DBX is a noise reduction system for tape recording. 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. 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 compresses the recording by a 2:1 ratio.
DBX did not achieve popularity in the consumer marketplace, as compressed recordings did not sound acceptable played back on non-DBX equipment; Dolby B won out instead, as 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 was used by Tascam in their Portastudio four-track cassette recorder for home studios, becoming standard on four-track cassette recorders of this type.