dbx is a family of noise reduction systems developed by the company of the same name. The most common implementations are dbx Type I and dbx Type II for analog tape recording and, less commonly, vinyl LPs. A separate implementation, known as dbx-TV, is part of the MTS system used to provide stereo sound to North American and certain other TV systems. The company, dbx, Inc., was also involved with Dynamic Noise Reduction (DNR) systems.
The original dbx Type I and Type II systems were based on the so-called "linear decibel companding" - compressing the signal on recording and expanding it on playback. It was invented by David E. Blackmer of dbx, Inc. in 1971.
A miniature dbx Type II decoder on an integrated circuit was created in 1982 for use in portable and car audio, although only a few devices took advantage of it, such as certain panasonic portable cassette players and sanyo car stereos. dbx marketed the PPA-1 Silencer, a decoder that could be used with non-dbx players such as the sony Walkman. A version of this chip also contained a Dolby B-compatible noise reduction decoder, described as dbx Type B noise reduction; this was possible after the Dolby patent (but not the trademark) had expired.
dbx was also used on vinyl records, which were known as dbx discs. While the earliest release is from 1971, their numbers peaked between 1977 until around 1982. Billboard noted in August 1981 that the total number of releases with dbx encoding was expected to approach 200 albums. Discogs mentions 1100 albums. When employed on LPs, the dbx Type-II system reduced the audibility of dust and scratches, reducing them to tiny pops and clicks (if they were audible at all) and also completely eliminated record surface noise. dbx encoded LPs had, in theory, a dynamic range of up to 90 dB. In addition, dbx LPs were produced from only the original master tapes, with no copies being used, and pressed only on heavy, virgin vinyl. Most were released in limited quantities with premium pricing.
The dbx k9 noise reduction card was designed to fit into the pro-dolby-A series A-361 frames, already in wide use in pro-reel-to-reel recording studios of the time. The dbx 192 was an elegant design made especially for the nagra IV-Stereo recorder. It had a single push-button for record/playback encode/decode and was integrated directly into the nagra's internal signal path. It drew power from the nagra supply.
dbx-TV noise reduction, while having elements in common with Type I and Type II, is different in fundamental ways, and was developed by Mark Davis (then of dbx, now of Dolby Labs) in the early 1980s.
dbx-TV is included in multichannel television sound (MTS), the U.S. standard for stereo analog television transmission. Every TV device that decoded MTS originally required the payment of royalties, first to dbx, Inc., then to THAT Corporation which was spun off from dbx in 1989 and acquired its MTS patents in 1994; however, those patents expired worldwide in 2004.
dbx noise reduction, capable of more than 20 dB of noise reduction, was used in the re-recording of the film Apocalypse Now in 1979. Dolby A-type noise reduction, capable of only 10-12 dB of noise reduction, was used only at the final stage for the mastering of the film's soundtrack to 70mm prints.
A modified version of dbx was also used in the Colortek stereo film system. In addition, dbx Type-II noise reduction was used in the Model-II and Model-III variants of MCA's Sensurround Special Effects System on the optical audio track and was a cornerstone of the entire system. MCA's Sensurround+Plus, used on the film Zoot Suit, employed dbx Type-II with the 4-track magnetic sound format on 35mm film prints, providing the motion picture with a stereo soundtrack capable of wide dynamic range and freedom from noise.
dbx for program delivery via the American NPR Public Radio Satellite System
The first generation Public Radio Satellite System (PRSS), introduced in 1979 and used by the American National Public Radio for delivery of network programming to their member stations via satellite, was a single channel per carrier (SCPC) system that had about 40 dB of analog (recovered) signal to noise. dbx modules that were set for 3:1 were used to increase the dynamic range of the system. Typically this worked well but for some low frequencies, the distortion exceeded 10 percent THD. Also, the dbx modules varied in how they tracked the compressed audio so the expanded audio was not an exact representation of what was compressed at the uplink. Still, the use of dbx allowed NPR to be known for its high fidelity standards on its satellite system as commercial broadcasters chose NPR to up-link a number of commercial radio music programs and concerts by commercial radio networks who demanded high fidelity in the analog era. Many of these problems were resolved when the PRSS moved to their second-generation system in 1994, the SOSS (Satellite Operations Support System), in which the feeds were sent digitally.