PreSonus ACP22 - Manual (Page 14)

Controls & Connections
+4/-10 Switch
This switch adjusts the internal operating level of your ACP-22 when it
is connected to line level (0dB = -10dBV) gear. With the switch in the '-10'
position, the signal is raised internally so that it is processed at the lower
noise floor of your ACP-22's internal circuitry. The signal level is lowered
on the way out to match up with your line level gear's input. When the switch
is in the '+4' position, the signal is not changed since this matches the internal
operating level of your ACP-22.
This function may also be useful in creating special effects with the
compressor and gate of the channel by 'overcompressing' or 'gating'.
Compression and Gating Sidechain
The sidechain jack on each channel interrupts the signal that the
compressor and/or gate is using to detect the audio signal for processing.
When no connector is inserted into this jack, the input signal goes directly to
the compressors control circuitry (normaled). When a connector is inserted
into this jack, the signal path is broken. If you have inserted a tip-ring-sleeve
connector, the input signal is sent back out of the ACP-22 via the ring of the
connector. This signal can then be processed by an equalizer for example to
reduce sibilance (de-essing) in a vocal track. The signal is then returned to
the unit via the tip of the connector. The signal sent to the tip could also be
that of a narrator or vocalist. In this application, the audio that you are
passing through the compressor will automatically 'duck' when the narrator
speaks or vocalist sings.