2-channel loop-back for sound cards with two physical inputs (e.g. line and microphone input)

If your sound card has two input lines, e.g. Line and Microphone input, no external device is needed to connect the loop-back. Choose different lines for Input Line and Loop-back Input Line found in the Measurement->Sound Card Settings dialog box. If your Loop-back Input Channel is mono, make sure the right channel is chosen as shown below.

Connect, using a cable, the output of the synchronization loop (which is on the right channel) 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.

Figure 7 Schematics for two-channel measurement using synchronization loop-back for two physical inputs.