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, B, C, S, and SR. Most widely used, by consumers, is the B type, which allows for acceptable playback on devices without noise reduction. Most pre-recorded cassettes available on the market 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 noise reduction systems available are designed specifically with either professional or consumer users in mind–Dolby A and Dolby SR were developed for professionals, as the equipment requires precise alignment in order to reproduce properly, while Dolby B, C, and S were designed for the consumer market, requiring less intervention by the user, but providing poorer noise reduction

Dolby has another noise reduction system called HX Pro, 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".

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 in the higher frequency registers is used to determine how much pre-emphasis to apply - a lower level signal is boosted by about 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 portion of the transfer characteristic of the tape. The high frequency signal, called bias, is inaudible on playback, as its wavelength is too short to be captured by the magnetic medium. An audio signal that contains a large amount of high frequency components has a tendency to partially act as a bias signal in itself, so the addition of a fixed bias can push the envelope a little too much for these signals, into the second non-linear region where the tape becomes saturated. Dolby HX modulates the bias level to allow for the self-biasing effect of high frequencies, thus preventing the signal from entering the saturation region. The effect is to reduce distortion of loud high-frequency signals, which can sound very harsh otherwise.

External link

• More information available at Dolby Laboratories, Inc. (http://www.dolby.com/)

ja:ドルビーノイズリダクションシステム