DBX (noise reduction)
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dbx is a somewhat generic term for a family of noise reduction systems. The family includes, among others, dbx Type I and dbx Type II for analog tape recording and, less commonly, vinyl LPs. A separate system, known as dbx-TV, is part of the BTSC system used to provide stereo sound to North American and certain other TV systems.
The original dbx Type I and Type II systems were based on 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. 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 Type I and Type II are types of "companding noise reduction". Companding noise reduction works by first compressing the source material's dynamic range (in this case by a factor of 2) in anticipation of being recorded on a relatively noisy medium (magnetic tape, for example). Upon playback, the encoded material, now contaminated with noise, is passed through an expander which restores the original dynamic range of the source material. The contaminating signal (tape hiss) is "masked" by the dynamic expansion process, resulting in a significant reduction in perceived noise.
Although it brought extraordinary dynamic range to the lowly cassette tape, dbx noise reduction did not achieve widespread popularity in the consumer marketplace, as compressed recordings did not sound acceptable played back on non-dbx equipment; Dolby B was already widely used when dbx was introduced. Although Dolby noise reduction also used some companding, the level of compression and expansion was very mild, so that the sound of Dolby-encoded tapes was acceptable to consumers when played back on non-Dolby equipment. However, dbx Type I was widely adopted in professional recording, and Tascam incorporated dbx Type II in their Portastudio four-track cassette recorder for home studios, becoming standard on four-track cassette recorders of this type.
An advantage of dbx Type I and Type II compared to Dolby noise reduction is that it did not require calibration with the output level of the tape deck, which could cause incorrect tracking with Dolby B and C, leading to muffled high tones. However, due to their high compression and strong high-frequency preemphasis, dbx-encoded tapes were, unlike Dolby B, practically unplayable on non-dbx systems.
A sometimes noticeable artifact of dbx was "breathing", as its compander rapidly increased and decreased the volume level of the background noise along with the music, which was most noticeable in quiet musical passages; this was a greater issue with dbx than with Dolby because its compander was more aggressive and worked across the frequency spectrum.[link]
dbx was also used briefly on vinyl records, known as dbx discs, which reduced dust and scratches to tiny pops and clicks and, in theory, gave them a dynamic range of 120db.[link] In practice, dbx companders contain electronic noise below about -60db, which is about halfway from the theoretical range. This meant that even with the best all-analog mastering, dbx discs still did not achieve the full 90db range.
dbx noise reduction, capable of more than 20db of noise reduction, was used in the re-recording of the film Apocalypse Now in 1979. Dolby A-type noise reduction, capable of only 10-12db of noise reduction, was used only at the final stage for the mastering of the film's soundtrack to 70mm prints.
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.[link] dbx marketed the PPA-1 Silencer, a decoder that could be used with non-dbx players such as the Sony Walkman.[link] It also contained a Dolby B-compatible noise reduction decoder.
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