Dolby noise reduction system

From Academic Kids

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.

Several types of Dolby NR were developed, including A (1966), B (1968), C (1980), S, and SR. Dolby A type is a multiband professional system. 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.

The Dolby systems are designed specifically with either professional or consumer users in mind—Dolby A and Dolby SR were developed for professional use. Dolby A operates in four bands providing 10 dB noise reduction across the full frequency spectrum. Dolby B, C, and S were designed for the consumer market. Dolby B was developed as a single band system providing up to 10 dB noise reduction on frequencies above 1 kHz. It was less expensive to implement but provided less effective noise reduction. Dolby C provides up to 20 dB noise reduction in the high frequency range.

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. (http://www.beoworld.co.uk/hxpro.htm)) While not a noise reduction system per se, Dolby HX Pro provides a cleaner original recording.

Dolby's analogue noise reduction systems, though still used in some professional applications, have been made obsolete by 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 recording.

How Dolby B works

Dolby B (and C which is similar) is a form of dynamic preemphasis. The background hiss of a tape white noise is unnoticeable if it is masked by a stronger audio signal, especially at higher frequencies. This is called psychoacoustic masking. 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 10dB (Dolby B) or 20dB (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. 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 processes cancel out as far as the signal is concerned, so it is reproduced faithfully, but only one process (the de-emphasis) is applied to the noise, which is thereby 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.

Dolby HXPro

HX or "Headroom eXtension" is a method for further increasing the dynamic range of a cassette tape. 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, it would be reproduced 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.

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