To elaborate:

Anything greater than 1:1 is expansion, and any device that does both compression and expansion is known as a compander.

The dbx company has been around for years, and has had, among other things, companders for use both for expansion of low-dynamic-range music and compression of overly-large-dynamic-range sounds and music.

They also have provided their circuitry for its noise-reduction capability when used in tape recording. During recording, the dynamic range is compressed at a straight 2:1 ratio, which means that the difference in loudness between the loudest and softest levels is halved.

The quietest passages are boosted to stay above the residual noise level (noise floor, or tape hiss), yet the loudest passages are reduced in level, so there is no chance of tape saturation, so the distortion that results from overload is eliminated.

During playback, the signal is subjected to a 1:2 straight expansion, which restores the original dynamic range to the audio. As a plus, along with the low-level audio signal, the accompanying tape hiss is reduced in level, so there is no audible background hiss left.

Dolby noise reduction is basically a frequency-selective version of the same thing; certain higher-frequency bands are boosted in level during recording, and reduced during playback, taking the noise down with it.

Playback of Dolby-'encoded' audio without Dolby engaged merely sounds a bit brighter, because the higher frequencies are not reduced in level, but dbx-encoded audio sounds very flat and lifeless, because the dynamic range is half of the intended level.

Larry
www.fineelectricco.com