In order to allow you to loop audio from one application to another, the iD4 MKII software includes the Loopback Mixer.
This allows you to blend the Mic and DI inputs with audio playing back from your computer into a stereo signal that can then be recorded or streamed. Simply raise the faders for the channels you want to include in the Loopback Mix.
The channels are as follows:
Mic 1 - The signal from the Mic/Line input on the rear of the iD4.
Mic 2 - The signal from the DI/Instrument input on the front of the iD4.
Main Output 1/2 - The audio currently playing back through the headphone and speaker outputs. This would be audio being sent to outputs 1+2 in your DAW.
Loopback 1/2 - The dedicated Loopback outputs of the iD4, which allow you to loop audio without affecting your main playback. This would be audio being sent to outputs 3+4 in your DAW.
Using the Loopback Mix
To record or stream your Loopback Mix, simply select either Input 3+4 or Loopback 1+2 in your audio software. The name displayed will depend on the DAW or application you are using.
If you’re unsure how to select this in your software, check the documentation for your application for more details. This setting will typically be found in its audio preferences or input settings.
Sending Audio to the Loopback Outputs
Most DAWs and audio applications allow you to select the audio output within the application's audio preferences or mixer. For details on how to do this, check the manual or documentation for your software.
For applications that don't provide an output selection, you can instead adjust the system audio settings.
macOS
To adjust your output channels, open the Audio MIDI Setup utility:
Finder > Applications > Utilities > Audio MIDI Setup
You should now see a list of audio devices currently connected to the Mac. Select the iD4, go to the Output panel and click Configure Speakers. Set the outputs to Loopback 1 and Loopback 2.
Click Apply. System audio will now be sent to the Loopback channels instead of the Main Outputs, ready to be sent to another application.
Windows
On Windows, you can change the system output by going to the Sound settings in the Control Panel. Go to the Playback device list and select Loopback 1+2 as your output device.