A as a single band system providing up to 10 dB noise reduction on frequencies above 1 kHz, primarily for cassettes. It 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, such as most inexpensive cassette players. However, Dolby B provides less effective noise reduction than Dolby A, generally by a factor of more than 3 dB.

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, although it allows for acceptable playback on such equipment. 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 C

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 C first appeared on higher end cassette decks in the 1980's.

[edit] Dolby SR

Dolby SR system, the company's second professional noise reduction 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. It is only found on professional recording equipment. [1] [2]

[edit] Dolby S

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, as 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. In any case, Dolby S only ever appeared as a feature on high-end audio equipment.

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. 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.[3]

[edit] Dolby HX

Dolby developed another system in 1982 called Dolby HX. Not a noise reduction system per se, it provides a cleaner original recording. HX or "Headroom eXtension" is a method for further increasing the dynamic range of a cassette tape by dynamically adjusting the ultrasonic bias signal, used by all analogue tape decks, to increase the headroom for high-frequency audio signals. This system was modified by Bang & Olufsen for consumer equipment and marketed by Dolby as Dolby HX Pro. (Reference.) HX and HX Pro are used only in recording.

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, known as bias, is mixed in with the recorded signal, which "pushes" the envelope of the signal into the linear region. With strong signals of fixed frequency and high amplitude, the amount of bias 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.

[edit] Technological obsolescence

Dolby's analogue noise reduction systems, though still used in some professional applications, as well as 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.

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