Dolby noise-reduction system
A Dolby noise-reduction system, or Dolby NR, is one of a series of noise reduction systems developed by Dolby Laboratories for use in analog audio tape recording.[1] The first was Dolby A, a professional broadband noise reduction system for recording studios in 1965, but the best-known is Dolby B (introduced in 1968), a sliding band system for the consumer market, which helped make high fidelity practical on cassette tapes, which used a relatively noisy tape size and speed. It is common on high-fidelity stereo tape players and recorders to the present day, although Dolby has as of 2016 ceased licensing the technology for new cassette decks. 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. Aside from Dolby HX, all the Dolby variants work by companding: compressing the dynamic range of the sound during recording, and expanding it during playback.
Product type
Audio technology
United Kingdom
United States
1965
Dolby Stereo (1976-present)
Dolby Digital (1986-present)
Worldwide
Dolby A[edit]
Dolby A-type noise reduction was the Dolby company's first noise reduction system, presented in 1965.[2] It was 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 frequency bands by four filters with 12 dB per octave slopes, with cutoff frequencies (3 dB down points) as follows: low-pass at 80 Hz; band-pass from 80 Hz to 3 kHz; a high-pass from 3 kHz; and another high-pass at 9 kHz. (The stacking of contributions from the two high-pass bands allows greater noise reduction in the upper frequencies.) The compander circuit has a threshold of −40 dB, with a ratio of 2:1 for a compression/expansion of 10 dB. This provides about 10 dB of noise reduction increasing to a possible 15 dB at 15 kHz, according to articles written by Ray Dolby and published by the Audio Engineering Society (October 1967)[3][4] and Audio (June/July 1968).[5][6]
As with the Dolby B-type system, correct matching of the compression and expansion processes is important. The calibration of the expansion (decoding) unit for magnetic tape uses a flux level of 185 nWb/m, which is the level used on industry calibration tapes such as those from Ampex; this is set to 0 VU on the tape recorder playback and to Dolby Level on the noise reduction unit. In the record (compression or encoding) mode, a characteristic tone (Dolby Tone) generated inside the noise reduction unit is set to 0 VU on the tape recorder and to 185 nWb/m on the tape.
The Dolby A-type system also saw some use as the method of noise reduction in optical sound for motion pictures.
Today[edit]
The widespread proliferation of digital audio in professional and consumer applications (e.g., compact discs, music download, music streaming) has made analog audio production less prevalent and therefore changed Dolby's focus on Dolby Vision, but Dolby's analog noise reduction systems are still widely used in niche analog production environments.