Dbx (noise reduction)

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 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 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 two) in anticipation of being recorded on a relatively noisy medium (magnetic tape, for example).

Because dbx Type I and Type II are broadband (single-band) companders (unlike Dolby-A's four bands), they are susceptible to audible noise modulation and other artifacts. To deal with this, both Type I and II use very strong high-frequency pre-emphasis of the audio signal in both the recording path and the control signal path. This causes the compander to 'back off' the gain in certain circumstances and reduce the audibility of noise modulation – even with this pre-emphasis, noise modulation can become audible when using very noisy media to begin with, such as the Compact Cassette format. In the control signal path, the dbx Type II process rolls off the high and low frequency response to desensitize the system to frequency response errors – since the roll-off is only in the control path, it does not affect the audible sound.

...
Wikipedia