The choices for MIDI Modulation Sources in Omnisphere include a variety of standard MIDI controllers such as Wheel, Legato, Expression, etc. You can use these options on the SYSTEM Page to map those sources to other MIDI controls.
For example, if you don’t have a particular physical controller such as an Expression Pedal, you can map Expression to Breath Control and use a Breath Controller in any Patches that use Expression as a Modulation Source.

To re-map any of these controllers, select a menu in the MODULATION column and move the controller that you want to assign to Modulation Type.

NOTE: The remapped control will only be visible on the SYSTEM Page. In the Modulation Matrix and Modulation Source menu of the Edit Page the original Modulation Source name will be shown. For this reason, it’s recommended to use the “User CC” source, since that’s not used in any Factory Patches.

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