Compress or expand audio dynamic range.

A description of the accepted options follows.

attacks, decays

Set list of times in seconds for each channel over which the instantaneous level of the input signal is averaged to determine its volume. attacks refers to increase of volume and decays refers to decrease of volume. For most situations, the attack time (response to the audio getting louder) should be shorter than the decay time because the human ear is more sensitive to sudden loud audio than sudden soft audio. Typical value for attack is 0.3 seconds and for decay 0.8 seconds.

points

Set list of points for transfer function, specified in dB relative to maximum possible signal amplitude. Each key points list need to be defined using the following syntax: x0/y0 x1/y1 x2/y2 ....

The input values must be in strictly increasing order but the transfer function does not have to be monotonically rising. The point 0/0 is assumed but may be overridden (by 0/out-dBn). Typical values for the transfer function are -70/-70 -60/-20.

soft-knee

Set amount for which the points at where adjacent line segments on the transfer function meet will be rounded. Defaults is 0.01.

gain

Set additional gain in dB to be applied at all points on the transfer function and allows easy adjustment of the overall gain. Default is 0.

volume

Set initial volume in dB to be assumed for each channel when filtering starts. This permits the user to supply a nominal level initially, so that, for example, a very large gain is not applied to initial signal levels before the companding has begun to operate. A typical value for audio which is initially quiet is -90 dB. Default is 0.

delay

Set delay in seconds. Default is 0. The input audio is analysed immediately, but audio is delayed before being fed to the volume adjuster. Specifying a delay approximately equal to the attack/decay times allows the filter to effectively operate in predictive rather than reactive mode.