Nitro does not allow to directly assign an editor control (knob, switch) to a MIDI CC. Instead, Nitro gives you much more powerful MIDI control. It is the MOD display tab where you set up all the modulation, including MIDI. You can choose virtually any MIDI message to be the modulation source (midi note no, midi note pitch, velocity, aftertouch and, of course, control change message). The main advantage of such method is that you can control more than one processing parameter with one MIDI message type, and each of the parameters can be controlled in a different way (you can set up the depth of modulations slots irrespective of each other - this allow f.ex. the filter cutoff to be increased and the resonance to be decreased as you turn the knob on your MIDI control surface). If you want simple one-to-on midi to parameter assignment simply choose the proper MIDI CC no to be the modulation source (use MIDI learn feature if you want), set the modulation depth to +100% and choose the modulation destination to be the parameter you want to control. Also you will need to switch the modulation on by clicking on the 'status' column next to the modulation slot.
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Hubert Pietrzykowski
DSP Engineer
PSPaudioware.com