PreSonus ACP22 - Manual (Page 10)

Controls & Connections
The Release control is only active when the Auto button is not
pushed in. When the auto button is pushed in, the compressor
automatically determines the appropriate release time for
compression.
Soft
The Soft button selects from soft knee and hard knee compression
curves. When this button is pushed in, soft knee compression curves are
used, otherwise hard knee compression curves are used. With hard knee
compression, the gain reduction applied to the signal occurs as soon as the
signal exceeds the level set by the threshold. With soft knee compression,
the onset of gain reduction occurs gradually after the signal has exceeded the
threshold, producing a more musical response (to some folks).
Auto
When pushed in, the auto button places the compressor in automatic
attack and release mode. The attack and release knobs become inoperative
and a pre-programmed attack and release curve is used.
2.3 Gate Controls
Threshold
The gate threshold sets the level at which the gate opens as indicated by
the 'OPEN' LED above the threshold knob. All signals above the threshold
setting are passed through unaffected, whereas signals below the threshold
setting are reduced in level by the amount set by the range control. The