Hi Robnev

you set the Key Input to "listen" to any input of your interface.

From page 5 of the manual (available here: www.serato.com) we have:

About the Key

Both of the Rane Series Dynamics plug-ins have key inputs that drive the gain reduction. The key input can either be internal (the input audio that is being compressed or gated), or external (some other audio signal). By default the key is internal; to switch to an external key simply choose a signal as the key input to the plug-in using the standard Pro Tools or VENUE menus. By deselecting the key input in Pro Tools or VENUE the plug-in will revert to an internal key.

Both Dynamics plug-ins also have key filters, the Compressor has a parametric EQ, and the Gate has a cut filter pair. To distinguish between the key input pre-filter and the key post-filter, this user manual uses the terms ‘key input’ and ‘key’ as follows:

Key Input is used to describe the pre-filtered, internal or external signal coming in to the plug-in that is filtered to become the key.

Key refers to the post-filtered signal that is then used to drive the gain reduction. In dynamic EQ mode the key might also be called the bandpassed key, because both the bandpassed key and broadband key (the key input) are used to drive the dynamic EQ gain reduction.

Hope this is clear, let us know how you get on.

Cheers

S