PreSonus ACP22 - Manual (Page 12)

Controls & Connections
frequencies at -12dB per octave. This feature is extremely useful when
gating drums where it would be great to stop the cymbals from opening up
the tom-tom gates, etc.
2.4 Gain
Gain
When compressing a signal, gain reduction usually results in an overall
reduction of level. The gain control allows you to restore the loss in level due
to compression. (Like readjusting the volume.)
2.5 Bypass & Link
Bypass
Activating the bypass effectively removes all processing being
performed by your ACP-22 and returns the signal to unity gain. You should
use bypass often when setting up your ACP-22 to compare the 'before and
after' results of the signal processing effecting your audio signal. Bypass
affects both the gate and the compressor. When in Bypass the Link function
is interrupted and defeats the stereo link.
Link
When the Link button is pushed in, channel One becomes the master
controller of both channels. Channel Two's controls become disabled and
metering becomes summed left and right, i.e. stereo mode.
IMPORTANT:
When the two channels are linked (link button pushed in), all of
channel Two's controls are inactive.