Loop-back using two physical sound card inputs

Go to Measurement->Settings… and click the  button.

Choose a different line Loop-back input than the line you have selected for performing the measurement. If your Loop-back input channel is mono, make sure the Left channel is chosen as shown below.

Connect the output of the loop-back (which is the right channel) using a cable to the line and channel you have chosen as input for the synchronization loop. Note that you may have to split the cable you are connecting to your output to access the output-signal that is on left channel (see the figure in the previous section).