Editing Dolby noise-reduction system (section)

== Dolby A ==
<!-- This section is linked from inside or outside this article. Do not rename without keeping in sync. -->

Dolby&nbsp;A-type noise reduction was the Dolby company's first noise reduction system, presented in 1965.<ref name="DolbyChronology"/><ref name="History"/> 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&nbsp;dB per octave slopes, with [[cutoff frequency|cutoff frequencies]] (3&nbsp;dB down points) as follows: [[low-pass]] at 80&nbsp;Hz; [[band-pass]] from 80&nbsp;Hz to 3&nbsp;kHz; a [[high-pass]] from 3&nbsp;kHz; and another [[high-pass]] at 9&nbsp;kHz. (The stacking of contributions from the two high-pass bands allows greater noise reduction in the upper frequencies.) The [[Companding|compander]] circuit has a threshold of &minus;40&nbsp;dB, with a ratio of 2:1 for a compression/expansion of 10&nbsp;dB. This provides about 10&nbsp;dB of noise reduction increasing to a possible 15&nbsp;dB at 15&nbsp;kHz, according to articles written by Ray Dolby and published by the Audio Engineering Society (October 1967)<ref name="DolbyJAES1967"/><ref name="DolbyAESConv1967"/> and Audio (June/July 1968).<ref name="DolbyAudio1968June"/><ref name="DolbyAudio1968July"/>

As with the Dolby&nbsp;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&nbsp;nWb/m, which is the level used on industry calibration tapes such as those from Ampex; this is set to 0&nbsp;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&nbsp;VU on the tape recorder and to 185&nbsp;nWb/m on the tape.

The Dolby&nbsp;A-type system also saw some use as the method of noise reduction in [[optical sound]] for motion pictures. In 2004, Dolby A-type noise reduction was inducted into the [[TECnology Hall of Fame]], an honor given to "products and innovations that have had an enduring impact on the development of audio technology."<ref>{{cite web|title=TECnology Hall of Fame, 2004|url=http://legacy.tecawards.org/tec/04TECnologyHOFdetails.html|website=TECawards.org|date=2004|access-date=December 12, 2024}}</ref>