Dbx (noise reduction)
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Template:Lowercase Image:DBXlogo.jpg dbx is a noise reduction system for analog tape recording, North American TV broadcasting, and, less commonly, vinyl LPs. It works by a process 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 works by increasing the perceptible dynamic range (difference between quietest and loudest elements) of a recording, i.e making the quietest sounds at noise prone frequencies louder than the noise background. The dynamic range of a live performance may be 90 to 100dB, whereas the best obtainable result from a vinyl record without compression is around 60dB. dbx Types I and II compress the recording by a 2:1 ratio.
dbx 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, and its use of preemphasis gave far more acceptable results when played back on non-Dolby equipment. However, dbx was widely adopted in professional recording, and dbx Type II was used by Tascam in their Portastudio four-track cassette recorder for home studios, becoming standard on four-track cassette recorders of this type.
An advantage of dbx 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 artefact 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.[1]
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.[2] 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 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.[3] dbx marketed the PPA-1 Silencer, a decoder that could be used with non-dbx players such as the Sony Walkman.[4] It also contained a Dolby B-compatible noise reduction decoder.