PAIA 9209 Ducker + Gate Stereo Dual Compressor

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"Controls include:
• On/Off
• 4 way mode switch: A/B, Gate, Duck, Bypass
• Threshold
• Hold
• Channel A amount
• Channel B amount

Back Panel Connections:

• Channel A In and Out
• Channel B In and Out
• Key input (audio trigger)
• S-Trig input (switch trigger)
• 12VDC power input

Here's what Paia says about the unit:
You can think of a Ducker as lo-tech automation - invaluable when hands to ride gain controls are limited. The PAiA Ducker+ is particularly useful because in addition to the ducking function, it can also serve as a stereo gate and has an unusual toggle mode that lets you use it as an audio router as well. Front panel controls consist of Trigger Sensitivity with LED "active" indicator, Hold Time to keep the outputs active during brief quiet intervals, individual Attenuation Controls on the program channels and 4 position Duck/Gate/Toggle/Bypass Mode select switch. A Footswitch Jack provides for manual control. Works with either line or low level signals.

Duckers automatically decrease the amplitude of program material in response to the presence of a Key signal, such as narration. Ducking the program source makes audio space for the voice-over.

In Gate Mode, the Ducker + boosts the level of the program channels in response to the key signal. When the program and key inputs come from the same source, the result is a noise gate that mutes the channel when the program level drops low enough to make the noise floor noticeable.

In toggle A/B mode, one channel Gates while the other Ducks. The footswitch jack makes this a handy manually controlled channel selector or switchable preset effects mixer."