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. Noise reduction is the process of removing noise from a signal. ... Dolby Laboratories, Incorporated (Dolby Labs) is a company specializing in audio compression and reproduction. ... Magnetic tape is an information storage medium consisting of a magnetisable coating on a thin plastic strip. ... A waveform before and after the compression stage of companding In telecommunication, signal processing, and thermodynamics, companding (occasionally called compansion) is a method of reducing the effects of a channel with limited dynamic range. ...

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. 1966 was a common year starting on Saturday (the link is to a full 1966 calendar). ... 1968 was a leap year starting on Monday (the link is to a full 1968 calendar). ... 1980 is a leap year starting on Tuesday. ... For the meaning of cassette in genetics, see cassette (genetics). ... FM radio is a broadcast technology invented by Edwin Howard Armstrong that uses frequency modulation to provide high-fidelity broadcast radio sound. ...

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. 1982 is a number and represents a common year starting on Friday of the Gregorian calendar Events January January 6 - William Bonin is convicted of being the freeway killer. January 8 - AT&T agrees to divest itself of twenty-two subdivisions January 11 - Mark Thatcher, son of the British Prime... Tape bias is a high-frequency signal (generally from 40 to 150 kHz) added to the audio signal recorded on an analog tape recorder. ... Bang & Olufsen is a Danish company that designs high end audio products, television sets, and telephones. ...

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. Digital audio describes sound recording and reproduction systems which work by using a digital representation of the audio waveform. ... Size of CD compared to pencil. ... MP3 is a popular digital audio encoding and lossy compression format. ... The Sony MZ1 MiniDisc player, the first to hit the market in 1992. ... Digital Audio Tape (DAT or R-DAT) is a signal recording and playback medium introduced by Sony in 1987. ...

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. In telecommunication, preemphasis is a system process designed to increase, within a band of frequencies, the magnitude of some (usually higher) frequencies with respect to the magnitude of other (usually lower) frequencies in order to improve the overall signal-to-noise ratio by minimizing the adverse effects of such phenomena... This article is about white noise as a scientific concept, see also: White Noise (novel), a 1985 novel by Don DeLillo. ... Audio can mean: sound that can be heard electronics or other signals of frequencies audible to humans (about 20--20,000 Hz) broadcasting or reception of sound high-fidelity sound reproduction sound recording and reproduction in general I hear in Latin This is a disambiguation page — a navigational aid which... Psychoacoustics is the study of subjective human perception of sounds. ... Amplitude is a nonnegative scalar measure of a waves magnitude of oscillation. ... Although it is widely used as a measure of the loudness of sound, the decibel is more generally a measure of the ratio between two quantities, and can be used to express a wide variety of measurements in acoustics and electronics. ...

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. Dynamic range is a term used frequently in numerous fields to describe the ratio between the smallest and largest possible values of a changeable quantity. ... Hysteresis is a property of systems (usually physical systems) that do not instantly follow the forces applied to them, but react slowly, or do not return completely to their original state: that is, systems whose states depend on their immediate history. ... For the Analog Science Fiction and Science Fact publication, see Astounding Magazine. ... Non-linearity is a slight distortion of a single frequency into multiple frequencies within the human or mammalian auditory system. ...

External links

• More information available at Dolby Laboratories, Inc. (http://www.dolby.com/)
• Commercializing the Dolby Noise Reduction System (http://www.zpower.com/invention_app3.htm)